Quote:
Originally Posted by opiesche
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? |
I think you put to much brain power to determine your heat demand. It doesn't really matter. If you have right controls you will only use just enough heat. Rest of heat will just be stored.
It is easy.
1. You should determine your highest temperature needed for your heating system. Let's assume you need 100F.
2. You have to check the lowest temperature your HWT can operate. Let's assume you will use regular DHWT (non condensing one) and set its thermostat @ 120F.
3. Thermostat will cycle on/off 120F-140F or 120F-130F or something close to this.
4. Now you have your heat source @ 120F-130F. If your HWT capacity 50000BTU or more or less and you only need 5000BTU your HWT will just seat and do nothing. If suddenly you need 40000BTU your HWT will start/stop more often.
I can't build my own control system so I used stock controls. I used taco 2 way mixing valve with outdoor sensor.
http://www.pexsupply.com/Taco-I050C2...Valve-w-Sensor
It allows to modulate floor loop temperature according to outside temperature. The colder outside the warmer loop water.
Also I added variable speed control for main circulating pump.
http://www.pexsupply.com/Tekmar-157-...peed-7955000-p
I put one temperature sensor on supply and one on return at manifold. I set 5F TD between supply-return.
Now mixing valve determines loop temperature according to outside temperature but often if you cook or outside cold but sunny day you don't need much heat. In this case TD will go down(if heat is not needed hot water can't loose it's temperature and return temperature goes up this lowers TD) and pump control will slow it down to keep TD @5F. If pump works at lowest set speed for set amount of time pump control will stop it for 5 min and restart it after.This is new cycle.
This 2 systems allow precisely control heat demand for house. It doesn't matter if my HWT is 40000BTU or 75000BTU my system allows to take just enough heat.