The Radiant Heat Experiment – Did it Work?

One whole year ago, I was in the depths of destroying and rebuilding a sagging 1950s brick ranch house, which has since grown up to become our actual home. Looking back through Google Plus’s automatic archive of my phone pictures, I can see the “kitchen” was still open to the great outdoors on that date:


Despite the lack of windows and insulation, I was already looking ahead with nerdy engineering glee to building a home-brewed heating system for this place, and I told you about it in the article called The Radiant Heat Experiment.

In a nutshell, this involved running thousands of feet of PEX pipe under my existing wood floor via the crawlspace and circulating hot water through it with a pump and this high-end Rheem tankless water heater.

The plan was met with both enthusiasm (generally from fellow engineers) and scorn (more often from plumbers), and since then people have been sending in emails and comments to ask how it all went. Although I’ve already dropped a few hints that I’m very happy with the end result, this experiment came with some good lessons and pitfalls which are finally ready to share.  I have also had a chance to measure the performance of the system (and the house in general) through most of a Colorado winter, and the numbers surprised me just a bit. So let’s dig in.

How it All Went Together

Last time I presented you with a daunting list of parts. The list makes a lot more sense when you stick everything together. Here is a picture of the heart of my setup as it stands now, with everything screwed onto a plywood board:


The funny part is that all of the brains of the system are right there on the board. All the research and shopping boils down to just that 2×3 foot rectangle. The input is hot water from your water heater on the left, 120 volt electricity for the pump through the orange cord, and a pair of small wires you connect to your thermostat to indicate “ON”. Then the hot water flows out through the zones, delivers its heat to your house, flows back into the cold side of the manifold, and returns to the heater for another round. If DIY radiant heat were more common, this whole setup would come as a single product for $199 at Home Depot instead of the $600 or so you see here.

It took only about two hours to attach all this together, and then I confidently crawled down under the house with it to get to work on the rest of the installation. Little did I know that the real work was yet to begin.

Running the Heating Tubes

This system proved to be quite torturous to build, but it was because of plain old physical challenges rather than anything technical or mental. The problem is that to install radiant heat below the floor of a wood-framed house, you need to thread a huge length of stiff, fussy pipe through an unyielding grid of tightly spaced floor joists. I divided my house into six zones, each one about 250 square feet in area. For each of those zones, I had to do the same steps:

  • Meticulously review the underside of each joist bay and clear out any remaining scraps of duct work, old plumbing, spider webs, etc.
  • Grind off a few hundred flooring nail ends poking through from the original Oak floor above using a cordless grinder with a cutoff wheel
  • Drill a 7/8″ hole through the end of each joist
  • Pull through the whole required length of PEX pipe, fighting the stiff tangly coil the whole way
  • Staple it up to the underside of the floor, using aluminum reflector plates
  • Run the ends of the tube back to the manifold and connect them into the system
  • Cut and fit R-13 insulation batts underneath the whole area to force the heat upwards into the floor instead of down into the crawlspace.

The end result in each bay looks something like this:

Here's the end of one joist bay. Tubing, aluminum diffuser plates (optional), R-13 insulation batts underneath (essential)

Here’s the end of one joist bay. Tubing, aluminum diffuser plates (useful), R-13 insulation batts underneath (essential). The fluffy spray foam insulation on the right is part of my new crawlspace insulation – not strictly related to a radiant system but handy for keeping the resulting heat from leaking out through the rim boards of your house.

I found that each 250 square foot zone took about eight hours of work. But not just a casual eight hours that flies by like it does when installing kitchen cabinets while your radio plays happy bluegrass music in the background.  This is eight hours of proper torture, crawling in a 40-inch-high space with sharp rocky dirt beneath and obstacle-laden floor joists above. Even the slightest movement stirs up thick dust, so you have to wear a full-face respirator. That’s handy, since the grinder also throws down hot metal sparks towards your eyes and face. Gloves and kneepads are essential too. And ear protection. It’s dark down there, so you also have a bright LED headlamp strapped over top of all the other accessories on your head. But the ground-driven temperature of 60 degrees is far too warm for the work pants and long sleeves you need to wear to avoid skinning your arms and legs, so you also sweat a lot. In general, I could only withstand about 2 hours of this work at a time, so each zone was done over four days.

But if the paragraph above sounds horrible, you’re just thinking about it wrong. This is voluntary hardship at its best. The physical and mental benefits of crawling and sweating and fighting with stubborn tools and materials for so many hours are incomparable. Every possible move is constrained, so you must overcome the constraints with strategy and strain. The feeling of suiting up and descending into the crawlspace each morning while knowing I could earn much more money by outsourcing the activity and instead simply typing a bit more shit into this computer was enlightening. The feeling of emerging two hours later into the fresh air and bright sunlight, stripping off the dusty clothes and seeing the beauty of the world again was life itself.

Even with all that struggle and joy, I paused the effort* after finishing four of the six zones. Those cover the primary areas of the house and are more than enough to keep up with our heating needs for the rest this year. I’m finishing up the main floor carpentry and a second bathroom, and those last two zones will go in before next winter.

Real World Performance

The Living Experience

This was the unexpected surprise – how nice it is to have warm floors. Your feet get a pleasant reward with every step you take, or as you rest them on the wood floor under the dinner table. On top of that, anything you leave on the floor gets extra toasty: a pair of wet winter boots, a forgotten coat, or even the socks you threw off before hopping into bed – perfectly warm and dry when you pick them up the next morning. The bathroom floor also dries quickly after a shower.

