Radiant Floor Systems

Image via Deiter Bros.

We originally planned to have a slab on grade foundation (lots of concrete) and a masonry stove heater for our house. It seemed obvious that we should install radiant floor heating, even if we weren’t planning to use it, given the finality of concrete. I’ve long been attracted to radiant heating because (a) I hate cold feet, and (b) it is a clean, quiet way to heat your house. It also has the benefit of evenly heating everything rather than just blowing warm air around. Even after our floor planned changed, we decided to keep the radiant floor tubing in the slab, and wait on installing a masonry heater. This will have an added bonus for us because the heating will be ready to use this winter, rather than needing to be installed later.

Here are the steps we took to install our system.

Image via AIM Radiant Heating

1) We had a plan designed for us. Radiant floor systems are made up of long loops of PEX tubing. A zone is an area that will all be heated to the same temperature, and there are often multiple loops of tubing in each zone. Each loop has to be as close to the same length as possible, and the tubing spaced a certain distance apart. As you can imagine, this requires some complex planning.

Our local plumbing supply company offers a free system design service that takes into account everything about your house: elevation, how many windows/doors, insulation values for walls and roof, and the number of zones you want. If you don’t have a company like this you can talk with radiant system companies in your area, or call one of the DIY companies. We were originally talking to the Radiant Floor Company and thought they seemed good.

2) Prepare for your concrete (this only applies if you are having a concrete floor poured). We are having a 3 inch concrete floor poured, so we installed a 3 inch toe up around the perimeter of the house, as well as in position for future walls. Since the tubing loops under some walls, we cut holes in the toe up for the tubing to run through. The second toe up will sandwich the tubing underneath it, but won’t squish the tubing.

3) Install the tubing. Depending on your system, this could be as basic as stapling the tubing to the floor (like we did), or zip tying the tubing to your reinforcement system. Note: do not staple the tubing. I did this twice roughly half way into a 200 foot loop, and we had to rip it all out. There are options for splicing two pieces of PEX together, but do you really want something in concrete that might leak? We had to use huge staples and a special staple gun, so if you go this route make sure you can get the right tools. We watched movies on YouTube to understand how to unroll and install the tubing.

4) Build manifold and attach tubing. I’m not sure how other places work, but we had to build part of our own manifold using copper pipe. Make sure you get clear directions for this step. We had to go to the plumbing company twice before we understood how this all fit together. You will have both a supply manifold (top in this picture), and a return manifold. Eventually this will all get attached to the water heater, along with pressure gauges and all sorts of stuff I won’t mention (the diagram is complex). At this stage though, all you need is a sealed system with a pressure gauge attached so you can pressure test your system.

5) Pressure test your system. Get an air compressor and fill your system until it reaches the pressure specified by your building inspector. Ours needed to be pressurized to 100 psi and hold that pressure over night. We quickly found several leaks in the manifold, but we spent a long time agonizing over the pressure. No one told us that it would take while for pressure to equalize, which lead to a loss of 10 psi over an hour period. After that, we also saw a drop of 2 psi overnight. This was due to changing pressures in the tubes as the air cooled. Once it warmed up the next day, the pressure rose back up over 100 psi and we had to let some air out. If you see these sorts of fluctuations, they may not be due to a leak in the system. I would suggest having at least a day before the inspection to see what happens. Do not pressure test your system with water! If it freezes, your pipes will break. I suppose that this isn’t a risk during the summer, but the back of the PEX box was very specific about this.

6) Pour concrete. Your system will stay pressurized during the pour.