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I’m not convinced separation will solve his problem. We both know it’s needed anyway but I still suspect there’s a problem on what would be the secondary side.
IF the boiler is suitably sized and separation is installed be it CCT’s or a LLH then there should be no mixing as the flow rates should be equal on primary and secondary. There has been no mention of UFH etc. A 25/80 is huge and I assume 8m head? That should be more than enough to cover even an abnormally large index circuit, yet they’re still complaining of Luke warm radiators at the moment.
I still maintain they should have the building and current pipework system surveyed. Without knowing the boiler output required and the current system configuration no engineer can accurately say this is the fix or that’s the fix. Once you have a picture of the current system (which I suspect was never designed properly at the start) then you can give them their options. There’s nothing saying the combi couldn’t stay in place and have a separate HW cylinder but with what little information we have all we are doing is just guessing, regardless of how experienced and competent I know we are.
Agreed but I suspect the boiler pump is fighting the fixed speed pump, the fixed speed pump is pulling the combi pump eg increasing the speed so the boiler is having a hard time as the water is flowing too fast through the hex or the other way round the boiler is strangling the 25-80