How does head pressure figure into a closed loop system? How do you calculate it? I have a pump rated at 500GPH @ 0ft, how would the output be affected by head pressure?
First the intake to the pump comes from the tank and enters the pump. The return line to the same body of water cancils out the height of run from the pump except for the slight amout of pipe above the water wich is minimul. So if your pump is on the floor the pump sees a PRELOAD of pressure equal to the AFTERLOAD the pump sees on the other side.
Now you will still have some resistance to flow caused by the frictional losses and bends etc... Just not head. So the output of your pump will not be exactly 0 head but not more than a foot or two.
Now some people try to run their circulation loops from the overflow. You do get some preload on the pump using this method but depending on the water level in the overflow it is less than the level in the tank or else the water would not flow into it. Also most overflows can not handle the flow from both the loop pump and the main pump with out backing up. Still not a closed loop because there is AIR between the output and intake bodies of water.
Now if your pump is above the water it is the same principal but now the pressures are reversed. Negative load on the intake ( pump has to suck ) and also negative pressure on the output side enough to cancil out the intake side ( water falling from the pump to the tank.)