Keeping up with the Cold

On a “normal” January day in my part of Colorado, daytime temperatures reach about 43F/7C, but the extremely bright sunshine makes it feel much warmer. The South-facing glass of the house sucks in about 10,000 watts of heat at high noon and it gets stored in the copious thermal mass of the various interior stone and brick walls. I blow it around with a ceiling fan to accelerate this process and the interior temperature reaches a peak of around 76F in the afternoon. Then the sun goes down, the stored heat is gradually released, and we make it through the night (low around 20F) with the house dropping to a comfortable 66. If you’re lucky, the sun rises into a clear sky the next morning and you repeat the cycle – with no heating required at all!

But weather adjacent to the Rocky Mountains is anything but consistent, and this winter we have also seen an all-time record low of -14F (-26C) as well as a daytime high of 77F (25C) just a few weeks later. This is why you still need a heating system with some juice.

With only four zones running at -26C, my house was a bit underpowered – the temperature would drop slowly unless we lit a fire (I also added a wonderful high-efficiency EPA woodstove to the house – another story). From a standstill, the system also takes about two hours to get the floors to their full operating temperature of 80F. However, the remaining two zones should provide the extra bit needed to keep up in worst case conditions.


To test this, I had to calculate the amount of natural gas I burned every hour, and compare it to the amount of heat actually being pumped into the house. I did this by cranking up the system on a cold day and taking “before” and “after” readings of the gas meter, and noting the flow rate and temperature drop** across the whole system:


Here are the things you need to look at to calculate system output and efficiency.


To make a long story short, the gas meter told me I used 40 cubic feet of natural gas over my 144 minute test period.  The gas bill tells me that each 100 cubic feet is 0.945 “therms ” (94,500 BTU) worth of heat. One therm costs 62.67 cents in my area. The net result is I was consuming 15,740 BTU per hour of gas, which is just under 10 cents worth per hour.

Next, I added up the (approximate) flows of those four flow meters and saw the system was pumping out 1.68 gallons per minute of water with a 16.5 degree F rise. You can calculate the energy delivered to the water with the “Universal Hydronic Formula” like this:

1.68 GPM x 16.5 degrees x 500 = 13,680 BTU per hour

Back in the design stage, this is roughly the heat loss I calculated my house would experience at a temperature of 20F, so the numbers seem realistic to me. Also, dividing the output by the input, we get a water heater efficiency of 88%, which is close to my unit’s stated efficiency of 94% (efficiency rises for lower input water temperatures, so I’m very happy it can perform this well with a 118F input).

On top of all this, I measured total electricity consumption (for the tankless heater and water pump combined) at only 55 watts, which is under $4 per heating month even if you run the system 24 hours a day. To add it all up, my home’s total gas cost this year breaks down roughly like this:

Gas company fixed monthly fee whether you use any or not: $12
Regular monthly gas use for showers, laundry, dishwasher, cooking, etc: $4
Heating for Oct 14 – Nov 12: $8
Heating for Nov 12-Dec 15: $55
Heating for Dec 15 – Jan 16: $58

And that’s probably the peak – here in February, the weather is already warming up and the system is off most of the time again.

So What’s the Catch?

When I started this experiment, I was optimistic that we could revolutionize home heating and make the forced air gas furnace obsolete. After all, the cost is lower, living comfort is greater, and you save a lot of interior space that would normally be consumed by ducts and chases – especially in multi-story homes. But until the industry advances a bit, there are a few flaws:

Building was Quite a Bitch

Installing this was near the limits of my skill and endurance, and I’m a not-all-that-old dude with lots of great tools who has been building things since I was a little kid.  However, it would be much easier if you installed it in an unfinished basement instead of a crawlspace. Also, recruiting as many friends as possible to thread the pipe will speed you up exponentially. Overall, I’d recommend it only for experienced handypeople.

Heat Output is Lower than Expected

I’m getting under 14,000 BTU per hour over the 1000 square feet I have installed so far. This works out to 14 BTU/hr per square foot. This place is pretty well insulated, so I should be fine. But an older and draftier house would lose more heat. The problem lies in the slow transmission of heat through the 1.5 inches of my subfloor and the oak floor above it. To increase that, I’d need to raise the water temperature further (it is already at 140F) or add some extra radiators.

On the positive side, you can get really creative with radiant heat, embedding the tubes into tile walls, or making heated towel racks in your bathroom that tie into the system. Each extra heated feature will deliver more BTUs. Also, installing under a tile floor instead of wood floor will increase heat transmission.

Not all Tankless Water Heaters will Work

In reader feedback, I heard stories of tankless heaters dying early or cycling constantly. Cycling is not a problem with the unit I used – it runs at variable speed so the system quickly reaches a nice loafing steady state where the pump is going slowly and the heater is barely murmuring to match the required flow and temperature rise. Time will tell how long it lasts, but I’m betting it will prove to be far more cost effective than a $3000 boiler.

The Open Loop System Has Drawbacks (as well as advantages)

I am using a single tankless heater for both home heating and domestic hot water – this is called an “open loop” configuration. It would be easy to add a second basic heater for $600 for the domestic water. This would separate the water systems, and I may do that someday.

The main drawback of combining them that you need to keep the water heater set very high (140F) to get enough heat output to the floors. This means somewhat fussy water temperature balancing in the shower, whereas with a dedicated tankless heater you just type 110F into its remote control unit, crank the hot water handle, and enjoy a computer-regulated perfect shower every time.

A second issue is that the hot water can sometimes smell like new plastic pipes. This effect faded to zero after about three months, but it is worth noting, especially if you are installing your system in a house with people likely to complain about this. All components I used are after all specifically designed for potable water.

On the positive side, I found that if you run hot water when the pump is off, water is drawn through the system through natural pressure differences. This means that in the summer, my floors will actually be cooled down by the cold water supply as it sucks unwanted heat from the house. So the floors will pre-heat the water before it hits the water heater. Double energy savings and free air conditioning.

