4300 Gallon Plywood Build (3600+ Take 2)

Let me try to explain something:

1 gallon of water needs approximately 10 BTU to raise its temperature 1 degree F

this is actually higher as you have to account for heat loss thru the pipes etc but lets ignore that for now........

Now, I don't know your average ambient temperature of your house or the target temperature that you wish to heat your tank but let me plug some numbers in.

Average house temp 72 degrees
Target tank temperature 78 degrees ?

Ok, so now we have to heat 4300 gallons of water 6 degrees so:

4300 x 6 x 10 = 258,000 BTU's to heat the water to temp

Now, we also have to account for the cooling properties of the sides of the tank, filters, piping, being exposed to colder (room temperature) air, as well. This will be working against your heating system. Calculating heat loss is quite a bit more complicated but I am going to guess it to be nearly 1/4 of the BTU's needed to heat the tank to temp. I could be wrong and others feel free to chime in here.

But, what I am getting at is I think your "on-demand" heater is going to be "under demand" most of the time, if not nearly all.
 
4300 x 6 x 10 = 258,000 BTU's to heat the water to temp
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And this is only after allowing <60 degree tap water to come up to room temperature

Now if your tank loses 1 deg F an hour in heat dissipation, you will need approximately 50,000 - 75,000 BTU an hour to compensate (depending on transfer efficiency).

Is your tankless heater going to be electric or gas fired? If electric, how easy is it to change the element and at what costs?
 
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:popcorn:
 
The heat loss is going to be effected by the temperature differential between the room's ambient air temp. and the tank water. The closer those two temps. are the slower the transfer. Once the contents of the tank are tempered (reach his desired holding temperature). If the tank looses 1 degree per hour as you project, the next missing variable is where did that heat go? Into the room or house as it were. Now the room or building's thermal efficiency comes into play, as well as air leakage and all sorts of other fun stuff. Assuming very low heat loss from the building the tank would eventually find an equilibrium where continued high levels of BTU input should not be necessary beyond the added demand placed by non-tempered replacement water.

Something big is being missed in your calculations because the amount of heat that you're projecting his tank to require would be double what I use to heat a 3,000 sq. ft. house.
 
Ya got me with all this?
 
nc0gnet0 said:
And this is only after allowing <60 degree tap water to come up to room temperature

Now if your tank loses 1 deg F an hour in heat dissipation, you will need approximately 50,000 - 75,000 BTU an hour to compensate (depending on transfer efficiency).

Is your tankless heater going to be electric or gas fired? If electric, how easy is it to change the element and at what costs?
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Not using tankless of any kind.

Hot water feeds the 100' Pex coils in the settling tanks.

The recirculating system sends the cooled water back to the hot water heater through the cold line.

How are you calculating BTU usage?

My hot water heater is natural gas.
 
foolishfish said:
The heat loss is going to be effected by the temperature differential between the room's ambient air temp. and the tank water. The closer those two temps. are the slower the transfer. Once the contents of the tank are tempered (reach his desired holding temperature). If the tank looses 1 degree per hour as you project, the next missing variable is where did that heat go? Into the room or house as it were. Now the room or building's thermal efficiency comes into play, as well as air leakage and all sorts of other fun stuff. Assuming very low heat loss from the building the tank would eventually find an equilibrium where continued high levels of BTU input should not be necessary beyond the added demand placed by non-tempered replacement water.

Something big is being missed in your calculations because the amount of heat that you're projecting his tank to require would be double what I use to heat a 3,000 sq. ft. house.
Click to expand...

Agree completely. It's not going to take anywhere near what he said to heat the tank.

The hot/cold runs to the settling tanks will be CPVC then that will connect to the two runs of 100' PEX which will be joined with CPVC. The CPVC will have the foam insulation sleeves on it. Very little heat loss along the way.
 
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