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Would like some feedback on my radiant design "plans"
I'm a long-time lurker, first-time poster.
I'm planning an underfloor radiant heat install in an ~1700 square foot well-insulated remodel. Total heat loss is ~18,000 BTUs at 0 deg F outdoor temp (I'm capable of and very confident of the calculation). Calc is room-by-room. Floors are either 1x6 pine topped with fir strip floors (and maybe 40% coverage with area rugs - average R-value with rugs of ~2.9) or 1x6 pine topped with 1/2" subfloor topped with linoleum (the real stuff, average R-value 1.9). I've used a variety of charts and tools from Healthyheating.org to calculate heat dissipation from fir strip floors as 12 BTU/hr and for the linoleum floors of 16-17.5 BTU/hr (depending on location and exact construction) with 110 degree water and 9" PEX spacing with good quality (thick) plates.
The heating system is a GSHP wtih a large water storage tank. I have an 8 manifold system set up. There is a Grundfos Alpha II pump to push water through the zones.
I've done complete calculations to understand how much heating I'll get at different water temperatures (I'm completely capable of this). Other calculations show an ~0.5 to 1 gpm flow with 0.5" inner diameter PEX (need 0.5" to keep head loss down). My longest runs are 400' total 0.5" PEX (head loss ~15 feet). Downstairs, I'm planning on one zone with a master thermostat (the floor plan is very open) and upstairs (three bedrooms and a bathroom) individual zones with a thermostat in each room. Two of the larger downstairs rooms with larger, older windows will be augmented with cast iron radiators (this is necessary to supply the required heat at cold temperatures). I can get the heat I need with 120 deg F water temperature at 0 deg F outdoor temperature (low water T with outdoor reset at higher outdoor Ts).
Here are my basic questions I'd like to get answered and then I'm happy to listen to any other suggestions/ideas as well:
1. If I reduce the PEX spacing to 6" from 9" (or 8") (effectively putting three 4" plates in one joist bay instead of two 4" plates), I don't seem to get much more radiated heat off the floor (I see maybe a factor of 1.125 more heat). Is that the case?
2. The pump I have already installed is a Grundfos Alpha II. If the head requirement at 1 gpm is for the longest loop only, then that has what it takes, from what I understand. But if all loops are calling for heat, will it just divide it's total ~4.25 gpm pumping capacity (at 15 feet head) by all the loops (assuming equivalent frictional resistance) or will it fall short for some reason? At 0.5 gpm, head loss of 400' of 1/2" PEX is a lot lower...which sounds like it works in my favor as well...
3. I calculate a delta T of ~9 degrees F at 0 degree outdoor T (0.5 gpm per loop) and 5 degrees when at 1 gpm (fewer loops calling for heat). Everything I read says that is reasonable. Can anyone confirm?
4. What would be recommended for balancing flow on the first floor loops, seeing that they are all on one T-stat controlled zone? I am designing for (mostly) balanced temps, but I want to plan for some miscalculations or adjustment after we've lived in the house for a bit.
5. There is 100 year old felt paper between the 1x6 pine subfloor and the fir wood strip floor. Given that my water T will be 120 deg F (130 deg F worst case at -30 deg F outdoor T), everything I've read says that this shouldn't be an issue. Any comments?
Thanks in advance. I've learned a lot already from reading the posts and plan on learning even more now.