Because the water supply is constantly refreshed and/or heated to 140 degrees Fahrenheit, bacteria growth and stale water in the pipes is not an issue at all.


It has been a worthwhile experience. Loads of learning, plenty of hard labor, a luxurious end project, and an $8,000 savings over having a new forced air furnace and duct system installed into this house. Although DIY radiant heat is not for everyone, I can declare this particular experiment a success.


* To finish up next fall, I will also swap out the manual adjustment dials (white knobs in the picture) for electrically controlled actuators, and use a multi-zone WiFi thermostat to control the whole house. This thermostat is being developed by an MMM reader who has started his own company to produce it – more details on that in a future story.

** The temperature drop is configurable with a little knob inside the computer-controlled circulator pump by Taco. I set my own pump to maintain a differential of 20 degrees F, which is typical for a system like this. Then if the pump starts seeing a drop of more than 20 degrees, the pump runs faster to compensate. If it is less, it means your house is already warm so the pump runs slower.


  • Dave C February 7, 2015, 8:06 am

    Really cool to see the results of your experiment. My wife and I live in a 100+ year old house in KW with hot water radiators. Love the heat they put out. I’ve been doing research on eventually replacing the boiler (21 years old).

    One option you could look at to change from an open loop system is to add another zone for an indirect hot water tank. You would just add a zone that would run to a hot water tank with a large copper heat exchanger inside. You could probably DIY the tank yourself if so inclined. The indirect hot water tank is commonly used in heating systems with a boiler.

    If you had to do this again would you consider using hot water rads instead? The plumbing would be much easier and the calculations for sizing rads are simple. Downside is that you don’t get the same nice toasty floors. You also have rads taking up space and probably need to run the water a bit hotter.

    • Teacher Lady February 7, 2015, 12:39 pm

      Yes! Hot water radiant is wonderful! So clean. Our old house had that system and I loved it. I don’t think the more modern baseboard radiators take up too much space!

  • Adam February 7, 2015, 8:28 am

    Looks like an awesome project though definitely beyond my humble DIY skillset. I’d love those warm floors on cold mornings.

  • Howie February 7, 2015, 8:56 am


  • TheGoyWonder February 7, 2015, 9:48 am

    1. If you’ve removed ducts – good luck ACing a house with massive south-facing windows.
    2. Ballparking that there are several times the number of failure points in this system than typical plumbing. If one thing goes wrong you’ve damaged the floors and have negative ROI.
    3. You’ve increased hot water temp: you’ve decreased your hot water efficiency and probably not really getting $4/month anymore. Maybe you’re well within PEX spec but heat is the enemy for your polymer lifespan. Also if somebody burns themselves one time you have negative ROI.
    4. Thermal resistance of subfloor & floor means you’re losing way more heat thru the bottom of the house, and you’ve lost the on-demand nature. If you let the temp fall at nite you’ll never get it back by morning.
    5. What’s so great about warm floors if you’re wearing shoes…if you’re trying to save on heat you’ll surely wear shoes/sox instead of running around barefoot like a child.

    • Dave C February 7, 2015, 10:00 am

      1. Easily doable with a ductless mini-split. Also appropriate shading and overhangs in the summer cut out the worst of the sun.

      2. Most of those loops can be run with minimal connections which is where a failure would occur. It would also just end up damaging the insulation under the floor.

      3. Oxygen barrier PEX is rated for high temperatures and regularly used in hot water rad retro-fits. It’s perfectly fine to run at a higher temperate. He’s not even close to the 55-70 degrees Celsius people run rads at.

      4. You’re right hardwood floors are not ideal for radiant heating. Tile or cement are what works best.

      5. Wearing shoes inside the house? Who does that? Even with socks or slippers on the warm floors are very noticeable and pleasant.

      • Chachatat February 10, 2015, 2:26 pm

        5. I certainly don’t wear shoes when getting out of bed in the morning…

        4. Although hardwood isn’t as efficient as putting the pex in a concrete slab, plus heat going through subfloor and then surface floor, it still DOES produce heat efficiently. Frequently the cost to get a couple more degrees of heat is not worth the added cost of having a more direct heat system, especially in a retro-fit.

        3. Most state codes require the showers have something to regulate the temperature so that you can’t possibly burn yourself. As for the sinks, most individuals run cold and hot together even if their tank water heater is set at 120deg. If the tankless is set at 140deg to account for the floor heat the individual will still do a cold/hot mix.

  • James February 7, 2015, 9:50 am

    I assume you have considered it, but the heat loss in all the components on that board would be quite substantial if it is not in the heated part of the building. I assume that is the crawl space, so all the heat loss from the manifold, pipes, pump, etc, is being lost into the crawl space. Since the fresh hot water goes through there, it will be at the peak of temperature, and constantly heating that lower manifold, valves, pump, etc (which are metal and radiate heat fast) to max temp is going to cost a lot of energy.

    I would suggest starting by insulating all the pipes, and maybe use foam blocks to insulate the board itself? The less heat loss before the water runs under the flooring the more efficient the system will be.

    Having said that, if I was installing in a new house I would place the components in a small utility closet within the heated space. The heat built up in that closet would simply radiate to the house, rather than being lost outside.

    • Emg03036 February 8, 2015, 4:31 am

      Alternatively, the crawl space can be sealed and insulated on the perimeter relatively easily (if MMM hasn’t done it already, which I imagine he has). Otherwise I would imagine you would want to insulate under all of the runs to guard against downward heat loss from the pipes into the crawl space. By putting the crawl space inside the thermal envelope, you get the benefit of all of the soil underneath acting as a thermal buffer.

