Heat transfer Tank to coil

Don't know if I'm posting this in the correct forum but...

I am looking for a calculation for heat transfer from a hot water storage tank to a coil mounted internally. I am trying to heat the water in the coil from the water stored in the tank.

I have specific's on temperature, coil flow rate and materials, but I thought I'd just ask the general question to see if anyone has any experience in calculating this type of condition.

I have calculations to go the other way, coil heating the water in a tank, but not in reverse.

Thanks