How to Calculate Propane Usage for Tankless Water Heater
I hope there are some techies following this forum who may be able to help me do some basic calculations regarding anticipated propane usage.
I'm building a new home this coming year and one of the indulgences I'm allowing myself is a luxury shower with 6 shower heads, each using 2.5 gpm. At full bore, it will therefore use 15 gpm of hot water. I know how incredibly wasteful this sounds, so I'm utilizing some technology to get my costs down on heating the water. It's also water coming from my own well and going back into my own land.
I'm trying to calculate the amount of propane I'll need to use on a daily basis in two different scenarios--summer and winter.
My basic set-up consists a pair of a 119 gallon indirect hot water heater (and holding tank) that is preheated by a boiler loop coming from the wood stove heating the house in the cold months. Additionally, I'm be using a GFX Drain Heat Recovery System which also preheats the water going into the on-demand water heaters by means of a copper coil wrapped around the shower drain. The preheated water will then be fed into a pair of on-demand water heaters.
In the summer months, my well water is around 56F and my goal is to be able to produce 15 gpm at 110F. The GFX coil on my drain will be able to preheat the well water from 56F to 80F, leaving a needed rise in the summer of 30 degrees to get to 110F.
In the winter months, my well water is around 45F. I'll have the benefit in the winter of preheating the water twice, once with the boiler loop between the wood stove and indirect hot water heater, and a second time with the GFX coil on the shower drain. The boiler loop should get the water from 45F to 65F, the GFX from 65F to 89F, leaving a needed rise in the winter of 21 degrees to get it to 110F.
I'm anticipating using the shower at full capacity (15 gpm) for a full hour every day. How do I calculate how much propane I'll need to use on a daily basis, both in the summer and in the winter?
By the way, the pair of on-demand water heaters I'm using are 94% efficient, if that needs to be factored in here. The also each have a maximum consumption of 199,900 btu/hr, for a total of 400,000 btu/hr for the pair.
I'm at a loss how to calculate the BTUs needed on a daily basis, and the subsequent amount of propane needed.
Anyone willing to help me calculate this? I'd greatly appreciate it!