  • ThriftyHamster February 7, 2015, 11:06 am

    Interesting experiment. I’m curious to see how it all fares in the long term.

    Personally, I wouldn’t use a combined potable/home heating system. Each to their own.

    A note about the pipe used, not all PEX is rated for potable water so make sure to take that into account when installing such a system. Depending on the type of system use, radiant floor heating is either low or high temperature. In a slab or over pour the pipes are embedded within the thermal mass so a water temperature of 120-130F will suffice. On the other hand, in a floor joist installation the pipes heat the air space (or inefficiently the wood structure) which in turn radiates above. To overcome these inefficiencies it is recommended to have a water temperature of 180F, same as in a typical baseboard system.

    Having installed my fair share of in-floor heating I can safely say never go at it alone if possible. Unless of course you really enjoy having the added workout. :)

  • Jon February 7, 2015, 11:28 am

    Thanks for this, would love to see more similar experiments in home efficiency. Maybe some sort of solar electrical system? At what point does that start to make sense, is there a way to do it cost effectively? MMM, I know your electricity usage stays nice and low, so maybe tough for you to benefit financially, but on the other hand I might even be willing to pay a premium just for the environmental benefits.

  • CL February 7, 2015, 12:13 pm

    Glad to see this awesome write-up on the impact of the radiant heat experiment! I look forward to the future post by the MMM reader who is doing a business with this stuff!

  • Lauren February 7, 2015, 2:38 pm

    Just wondering, what is the environmental impact of a radiant heat system? With MMM living in a fairly arid area, I would imagine that water use is a concern.

    • woodnclay February 8, 2015, 11:35 am

      Lauren, the system doesn’t consume water. As with all radiant heat systems, water consumption is negligible. Environmental impact of plastics pipes *may* be of more concern but this could be case of TDES squared. Overall MMMs environmental impact has been dramatically and impressively reduced over the years of this blog.

  • just call me al February 7, 2015, 10:20 pm

    I can’t take any credit for my home’s system since I bought the home from the original owners. Hell, I can’t even explain this fancy shit. But, my lower level is heated by this stuff you explain here. Mine is embedded in the concrete floor and hooked up to a normal gas water heater (I have 2 water heaters; one for house use and one dedicated to the in-floor heat on the lowest level). I also have this box thingamajig hooked up to the dedicated in-floor heat water heater, it’s called an airtap that uses heat from the air to assist the water heater– by taking heat from the air and somehow transforming it to economical and efficient heating for the water heater. I just say…what? But it works. It’s great. It’s toasty. And any plumber or furnace guy that comes to my house always drops their jaw in awe of it.

  • Emg03063 February 8, 2015, 4:46 am

    Thanks for the great follow up article! Out of curiosity, do you have an operating cost comparison of your system vs. forced hot air gas you can share?

  • jestjack February 8, 2015, 12:32 pm

    Thanks for sharing the results of your heat system. You are a better man than me….I worry about insurance issues should the system fail…have you considered this….and if so what did you find and what type of “certs” are required? In addition I worry about the long term affects on the flooring and condensation issues…I’m with other readers that would have chose to heat with wood. A new efficient woodstove in a “super insulated house” like yours , that’s only 1000 square feet should be real easy to heat. I heat with wood and get wood from the local saw mill for a great price. Haven’t used heating oil in over 8 years at home…Gotta tell ya I just pulled radiators out of a rental and installed electric baseboard with individual stats and I should have done this years ago. Boiler replacement would have been $8K…Love the simplicity of baseboard and the scrap from the radiators paid for the heaters. I’m truly happy for you and make no mistake you are a “pioneer”….But remember the “pioneers got the arrows”….LOL…Thanks again for sharing!

  • Matt February 8, 2015, 4:41 pm

    We have in-floor radiant heat, via copper pipes that contain heated water. The problem with the system that I discovered has to do with sleeping difficulties. I am a very light sleeper, and am very sensitive and prone to overheating (which prevents me from sleeping). The radiant heat shoots right into the bed, turning it into an oven. On top of this, radiant heat tends to go through more dramatic highs/lows as the thermostat struggles to reach the set point; generally the response time is much slower than forced air. This can be circumvented by getting a smart thermostat like a Nest, though we have not done this yet.

    Anyways, my overall opinion is that radiant heat is not a good idea that tends to overheat in bed. That said, I will admit that the warmed floors do feel awesome on your feet.

    • woodnclay February 9, 2015, 7:16 am

      How about turning the heating off at night? Or whenever you are in bed.

    • Chachatat February 10, 2015, 2:31 pm

      A family friend of mine installed his own system roughly 15 years ago. The bedroom is upstairs and is on a different zone than the living room downstairs. He installed valves on each zone for temperature control. He usually has the bedroom zone valve closed so that the hot water isn’t running under that room and leaves the valve for the living space open. He finds that the downstairs zoning will heat the upstairs to a comfortable level that doesn’t effect his sleep. Basically, the bedroom heat is off.

  • Simone C. February 8, 2015, 9:57 pm

    Hi everybody,

    I’m from Italy and live in the US for almost 10 years now.

    Great job MMM! Radiant heat is quite common in Europe. My parents restored an old apartment and had it installed there too. The apartment is all made of bricks and cements and tended to be quite humid. Now, with radiant heat, the bill is much lower and the temperature is comfortable during the winter.

    Underneath the marble tiles, downstairs, and the hardwood floor, upstairs,, the workers installed amolded insulated underfloor, with “tracks” for the pipes. I don’t know whether you can find it here, but for those who are planning on installing a radiant floor, it might be helpful to see whether it is available.

