Non pressurized solar water heater with boosting pump

[samiam]

I have a non pressurized thermosyphon solar water heater connected to a booster pump.

I have a 1/2 inch pex pipe coming from the heater connected to the booster pump. The pump keeps kicking in and out - I placed a flash light behind the hot-water-supply-line and I could see that water was flowing back-and-forth which suggests that there is not sufficient pressure to even get the water to the suction side of the pump and the pump is actually working to pull the water out of the supply line.

There is about a 35 ft run between the heater and the pump. How do i determine if the pump is getting enough pressure from the solar water heater line? Pressure coming into the house is 30 psi? i would appreciate any help. Thanks

[PipeFlowCalcs]

Is pump above or bellow the tank? If it is above than that might cause the problem.

NPSH of the pump - Net Pressure Suction Head is pump parameter that tells how big head in front of the pump should be in order to pump work without cavitation. You should calculate head in front of the pump and compare it to its NPSH.

Pressure drop from tank to the pump you can calculate with pressure drop calculator.

Also height difference should be added or subtracted if pump is above or bellow the tank.

[samiam]

Thanks Zoran
I am a layman so i am unsure how to use the calculator - also i have looked over literature and cannot find the Net Pressure Suction Head value
the pump is about 3 ft below the tank.