@william1066 so my system did the experiment for me today.

I currently have a commissioned ASHP, the flow side goes to a 50L volumiser, then there is temporary shortcut (i.e. a bypass) back into the return side. I temporarily created demand so the ASHP has warmed up the water in the circuit and volumiser. the routes from the ashp's f+r circuit into the house are currently capped - there is no load on the ashp. the house is  still on the boiler.

the graph starts when I got modbus working yesterday evening. RWT goes below 25 right at the start but nothing comes on. When Outside temp goes below 4C, the system switches the circulation pump on. But not the compressor. the lukewarm water in the circuit must be enough to satisfy the warmth needs of the outdoor unit. After that, the pump is leaving  "off" gaps of 60 minutes then running for about 6-8 mins each time. I'm not sure what the "off" trigger is at the end of the "on" period. It'll be interesting to see how cool it gets before it puts the compressor on.

Posted by: @iancalderbank

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I'm not sure what the "off" trigger is at the end of the "on" period.

Probably just "x" amount of time

Hi, 

I have the same Error F788 did you finally fix it? 
Coule you please tell me Howell did you do? 
thanks for tour help

@bobshopsupreme did you ever get this matter sorted out?

@editor Hello. Yes and no! The original problem was indeed a dead water pump, killed by contamination in the circuit and possibly the power cut during freezing conditions (we do have anti-freeze valves). It took two months to get a replacement part.

We have then had another 2 years of fun with shutdowns caused by low flow due to under-dimensioned pipes, and then we still have a contamination issue which seems to be coming from the radiators. At the moment the flow rate gradually decreases over several weeks until the system cuts out with a low flow error, and then is fine again after cleaning the filters. They are trying to find the right product to protect the inside of the radiators because whatever inhibitor they have in there at the moment clearly isn't working.

@bobshopsupreme very interesting. Do you have a buffer tank or is it an open zone?

@editor We have a 200 litre buffer between the hydraulic station and the radiator and underfloor heating circuits. The flow rate problem was due to undersizing of the pipes between the outdoor unit and the hydraulic station. Those pipes have now been changed and it now meets the >2000l/hr nominal flow rate when the filters are clean.