Quote:
Originally Posted by Vlad
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.
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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.
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I see what you're saying - clearly, any system will only use as much heat as it needs based on turning on or off the circulator; what I'm getting at with the calculations is to figure out how much heat I'm going to need in the first place to properly size the heater. If I have a 75k BTU water heater with only 5k BTU of heating demand, the water heater is much more likely to switch on and off all the time (short cycle), as it inputs much more heat than is needed to reheat the water back to target temperature.
The most ideal setup would be a heater that uses exactly the amount of gas needed to heat the water to target temperature. Of course, all water heaters are either on or off in terms of firing, so the higher the rating of the water heater, the more likely it is to short cycle.
For example a 75k BTU water heater with 5k BTU/h of heat loss, will turn on and off so frequently that it'll only be on a little more than 6% or so of the time. This presents a problem because of the mentioned efficiency loss through short cycling, so choosing a water heater with a rating as little as possible over the demand should increase efficiency (and save money).
Am I making sense?