    Here you can see some pics: http://www.archiexpo.it/prod/amvicsystem/pannelli-isolanti-polistirene-espanso-riscaldamento-pavimento-senza-barriere-vapore-forte-densita-67110-1497197.html

  • Emil February 9, 2015, 3:07 am


    Water heated floors are more or less the standard for high and midlevel new houses and refurbishments here in Norway. Everything you need is available off the shelf, and many plumbers have experience with water heated floors if you don’t want to DIY. Putting the pipes underneath the existing floor is not a common solution, because of poor perfomance. The usual method is using particleboard with grooves precut for the pipes, and then hardwood or laminate floors on top, like this: http://www.interplywood.no/wp-content/uploads/2014/05/Byggelit-Tempo-Va%CC%88rmegolv-25-mm.jpg

    One of the big money savers with floor heating is that it allows you to keep a room temperature about two degrees C lower than without heated floors, because the room feels warmer with heated floors.

  • Jim February 9, 2015, 9:37 am

    There’s no next button here!

    For the past ~2 weeks I’ve been able to binge on badassity – and no more.

    To the forums I suppose!

  • Charles February 9, 2015, 10:14 am

    you should check out


    Might help generate more heat and be more efficient.

  • Money Saving February 9, 2015, 2:52 pm

    Awesome that it worked well for you. I remember back to the original post that I was worried the plastic tubing would not transfer heat well. It looks like that hasn’t been an issue for you.

    Awesome project!

  • Jeremiah February 10, 2015, 10:48 am

    Awesome! So glad it worked well for you. I’ve been thinking of this for years, but the payback on replacing my furnace isn’t there.

    Also helpful to know you didn’t get a lot of heat through your subfloors. That would be a game-stopper here in Wisconsin. Thanks!

    • tlars699 February 13, 2015, 9:08 am

      Fellow Wisconsinite here.

      My Grandfather installed radiant heating pipes underneath his rentals tile and carpet floors in Northport, WI, and those were very toasty.
      There’s also different links in other people’s comments about stuff that you can put in to replace some sub-floor that would increase the heat’s diffusion properties.

      Also, is 1.5″ of sub-floor normal? That seems to me to be an excessive amount of plywood before you get to your flooring (Which can also add an inch to your floor’s depth or so). <- IANACarpenter, so forgive me if I'm clueless in this regard.

  • Stan February 11, 2015, 8:09 am

    I work for an electric utility and you have confirmed what I have seen many times. In floor heat is not the warmest or easiest way to heat a house. The slab under the house will maintain cold which takes away heat from the system and a domestic water heater isn’t made to be running constantly. Depending on the hardness of the water lime will build up over time making less volume inside the water heater. I had someone say their water heater wasn’t working and when I tried to remove the lower heating element it turned hard, real hard. When it finally came out the entire element had been covered in lime. This doesn’t happen in a closed system with a boiler. If an electric water heater would have been used the elements wear out fast. Just my observations.

  • Kyakerdog February 11, 2015, 10:09 am

    I am working on a similar radiant project on a recently purchased home (my future home as part of my mustache-ian plan… that is for another time). I am also planning for an open system as my current home has radiant heated concrete floors (12 years old) with no ill health effects. I wanted to keep the option of converting to a closed system in the future which means an oxygen barrier in the PEX. I see you used this type of PEX in your system but it is not certified for potable use. Are you concerned about this? There are 2 issues here: 1. Potential but unlikely health concerns and 2. resale and inspections for using non-potable PEX for Potable water.

    I already purchased the Oxygen Barrier PEX but I am considering returning it. Any help here would be greatly appreciated!

    • Mr. Money Mustache February 12, 2015, 6:00 pm

      I ended up using potable water PEX to accommodate my open loop system, rather than the Oxygen barrier (non-potable) type.

  • Tom February 13, 2015, 3:55 am

    Thanks MMM, been waiting to hear from you on this experiment. I’m a big fan of the Tiny House movement and most tiny house builders install tankless water heaters. I’ve made mention your idea countless times in varying forums that it makes little sense to purchase a second heating unit like the insanely expensive ($1100) Dickinson propane unit, so common in tiny house builds, to heat the space.
    I said, why not just install a radiant system as you’re building it? I found this product ( http://www.warmboard.com/ ) an aluminum clad plywood with routed channels that IS the subfloor. Combine it with a tankless heater to heat 120 -200 sq ft. seems to me at first glance more efficient and less costly than purchasing a second heat source.

    • chachatat February 13, 2015, 9:52 am

      I looked into warmboard and for a 2100 sqft it would come out to $7.50/sqft (includes designing the number of zones and where). Which is a bit on the spendy side for me.

  • Kyra February 15, 2015, 4:22 pm

    Hey mom, I have a question for you. I believe I read in a post somewhere about your occasional love of lighting up a bowl. I live in Washington where it is also legal, and I would love to hear about how you keep your spending realistic in this aspect. I usually find that I have to spend quite a bit for quality bud, so what would be a good way to remain mustacian without giving up one of my favorite things?

    • Mr. Money Mustache February 16, 2015, 1:15 pm

      I’m finding Marijuana is almost free on a per-serving basis, because it is so strong and it takes so little to have a good time.

      On top of that, it is legal to grow your own here, which has resulted in a general gifting culture where bags and jars of excellent weed tend to just materialize and circulate among friends. Although I consume very little (maybe 1-4 Good Times per month), I always have plenty around. Stop by if you’re my town someday!

  • Bob W February 16, 2015, 1:58 pm

    Thanks for that update. Will definitely consider a system like this for future projects and for the anticipated enclosure of my 400 sq ft deck.

    I had given up on this idea as a local manufacturer of floor based heating systems (one of the nations largest) had been sued so many times by failing systems. Apparently they lead a lot. But in your application when they eventually leak there will just be some mudd to deal with and not a completely ruined lower level.

