I somehow got away with 1/4 HP, and it offers 35 ft of head while operating at 14.2 PSI. We are able to push it through two 50 ft cooling coils and two jacketed 12"x30" cylinders simultaneously (inlet at the bottom of jackets). For any seriously large application, surely a 1/2 HP or greater would be minimum.
Thanks, I'll take a look at the smaller pumps.
A foot of head is 0.4332 psi (0.0361lbs/in2 X 12") and for the pump to reach that pressure, requires that much back pressure.
Back pressure is a function of flow, so the pressure required will depend on how much liquid is being pumped through what resistance. As the pump size and flow goes up, so does the pressure and horsepower requirements.