Hi all,

I'm planning on using a 80 gallon Bradford White Ecostor indirect tank for DHW heated by my GARN. The manual has a section "AHRI certified water heater ratings" which indicates the minimum flow, boiler temp & heat output for the rated output as 13.7 gpm at 180* and 157,000 Btuh. One big difference I see from operation with a normal boiler is that with the GARN, the return temperature can be as low as possible. I can design the DHW heat supply loop for the widest possible delta, to minimize the pump size. So I'm trying to think theoretically what the best flow rate for the DHW circulator would be. My figuring is as follows; can someone check me here?

AFAIK, Bradford White doesn't have published numbers on the overall heat transfer coefficient of the coil, so I'm going with u = 80 btu/ft^2*hr*degf based on this page. The area of the coil is A = 14.2 ft^2. Let's call the difference between the boiler water temperature and the current water heater tank temperature = dT. Then:

let F = heat flux through coil (Btu/h) = A * u * dT = 14.2 * 80 * dT

then, flow rate to deliver that amount of heat with the widest possible delta (the return temp to the boiler is equal to the current water heater tank temp):

F / 500 / dT = 14.2 * 80 * dT / 500 / dT = 14.2 * 80 / 500 =~ 2.3 gpm.

Note that for 180 boiler water and a tank of 40* water, dT is 140* and F = 14.2 * 80* 140 = 159040 Btuh, which is quite close to the manual's required heat output for their rated recovery, so my coil heat transfer coefficient of 80 seems to be quite close.

So 2.3 gpm is quite different from the manual's suggestion of 13.7 gpm, but the Btu's work out nearly the same. My thought is that their flow rate is based on limiting the temperature drop for a conventional boiler, since if you rearrange the equation, their specs would give a delta T of 157000/500/13.7 = 22.9*F.

Any thoughts or corrections? Thanks!