Active Direct Solar system not circulating correctly

by Nigel

I have a DIY Active open loop solar water system which comprises of following:

1. 20 58mm Evacuated tube collector
2. 145 litre Hot water cylinder with single coil for boiler
3. Resol B controller (12V version)
4. Laing Ecocirc d5 Pump

The system is plumbed in mainly 15mm copper pipe, which is roughly 8 to 10 meters total length for flow/return. The pipe work has 13 90 degree elbows and the pump is situated about 1 meter above the hot water cylinder. The cold water feed tank is roughly 2 meters above the hot water cylinder and the top of the collector is roughly a meter above the cold water tank. The hot water return from the collector is fed into the tank via the hot water outlet using a salamander s flange with some compression fittings. The cold water feed is connected through a modified immersion heater blanking plug which has a compression fitting attached and a long 8mm copper tube to the bottom of the HWC.

The system initially functions correctly, with the controller kicking in the pump when the differential temperature is reached. You can hear the pump kick in and hear and feel flow through the pipework. The hot water return pipe from the collector gets very warm (when the weather is good).However, quite often you can hear the noise from the pump change note and flow stops through the pipework. I installed a flick switch to the supply of the pump and when I toggle this the pump restarts and water flows correctly, only for it to stop again shortly after. The problem seems to get worse when the water temperature increases.

I have tried another cheap pump with exactly the same results. If I open the isolation vale to the pump, the water flow is very good, in excess of 5 l/m, so I am very confused as to what the problem is.

Any advice gratefully appreciated

Nigel