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opiesche · 09-23-12, 04:34 PM

Going a little further with this: say we more than double the heat loss for our coldest nights, to 4000BTU/h.
I've got 980 ft^2 of heated floor surface, that means I'll have to put at the most 4 BTU/h/ft^2 into the floor.

Each square foot of flooring contains on average 13 inches or so of tubing, at 1/2". The volume of a cylinder is pi*r^2*h, so 3.14*0.25*13 = 10 cubic inches or 0.163l of water.
At 1kg/l, that's 0.163kg or 0.35lb of water per square foot.

Water has a specific heat of roughly 1 BTU/ lbdeg (1 BTU for each pound of water that is 1 degree warmer than ambient). So, to get 4BTU out of 0.35lb of water, it'll have to be 4/0.35 = 11 degrees warmer than the ambient temperature. If I'm considering 70 degree ambient temp, I'll need the water to be at least 81 degrees to satisfy the highest heating demand - take losses into account, and the 85 degrees I was originally considering don't sound too far off

Again, I'd appreciate if someone could double check my math here.

Also, what does that mean for my water heater? I'm seeing that most of them rated somewhere between 30,000 and 60,000 BTU/h - seeing how my need should be around 4000-5000BTU/h, and taking the energy factor of 0.6 of most gas water heaters into account, I shouldn't need more than 7000BTU/h in gas input during the coldest outside temperatures.
My guess is, to prevent short cycling as Vlad and AC_Hacker mentioned, that I should go with the lowest rated water heater in terms of energy input, that I can find. The 75,000 BTU tankless I mentioned would definitely not be a good choice - a 35k BTU tank water heater would probably do the trick nicely. Any thoughts?

09-23-12, 04:34 PM	#20
opiesche Helper EcoRenovator Join Date: Jul 2012 Location: Rohnert Park, CA Posts: 99 Thanks: 4 Thanked 14 Times in 9 Posts	Going a little further with this: say we more than double the heat loss for our coldest nights, to 4000BTU/h. I've got 980 ft^2 of heated floor surface, that means I'll have to put at the most 4 BTU/h/ft^2 into the floor. Each square foot of flooring contains on average 13 inches or so of tubing, at 1/2". The volume of a cylinder is pir^2h, so 3.140.2513 = 10 cubic inches or 0.163l of water. At 1kg/l, that's 0.163kg or 0.35lb of water per square foot. Water has a specific heat of roughly 1 BTU/ lbdeg (1 BTU for each pound of water that is 1 degree warmer than ambient). So, to get 4BTU out of 0.35lb of water, it'll have to be 4/0.35 = 11 degrees warmer than the ambient temperature. If I'm considering 70 degree ambient temp, I'll need the water to be at least 81 degrees to satisfy the highest heating demand - take losses into account, and the 85 degrees I was originally considering don't sound too far off Again, I'd appreciate if someone could double check my math here. Also, what does that mean for my water heater? I'm seeing that most of them rated somewhere between 30,000 and 60,000 BTU/h - seeing how my need should be around 4000-5000BTU/h, and taking the energy factor of 0.6 of most gas water heaters into account, I shouldn't need more than 7000BTU/h in gas input during the coldest outside temperatures. My guess is, to prevent short cycling as Vlad and AC_Hacker mentioned, that I should go with the lowest rated water heater in terms of energy input, that I can find. The 75,000 BTU tankless I mentioned would definitely not be a good choice - a 35k BTU tank water heater would probably do the trick nicely. Any thoughts?