    I would be concerned about putting something like this on a second story.

    By the way, my neighbor growing up had a similar system in his concrete basement, which was built in the 1940s. (copper tubing was cheap then)

  • afox February 18, 2015, 1:47 pm

    Very surprised you did’nt mention latency as a drawback of the radiant floors. In my radiant floor research latency was listed as probably the biggest drawback. The fact that it may take between 2 and 5 hours between the time that you change the thermostat and the time that the temperature of the living space changes significantly. This is a problem for fellow moustachians like myself that aren’t home much and turn the heat way down (off) when they are not home.

    Love that you built the simple s. facing solar thermal heating system (windows + sun) into your home. This is by far the cheapest source of heat available. All houses should be designed this way when possible.

    Love that your system does not require duct work, its takes up way too much room.

    If i were starting from scratch id use wall mounted heat pump units. No ductwork, heats the home up quickly, inexpensive, and relatively easy for a DIYer to install.

  • Erik February 23, 2015, 10:30 am

    Never had to worry much about heating growing up in SoCal, but then I moved to South Korea, where this kind of system is the standard(ondol), and I LOVED it. Cheap as all hell, and far more comfortable than hot air. Living now in Shanghai, pretty much the only thing available is reverse-cycle air conditioning, which means if we run it, its expensive and the house dries out. Ironically, it’s my Canadian other half that runs it the most. I almost never turn it on voluntarily, except on the coldest of days, which really aren’t that cold.

  • JT February 24, 2015, 1:32 pm

    Installing as a retrofit under a crawlspace where the heating tubes are supposed to transmit heat through subfloor and finish floor is basically the worst case scenario for this application, so the fact that it works at all is fantastic.

    Sadly, natural gas isn’t offered by Nashville in my neighborhood, which means hydro-radiant in floor heating isn’t really feasible for me.

    I *do* plan to try it at my off-grid cabin, with some combination of wood-powered boiler/passive solar heating for the water.

  • Jim Sunderlin February 28, 2015, 3:06 am

    Fantastic system and writeup. Another benefit of your system over typical hydronic heating systems such as the one I had in my Eastern Idaho home is the reduced noise from the boiler.

    When I bought my 2001 built home in 2005, I was very skeptical as the 2 story built on slab house was 100% radiant heat and there was nothing special about the house regarding any type of passive heating/cooling. And the boiler looked like a half size water heater. So I thought “how could this possibly keep a house warm in Idaho. But after the first winter with one day hitting minus 40, that house was “toasty warm” from top to bottom. My only issue with the radiant heating system was that noisy boiler when it was firing.

    Great job and hope you keep everyone updated with any modifications or recommendation for future installs.

  • JJQ March 6, 2015, 1:06 pm

    Hey.. Nice article.

    Have you seen these:


    Check out the “integrator panel”. Looks a lot like your DIY.

    Might take the “fun ” out of serious DIY build from scratch… But…
    Guaranteed to work at about 7-10x the cost.

    • Mr. Money Mustache March 9, 2015, 4:08 pm

      Thanks JJQ, I didn’t know integrated setups exist. But wow, that is a lot more dough for a MUCH lower quality water heater. Makes me feel glad I did the homebrew special!

  • Frugal Dutch March 12, 2015, 3:53 am

    Hi all, I am a vivid reader of MMM blog and all of your comments- thank you for sharing truly inspiring.
    Nothaving been aware of but my family of 4 led already a MMMM inspired lifestyle and located in bike friendly Netherlands makes it quite easy to pick up bike only lifestyle.
    We are well on our way to financial independence; might be already be there if I include the value of the house we live in as MMM suggests in his post. But it seems counterintuitive to me as I can monitize my home or earn interest on it. Sure value climbes but as long as I don’t sell it is virtual profits.
    Please share your thoughts and correct if I miss a vital part of the equation.
    Thanks a lot and in case you’d like to get some inspiration how to solely rely on bike and public transport check out Holland!

  • Irene March 12, 2015, 8:11 am

    This is a very interesting DIY. My parents have infloor radiant heat in their home in Calgary. They installed a glycol-water system and it works very well. The spacing of tubing is very important so that you have adequate control of your zones. And if you live in very cold climate or have areas of the house that are colder (north facing rooms for example), you might want to calculate the spacing.

    If you ever expand upon this system, you should have a look at gypcrete. It’s a self leveling concrete type floor. My parents installed the tubing on top of the subfloor with plastic clips, and then had this company come in and spray a sealer, and then poor gypcrete. They tiled afterwards. The radiant heat conducts well in gypcrete and you have a perfectly level very solid floor. It also cuts down on noise between the basement and main floor.

    In floor heating is the way to go in my opinion. It’s cheaper, cleaner, space efficient, more enviromentally friendly, and way more comfortable to live in. If I ever have the option to install something like this in my own house, I would jump at the chance.

  • Jason April 1, 2015, 7:00 am

    Interesting idea and something I’ve aspired to, but I’m too timid to pull the trigger in an already constructed house with a finished basement.

    One question about your design – is there any material heat loss using PEX vs. copper pipes? I would think the copper would transfer the heat much more efficiently. Could also make install easier since you wouldn’t have to feed and also provide some opportunity for baseboards (along the north wall?)

  • Dave April 1, 2015, 10:32 pm

    Thanks for sharing info from your experiment, it is very helpful. You mentioned that someone is delvelping a multi-zone WiFi thermostat that you want to use. Is there any news on that? I would be very interested in it as I want to add thermostat zoning to a basic hydronic system that is currently installed..

  • Krishanu April 21, 2015, 4:22 pm

    Take a look at this: http://www.uponor-usa.com/Residential-Radiant-Floor-Heating.aspx and watch the video at the end. They sell the entire pre-assembled radiant system!

