About: I'm a writer and illustrator of books for children and Marvin is a craftsman, carpenter, and retired building contractor. We build various things for our Funny Farm and I write about them.

This is about installing hydronic radiant floor heating in a slab floor system. While it was done as a new house was being built, in my last house I poured a concrete floor on top of a wood framed floor system and could have put radiant heat in that as well. The system works via pex pipe, a small pump, a water heater, and a thermostat.

Step 1: Design the System.

I obtained a design for the entire system from Supply House (aka PexSupply.com) where I also bought the components. They will provide, free, a layout according to your specs and also give you a list of parts you’ll need for the heating system. I was asked to provide a floor plan along with my intended insulation values for the floor, wall, and ceilings.

You can also download a free program from Uponor and read more information there than I will include here. I opted for a single pump, single zone system for our 980 square foot single-story house, but opted to have each room a separate system zone that could be controlled by lowering the flow of the water supply.

Step 2: Provide Ample Insulation.

The first step was to install a vapor barrier over the gravel base inside the stem wall, and then insulate. I used extra (rigid) insulation – R15 – to encourage the heat to go up, not down into the dirt.

Step 3: Install the Reinforcement Wire and Get Ready to Lay Out the Pipe.

I added 6×6 welded wire fabric (WWF) aka concrete reinforcement wire and used that to tie down the pipe. I bought a 1,000 foot roll of 1/2” pex pipe. You can get smaller rolls, but no way did I want any connections under the concrete floor. The pipe is very unwieldy especially in a coil that long, so I built a big spool for it and added dolly wheels so it could follow me around as I unwound pipe. The same contraption came in handy later when I wired the house.

Step 4: Continue Laying Out the Entire Space.

Each length of pex begins in the utility core, makes its circuit around its designated zone, and comes back so that it will circulate the water in a closed-loop heat system. I used zip-ties to tie down the pex pipe. As per the diagram/plan, all the pipes end in the utility core to be attached to the radiant floor system inside.

I slipped the pipes through 3/4" electrical conduit angles where the pipes needed to turn up the wall, and then slipped on 3/4" PVC pipe to keep them straight and protected during the building process. I labeled the PVC pipes according to zone (kitchen, bathroom, living room, etc.).

We took a lot of layout photos so we could remember exactly where all those pipes fell. I did not want to be nailing down a plate for a closet and poking a hole in my heating system.

Step 5: Get Ready for the Concrete Slab!

Finally, I installed concrete doobies and tied down rebar, all just standard practice for a concrete floor.

Step 6: Test for Leaks.

I also tied all the pipes together and pressurized them with air. I wanted to make sure there were no leaks before that 4 inch thick concrete was poured on top of them.

Step 7: Pour the Slab.

And now it all gets covered up! The guys who know how to do that showed up and poured our 4 inch slab. They did an outstanding job. After this was done, I moved on to building the house. Since I worked pretty much alone and at my own pace, it was a couple of years before it was time to set up the heating system.

Step 8: Set It Up!

I built a large utility core with plenty of room for two water heaters as the one designated for the floor would be set low, and space for me to get in to build the system and make adjustments. The option for an instant-flow water heater was there, but I didn’t want to spend that kind of money. I just got a standard 40 gallon hot water heater for a couple hundred dollars.

Step 9: Hook It Up!

The heating system consists of a thermostat which is wired into a relay transfer switch. I located the thermostat in the living room, which in our house is a central location. Since I was building the house from scratch, I could easily run the wire in the walls and over to the utility core. The relay tells the system when to start up. A pump kicks on and water runs from the water heater and into the floors. Water from the floors runs back into the water heater. If you can see the numbers on the gauges in the photo, the water going out is about 100 degrees and the water coming back is about 80 degrees.

Step 10: Troubleshoot If Necessary.

The first issue once everything is connected up is getting the air out. "Air in the lines” comes up as the #1 issue setting these up the first time. I futzed with it until the air was gone. The second issue was the size of the pump. I trusted the supply house to provide me with a pump adequate for the system they designed, but that didn’t happen. The pump has to be strong enough to deal with the resistance in over 900 feet of 1/2” pipe.

