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I was thinking of running oxygen barrier PEX under the floor between the joists in a cold room with a crawl space below and then putting rigid foam insulation below to keep the heat in. I have an existing boiler that is used to supply the hot water radiators. Can I hook this system up to my existing boiler?

The systems uses a Weil-McLain gas fueled boiler that circulated the water at 140 degrees F. This boiler used to run 10 large radiators but we remodeled and added an in attic forced air unit and it now only runs 5. The 9 x 15 room already has forced air heat but because it has a crawl space below, three exterior walls, 8 windows, and is located in Minnesota means that that the floor gets very cold in the winter. I thought I could use one of the empty runs to heat the floor of this room and make it more comfortable.

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Given that you are now running only half as many radiators, you might be able to "re-purpose" the surplus to provide hot water for your staple up radiant floor heating. An important factor to consider is that maximum floor temperature should never exceed 85 degrees Fahrenheit, so you might need to add a mixing valve and a separate circulator pump. Although, (just thinking out loud) I wonder if you could supply the PEX tubing with "less-hot" water being returned to the boiler. Thereby eliminating the need for a mixing valve and separate circulator pump.

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I've used PEX for baseboard heat in MN. Baseboard heater water is usually around 180 degrees. Looks like you can easily find PEX rated for 200 degrees at Home Depot even: http://www.homedepot.com/p/SharkBite-3-4-in-x-100-ft-Oxygen-Barrier-Radiant-Heating-PEX-Pipe-U870O100/202033002

So temperature isn't an issue. What will matter is how your current system is set up. If it's a gravity-fed system, you likely won't be able to use this for under-floor heating. If it's pumped-based, you will likely have to create a new circuit for this room.

On top of that, you can't just place the pipe under the floor. You need a way to efficiently dissipate the heat from the pipe directly to the floor. For that, you want to install these under-floor 'fins' for the pipe:

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  • This boiler is pump-based and has 5 open runs due to the removal of 5 radiators. I found 1/2" Oxygen Barrier PEXA at Menards. They also have the "fins". I was wondering if I should make one run or two runs in a room this size. Also, as this crawl space is cold in the winter, I was thinking of putting rigid foam across the bottom of the joists to hold in the heat and insulate the floor form the cold. – Tom Oct 2 '15 at 21:29
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I have installed radiant floor heating in a number of houses. It is important that the space where the tubing is running to be insulated and also sealed to prevent infiltration of air. When the radiant heat is running, this confined space gets quite warm. I'd recommend using Hi R or one of the other foil faced 1" thick insulation materials, and placing the reflective side up toward the floor. These materials cut easily with a razor knife so it is easy to get a tight fit. The other option is to use double bubble, or other aluminum faced insulation. This insulation can be stapled to the side of the floor joists to create a ~2" pocket under the floor. With double bubble, you still need to place other insulation under the double bubble. But the DB seals the cavity and also provides a reflective surface for IR.
It is best to run radiant floor heating at a lower water temperature, like 130 to 150 F. The temperature needed depends on how much tubing is between each joist (1,2 or 3 runs). If you run the floor heat at too high a temperature, it becomes difficult to control the room temperature

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