  • Patte May 10, 2015, 10:02 am

    No one has commented on leaks when PEX is embedded in concrete! When I had an addition built, PEX lines were embedded in new concrete slab & were run to a reg. HWH. Guy who built never completed installation & it was hard to find someone who understood how to make the set up work; they’d look at the HWH & say I needed a boiler and where is the manifold? I didn’t know what they were talking about. Then I found an independent guy, such as yourself, that made it work and it was wonderful! Until it quit working 2 years later! There’s no direct water line, it just circulates from/to HWH and pressure gauge said it was low. Guy who got it to work initially said that means there’s a leak and he wanted to dedicate a water line. I thought if there’s a leak, how would that help? I added water to the HWH for awhile, but that was a bit tiresome. I have kinda given up thinking about luxuriating in the warmth of the heated floors –until I read your ingenious article and all the intelligent comments. Would you look at pics I have of my system and post for any thoughts/ideas from your way-smarter-then-me readers?

  • RethinkingGreen August 3, 2015, 1:26 am

    Thanks for all your work in simplifying what most paid providers have overcomplicated. Here in Texas we don’t need boilers and attendant oxygen-barrier PEX and my system shouldn’t need lots of complicated mixing and balancing. The system wouldn’t be worth the $16,000 quoted by one provider, but should be well worth the amount shown on your order plus another roll of PEX. I still need a few more details, though.

    Will you please add another post with labels for all the parts on your mounting board? Are those two ordinary ball-valves on either side of the pump? What is the next valve to the right? Why would you need two valves in such quick succession? What’s between those two valves? Indeed, why did you add a pressure valve (in addition to another T&P valve) when the Taco pump says it “Eliminates Need for a Pressure By-pass Valve”?

    Do the temperature-sensing leads come with the pump? I don’t see them or a thermostat on your order list.

    How did you judge the size of your expansion tank?

    Thanks again.

  • Ben Q September 18, 2015, 8:36 pm

    Just a few notes about your radiant heating system. I had a friend install a tankless boiler unit to run our hydronic heating and domestic (your taps) last year and so I learned quite a bit about how to install one and the necessary components involved in making sure the system works right.

    1/ We used a heat ex-changer to separate the heating part from the tap water portion. We decided to take this route to use a single unit rather then split unit because they were a much higher cost. The heating portion needs treated water or parts of the system will begin to corrode. It was important for my system especially because I have the wall base boards with radiators and my system is mostly copper – but all the pumps, valves and stuff begin to corrode with out treated water. (my old boiler and the rest of the system started to leak and break down rapidly a year after I decided to drain the system so that I could eliminate one of the heating elements in our kitchen where I wanted more cabinets rather then a heating baseboard – I let new water into the system and didn’t know I needed to add treatment)
    2/ If you use a mixing valve going out to your domestic it can always go out at the ideal 120 temp. This way with a split system you bring the temperature on the unit to roughly 160 for heating months and then when heating season is over you can bring it back down to 120 to match the mixing valve. A tankless unit runs much more efficiently at the lower temp.

    That info may or may not be useful to you.

  • Dave September 25, 2015, 2:16 pm

    What would your experience and expertise say about the following idea?

    We have a 2-year old geothermal heater/cooling system including a geothermal water heater. BUT, there is one room in the house, the family room that is uncomfortably cooler. It is the only room over a crawl space. (It’s a crawl space that is easy to move around in, and in some areas we can walk in it if we stoop.) It was a room addition and therefore the farthest room away from the geothermal furnace.

    I had thought about low-temperature electric radiant heat which is installed between the joists in the crawl space to make this room more comfortable. But what if we installed a second geothermal water heater, and a water pump on a timer to pump the water to this floor in a closed loop system. We would pump water through the system every 30 minutes so that the water heater had a chance to heat up the water in the tank.

    Or should we use a tankless electric water heater? I have been told that geothermal hot water is very efficient (and “almost free”) What do you think?

  • Bill Pierce October 28, 2015, 3:29 am

    How are the different zones controlled? By looking at the drawing you posted the whole house acts as a single zone heating everything to the same temperature. Just a curious mind trying to visualize everything before I start engineering my own system for my first floor

    I definitely enjoyed the read and the follow-up posts. Have bookmarked this page for future reference.


  • Eleanor November 11, 2015, 11:06 am

    I looked into your pump with great sensors- and don’t understand how this control system dynamically responds to changes in temperature in the rooms being heated. The sensors keep the water at a certain delta, but what triggers a change in flow if one room grows colder or warmer due to outside temperature change? What turns the pump on and off? Ate thermostats used at all?

  • Sandy H December 1, 2015, 8:53 pm

    Love your article! I am currently in the planning stages of building a 50 x 80 x 22 metal barn. The barn will be used for my house. My question is if I am pouring a 4000 sq ft slab all in one shot, which will eventually be used for many rooms i.e. bedrooms, kitchen, baths. Should I just lay all the PEX thru out the entire slab? or should I try to do it by rooms, so I will have different zones. Also Can this be used for the second floor will I even need it for the second floor since I’m in AZ? I am building in Arizona about 40 south of Phoenix. Our winters can get down to 19 at night and 70-75 during the day. I mainly turn my heat pump on when I go to bed and turn it off in the morning. My main reason for wanting to do the heated floors is because my entire family can’t stand the blowing of dust and hot air in our faces. We all wake up with dry mouth, itchy eyes. What are your or anyone else’s suggestions.

  • Doug December 28, 2015, 8:45 pm

    MMM, Any issues with mounting the manifold upside down, seems like a basement install will always require the pipes to go up so not sure why the designs of the manifolds is out the bottom? I was to replace my current custom system with an easier to maintain design parts of yours will do nicely into mine.