You can determine the needs in advance (I now realize) by calculating the feet of head. I finally did that and the pump came up short! I ordered two sizes up and the system is now working beautifully. The pump doesn’t kick on very often and the house stays evenly heated. I use a Cen-Tech infrared thermometer to see what’s going on, but never in front of the pets.

Step 11: Enjoy Your Cozy House!

The pets have figured out where the supply pipes are and love to lay down on the nice warm floor. The inside of the house feels like a nice day all winter long, staying at a consistent 68, our chosen setting. There is no noise other than the quiet hum of the little pump and the usual water heater sounds. There is no hot air blasting dust from vents, and no more feeding a wood stove and having a house that's either too hot or too cold. And as for efficiency, in our 980 square foot well-insulated house, we average 33 KW a day during the coldest months in western Oregon. I mentioned that fact to our electric company energy expert and he said, "Wow!" He meant that in a good way.

I bought a spare pump just in case of an emergency, and the only other component that can wear out is the water heater which I made sure is easily accessible. With me doing all the work, the total cost including everything involved was about $1,700.

A shorter version of this story originally appeared on my blog: Wildcat Man.

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    67 Discussions


    3 months ago

    why cant we run boiler on electricity? Would it become more costly?

    3 replies

    Reply 3 months ago

    yes, i want to use. please give me specification of all components needed for it?

    Wildcat Man and RobinAbrarS13

    Reply 3 months ago

    Try contacting the company we listed in the Instructable. They can probably guide you to hook this system up to an instant water heater. Since that's not what I used, I can't be of assistance.

    Wildcat Man and RobinAbrarS13

    Reply 3 months ago

    Hi and thanks for your question. Do you mean an instant water heater, which is basically an electric boiler? If so, we mentioned it was upfront cost-prohibitive for us at the time, but if our water heater dies we might make the switch because they are not as expensive these days. It would save energy because it's on-demand and not heating water that isn't being used. But, in our case the heat lost by the water heater's stored hot water is gained in the house anyway and, a water heater is cheaper to replace. Lots to consider! Good luck and let us know if you install a system!


    1 year ago

    I had a heated driveway put in a number of years ago....yes, it's expensive to run but at the time I was concerned with getting to work on time and since my garage faces north, it wasn't uncommon for the bottom of my garage door to get frozen to the cement making it impossible to get my car out of the garage. Now that I'm retired, I only turn it on when we get a freezing rain that leaves an ice coating on the driveway. I appreciate your instructable and your undertaking such a project. I, too, took plenty of photos as the system was put in incase I needed the info later. I did the same when my house was constructed so I knew where interior pipes and wires were run. I have needed to refer to those photos often over the years. If I were to have another house built, I would have a heated water floor system included, as my feet are always cold in the winter. Not something that is easily, and inexpensively done, as an afterthought. I have a question about the flow direction of the water. Wouldn't it make more sense, for comfort, to reverse the direction so the hotter temp water is coming into the center of the rooms and the colder is going out around the perimeter, or, is it done the way you have it because the colder temp of the room would be around the perimeter because of exterior walls and windows? Again, good instructable!

    1 reply

    Thank you very much for your nice comments. Wow, a heated driveway! You might like knowing about Solar Roadways, if you don't already - http://www.solarroadways.com/. To answer your question, yes the perimeter is the biggest heat-loss so that's why they recommend starting from the outside and working in, but you have an intriguing idea. This theory is possibly based on a more traditional heating system i.e. heat pump and ducting, where all the ducts are along the perimeter walls. Why would it make much difference in a slab of concrete and in fact, be more efficient the way you propose? But I followed the standards, having no experience with the system, and no willingness to take a jack-hammer to the floor later to redo it all! :-)


    1 year ago on Step 11

    Radiant heating is the worst invention in history, our house gets colder then outside in the winter, and it's expensive, and there is never hot water.


    Question 1 year ago

    Do you find one side of each zone is warmer the the other with hotter water coming in and slowly cooling as it flows thru the system. We laid our pipes in spiral to try to prevent this but it may have been a lot more work for no benefit if yours works well. Finished house but yet to connect up underfloor pipes yet so thanks for the instructable

    Face book 2.JPG
    1 answer

    Hi thanks for the question and cool photo! It's confusing how many zones you have? I have six, so that meant each zone (room) only had x number of feet of pipe out of about 900 total. The longer the run, the more heat loss, as you know. I don't know if a spiral pattern as you describe would make any difference, and it's an intriguing question. The only heat loss of any note is along the exterior walls, otherwise we find no significant difference in different areas. Sunlight does play a role, when we get any that is. I installed large south-facing windows for some solar gain and that heats the floor up nicely as well. The cats love it.


