DIY Radiant Floor Heating System

This is about installing 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.

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I obtained a design for the entire system from Supply House where I bought all the parts. 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. You can also download a free program from Uponor and also read more information than I will include here.

I opted for a single pump, single zone system for our 980 square foot little house, but opted to have each room a separate system zone that could be controlled by shutting down the water supply. For larger spaces, they would recommend more than one zone with a pump and thermostat for each.

First step was to install a vapor barrier and then insulate. I used extra (rigid) insulation – R15 – to encourage the heat to go up, not down into the dirt.

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Next 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.

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I used zip-ties to tie down the pex pipe. Here are a few photos of the process. We took a lot more 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.

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

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

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After the floor was poured, 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. 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 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.

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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.

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The first issue once everything is connected up is getting the air out. If you troubleshoot a radiant floor system by googling, “air in the lines” comes up as the #1 issue. 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 by calculating the feet of head, which I did 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, ranging from 69-72. I use a Cen-Tech infrared thermometer to see what’s going on.

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Meanwhile, the pets have figured out where the supply pipes are and love to lay down on the nice warm floor.

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With me doing all the work, the total cost including everything involved was about $1,700. We haven’t got an electric bill yet, but considering that the house is so warm and the water heater is well insulated, plus the water going back in is almost as hot as it needs to be anyway, we suspect we’re not using very much energy for this system. It is a wonder that at least in Oregon, there are no Energy Credits for putting in this efficient system. I hope to change their minds about that.

Happy winter!

 

 

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