I gathered some more info elsewhere: how many feet of copper tube 3/4†do i need to put in a 800 gallon tank to heat it to 180 in 4 hours with a boiler that puts out 700000 btus
So, it looks like you want to heat 800 gallons from 130F to 180F (delta T = 50F) in 4 hours
How many BTU's you need?
![shade smirk >> >>](/talk/styles/default/xenforo/smileys/shade_smirk.gif)
> 800 gallons x 8.35Lbs/gallon = 6680 Lbs of water x DElta T of 50 = 334,000 BTU overall
Or 334,000 BTU / 4 hours is 83,500 BTU/hour.
This is not a big number, so instead of using copper I would suggest to use a water/water flat plate heat exchanger that can handle this load of 83,500 BTU/hour,
You just need 1 additional pump/circulator and control.
To design it for a copper coil you need more info: tank dimensions, insulation value for tank, Temp of area where tank is located, ...
An "educated guess" would be 3 to 4 pancake coils 100' long (OD 5/8") connected in parallel and a balanced flow path.
These 4 coils would run you already $300 to $400.
The capacity of the tank seems a little bit undersized for the boiler, or vice versa.
Curious what others have to say.