    1 year ago on Step 11

    Great article! I designed and installed these systems for years in the Midwest. I see the thermal expansion tank in one of the pictures. This is a must. I did not see a temperature/pressure relief valve. This is a safety item that must be installed, preferably at the heater discharge. Finally, I always used a 50% propylene glycol and water mix for freeze protection. It is food safe and good to -50 deg. F. Thermal conductivity loss is negligible and the increased viscosity results in about a 6% greater pumping requirement/energy. Good job!

    Chuck Black

    2 replies

    Thank you, Chuck! That's quite a compliment coming from an experienced installer of these systems. I figured the temperature/pressure relief valve on the water heater would be sufficient, but will consider your suggestion!

    Hey Robin,

    The water heater TP relief valve is sufficient. I just did not see it. Good job.



    1 year ago

    I also installed my own system and congratulate you on a neat system. Mine differs in a couple of ways. I decided to reduce the thermal mass of mine (to reduce heat up time) so I only laid a 40mm slab, to do this I decided to reinforce it with chicken wire. There are a few micro cracks, but I will be laying a tile isolation mat to ensure these don't carry through to the tiled floor when I finally get around to it. We are living in the house and the system works great. I laid my own floor, in 1.5m wide strips using temporary edges to run a spreader bar along. The concrete was vibrated in using a hand held vibrator whilst being levelled.

    Also I have allowed for thermostats in a number of locations, with electrically controlled valve actuators as well as the pump start up to control it. All dealt with by a control panel.

    My primary heat source is a wood burner stove with wrap around boiler. This heats up a thermal store like yours (by gravity feed), but I also have a thermostatic backup oil fired boiler that cuts in if the wood burner isn't running or isn't providing enough heat. The oil is typically only to keep the background temp overnight, provide our hot water requirements for showers in the morning, and any daytime running before we start the wood fire. I am in Scotland and we use around 2.5L of oil, and a basket of wood a day in the depth of winter. Based on usage this first winter a typical 1000L oil tank will I believe last us for two years between refills.

    7 replies

    You have an awesome off-the-grid system, sounds like! Ours relies on electricity, though if we were in a good area for solar, we could heat the water with the sun. I didn't mention we did lay slate or travertine tile in the two bathrooms and (real) wood floor in the bedrooms. Doesn't seem to make any difference about the heat in the rooms.

    It isn't off grid, but could be used that way. In the UK electricity is the heating source in a relative minority of homes. With the tax rate we have on base fuels, adding in that layer of inefficiency (conversion to electricity is less efficient than a direct use as a heat source) means most people use a direct fuel, such as gas (mains is preferred if available) or oil. Also we have little if any need for air conditioning as it rarely gets hot enough, so there is no need to take that aspect into consideration.

    I am surprised that the wood floor doesn't noticeably take longer to warm up. I presume you had the wood thoroughly seasoned before laying it, otherwise it would split. Engineered wood is less liable to splitting in that way, but perhaps that is what you have, it is still real wood.. ;)

    It's interesting hearing this information from outside the states, thank you! I live in a bubble. As for the wood, yes seasoned indeed - I pulled it out of a house remodel about 25 years ago and it had been aging more in the barn for a number of years before I installed it. Here's a photo of it during the installation. I sanded and did a clear finish, didn't want to hide that beautiful aging.


    Thanks for the question. I had planned for the bathrooms and bedroom to have flooring, so the foundation was formed accordingly. I built a sub-floor on top of a vapor barrier then laid the wood floor in the conventional manner. And wow, no squeaks!

    Thanks for the quick response. By “foundation was formed accordingly” do you mean these rooms were formed below the level of bare floor to accommodate the extra height?

    Yes, and the photos in Step 7 show the two bathroom areas. The person in the upper left is working in the bedroom. Here's a photo of the bedroom, which looked like a swimming pool until we got a roof on!