RADIANT FLOOR HYDRONIC (water) HEATING SYSTEM

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.