    • Mr. Money Mustache December 28, 2015, 9:10 pm

      Hey Doug – no, the manifold is happy either way and in fact the two halves are designed to rotate to allow either type of install.

  • Jay January 29, 2016, 11:57 am

    So, been reading on and off on your project, and one question keeps coming to mind. how do you get a 140 degree flooring temp out of a water heater that has a built in max setting of 120 degrees? I am sure I am missing something here.
    For the record, I love the project, and the amount of detail you went through to really let readers decide if it is “worth” it. I personally plan to do something like this eventually, although mine will only be supplemental heat of a couple of cold spaces, this blog has helped me decide this is the way to go for my case.



    • Mr. Money Mustache January 30, 2016, 8:50 pm

      Hi Jay, it sounds like you might have just missed my explanation about temperatures:

      – default max temp on the heater is 120F
      – but you can easily override this just by holding the “up” arrow a little longer, and run it at 140F
      – floor temperature tops out at about 85F in my house

      Note that the heater can be overridden even further and go up to 180, but it requires you to take off the cover and press an internal button. I haven’t needed the extra heat so I didn’t do this override. But I sure wish it came with all options open right out of the box – I hate safety nanny features.

  • Christopher February 5, 2016, 10:49 am

    We’re about to install a system like this, as it seems to fit the needs in an old Detroit home. The one question that I keep coming back to, is when the system (open-same as what you showed in your first radiant heat post draft) does not have an open faucet/dishwasher/bath running, how is the cold inlet not leeching? Is it the Pressure regulator valve you’ve got on there?

  • Gary hydock February 12, 2016, 9:48 pm

    Have you folks ever looked at a magazine called PME . It’s on line and could help . Professional mechanical engineer . It has a lot about radiant and it’s free.

  • steve poling July 12, 2016, 2:41 pm

    You said, “Because the water supply is constantly refreshed and/or heated to 140 degrees Fahrenheit, bacteria growth and stale water in the pipes is not an issue at all.” What about bacteria during summer months? Do you drain the system at the end of each heating season? I imagine you have to flush out the drained system with compressed air like they do for underground sprinkler systems in late fall around here (Michigan).

  • Ben September 5, 2016, 6:25 am

    I would think a small solar HWH exchanger -feeding your in “return to zones” but before the tankless. Seems like a ‘preheat’ tank would allow you to capture additional thermal heat and support the system in cooler times. Just a thought.

  • Robert September 29, 2016, 10:07 am

    Hey Mr M,

    stumbled across your blog when doing research for radiant floor heating systems. Just out of curiosity: have you already swapped out the thermostats for WiFi ones? Which ones did you use?
    I used a Loxone home automation system back at my house in Germany and recently moved to the US. Radiant floor heating is very common there and they are pretty big when it comes to controlling them.


    • Mr. Money Mustache October 1, 2016, 9:50 am

      Hi Robert,

      So far, I’m finding the house is much easier to heat evenly than I expected – so I have left it all in a single zone. So I use a single fairly basic wi-fi thermostat called the LUX Geo: http://amzn.to/2dkruwR ($95 from Amazon at the time of me posting that link)

  • Eric Boyer November 23, 2016, 6:58 pm

    Your tale of voluntary hardship while installing the pipes reminds me of the time that I stained the underside of my newly-built cedar plank deck. I had already stained the top side using high-quality stain and followed manufacturer and expert direction very closely to get excellent results. However, to ensure that the deck boards last their longest, I decided to stain the underside as well, to give the boards protection on all sides. The deck is 130 sq ft. The challenge? The height from the dirt ground to the underside of the joists ranges from 8” to 18”.

    So, I suited up in overalls (thankfully it was a cool day, so I didn’t get too hot), a full-face mask (this stain was very stinky, and when applying stain to the underside of something, there’s going to be a lot of drops coming down), shoes I didn’t care too much about, and crawled under there. It was a moderately painful experience, and it’s quite scary to have oil-based stain dripping right into your face. None of it ever got past the face shield, but it’s still unpleasant.

    It took about 2 hours to do the job, but afterwards I was very satisfied that I had given my nice new deck this extra level of protection. I built the deck 4 years ago and I checked under it just a few months ago. The stain on the underside is still in great shape (I’ve re-stained the top side twice in this time), and seems to be doing a great job of preventing algae and dirt from building up at the edges of the underside of the boards, something I commonly see on plank decks like this.

    It can be unpleasant at the time, but I agree that voluntary hardship has all sorts of great rewards.

  • Chris December 5, 2016, 3:59 pm

    How did you tie a thermostat into that particular Taco circulator?

    • Mr. Money Mustache December 8, 2016, 11:20 am

      Hi Chris,

      The Taco has it all built in – two thermocouple connectors which let you measure input and output water temperature, plus a pair of terminals that you simply connect together to turn it on – called “Heat Request” or HRQ. You connect those to the standard heat request terminals on any thermostat, and you’re running.


Leave a Reply

To keep things non-promotional, please use a real name or nickname
(not Blogger @ My Blog Name)

The most useful comments are those written with the goal of learning from or helping out other readers – after reading the whole article and all the earlier comments. Complaints and insults generally won’t make the cut here, but by all means write them on your own blog!


welcome new readers

Take a look around. If you think you are hardcore enough to handle Maximum Mustache, feel free to start at the first article and read your way up to the present using the links at the bottom of each article.

For more casual sampling, have a look at this complete list of all posts since the beginning of time or download the mobile app. Go ahead and click on any titles that intrigue you, and I hope to see you around here more often.

Love, Mr. Money Mustache

latest tweets