I came across this interesting comment regarding radiant floor designs
HERE.
So far as I can tell, the guy who did the post lives in Worcester, Mass, and has an average Heating Degree Days calculated at 7510, assuming a base temp of 68F.
He gives some info on his house construction, which is useful. He also really understands the efficacy of spreader plates, and also the superior performance of extruded plates over stamped ones. He didn't spell it out, but it sounds like he has staple up with stamped plates, and is switching to extruded plates, because he wants to keep his boiler running cooler, so that it is functioning in condensing mode.
If he had built his radiant floor above his subfloor, like Vlad did, he would probably not need to go to extruded plates.
Interesting too, his comment on radiant barriers not being as useful when running cooler floors.
Quote:
You will not achieve 85F floor temps through 3/4 " subfloor and 120-130F AWT in suspended tubing or staple-up PEX, but you might get better than 75F with staple-up Onix (which I don't recommend, BTW- there's a long and mostly bad history of EPDM tubing in radiant systems.) For any staple-up you'd need considerable hotter water than that to hit 85F floor temps.
But you don't NEED 85F floor temps either (and 85F floors are rather uncomfortable in the first place!) Start with a Manual-J heat loss calc for that room, and the number of square feet of available floor space that will have tubing underneath it. You get about 2BTU per square foot for every degree the floor is above room air temp. If your design condition heat loss in the room is say, 1500BTU/hr and you have 250 square feet of radiant floor, the max floor temp you'd need a floor temp of about (1500/250)/2= 6F above room temp, so at 70F room the floor would need to be 76F at peak temps, which is achievable with ~125F water using better-quality sheet-metal plates and 8" o.c. tubing, but not as a staple-up or suspended tubing without much higher water temps. If you need anything more than that you need to go with extrusions, or higher water temps.
I've starred in the movie- I'm ripping out the thin plates under my lossy family room and am replacing them with extrusions to be able to keep up at design-condition, rather than boost the water temp above condensing range just for one room.
The biggest heat-transfer problem in radiant is getting the heat out of the PEX with such little surface area. Extrusions grip the PEX very well, and because of the higher cross sectional area, present a lower temperature to the PEX/aluminum interface for higher heat transfer rates at any water temp. Thin plates don't grip as reliably and transfer less heat at the same delta-T. Staple-ups have very little contact between PEX & subfloor, which leads to noticeable temperature-striping on the floor when you crank up the water temps to be able to deliver much heat. Systems that use tubing in grooved material sandwiched between the subfloor and finish flooring (eg WarmBoard) with thin-metal plates work VERY well with low water temps and are very responsive- you can use overnight setbacks and get similar response as with baseboards or radiators, etc.
At the water temperatures you're talking the radiated heat out of the PEX is actually pretty small, even in a staple-up, and just snugging up cheap low-density fiberglass batts works better than any kind of reflective goods. For 180F water and suspended tube, maybe, but its kinda academic at that point. With tubing in any kind of plate, extruded or sheet metal, the low emissivity of the aluminum renders radiant-barrier type goods useless at any water temperature.
Without a careful heat loss calc, you'll have no idea what you really need, but unless you have a very low window/wall fraction and R19+7.5c.i. walls or better you're probably looking at sheet-metal heat spreaders at a minimum. If this is batt-insulated 2x4 construction with lots of U0.34 or higher windows you'll likely need extrusions. For reference: My family room is 2x6 R19 construction, no exterior foam, and a rather large glazing fraction- it doesn't quite cut it at sub 15F with just the thin plates, though it's still pretty comfortable sitting on the warm floor even when it's 62-63F in the room, which is about where it hit's when it's 0F outside. With extrusions it should keep up just fine without boosting water temps.
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Best,
-AC