Hi.

I have 2 thermal solar panels providing heat (alongside a heat pump) to an underfloor heating buffer tank and a domestic hot water tank through 2 individual RM Solar DC12 controllers. If either one of the controllers is turned off, the collector temperature indication over-reads significantly; apart from that (and shared error codes) the two systems are independent.

The domestic hot water system pump frequently fails on automatic and, when it is running, it fluctuates, speeding up and slowing down about 2 times per second. Furthermore, the pump fails to run on automatic unless set on top speed (3). However, it runs without any problems at all on manual with any of its 3 speeds selected. Also, I have noticed that the buffer tank system pump's output does not fluctuate on either automatic or on manual. I have seen quite a few Error 61 failures, which I clear by resetting the power to both systems.

I suspect that the output demand from the RM Solar DC12 controller on the domestic hot water system is faulty/intermittent. Is that likely to be the case? If so, can anyone recommend a suitable replacement for my RM Solar DC12? My research so far suggests a RESOL DeltaSol CS/4 could be best.

I am also in the process replacing my 18 year old pumps (Wilo RS15/6-3 4516688) with Wilo Varios PICO-STG 15/1-7 pumps. These have a PWM capability as does the RESOL DeltaSol CS/4. Is there a significant benefit in using PWM over simply setting the circulation pump to run at a fixed speed? If so, would it best to replace both RM Solar controllers rather than just the faulty one? 

Please note that it is the thermal solar heating of the domestic hot water tank that is most important for me, as the heat pump is very efficient at heating the underfloor heating buffer tank (with a COP of 4, compared to a COP of 3 for the domestic hot water) and the underfloor heating is normally off when its very sunny.

Any advice and suggestions will be gratefully received. Thanks!