I have a heat pump water heater that cycles the refrigerant in reverse, thus using the cooling portion of the evaporater to chill the water rather than to heat it (basically, a 50-gal storage homemade chiller). The problem is, I need to circulate the water throughout the tank, or else all of the chilled water will sit near the bottom. There's an opening/valve near the bottom of the tank (normally used for space heating I believe, not sure) and the tank inlet isn't being used - so there's my recirc path. But the path will contain some bends and twists since the valve is at the bottom pointed one way and the inlet is on the top pointed another way. Also, the pipe will probably need to reroute a little to accomidate the place where I'll screw in the pump to the outside of the tank. So that's my question - I'm going to use a pump to recirc the water. But how powerful of a pump do I need? I know it has to overcome the head in the pipe - but how exactly do you figure that out? I'll be using copper pipe with a given diameter and I'll be able to measure the length/height of the set up... I know bends and vertical lifts contribute to loss, but how, exactly? I don't want to get a pump that won't do the duty, but I don't want overkill either. Thanks, guys and gals.