Hi Stephen @Stephen_Crown
Did you delete that post with the pic of the exposed pipes, and a guesstimate of the heat loss in the outdoor unit? I’ve had the email notification but it doesn’t seem to be in the topic here.
If we assume my R1T and R2T are not lying/faulty, then there is always a 2C loss in the BUH and associated large bore piping in the unit. At 30l/min, that’s approximately 2 x 30 x 4.2 / 60 kW or 4.2kw. More importantly, 10l/min is 1.4kW, which is a huge chunk of the output.
The simple graph above, plotting R1T temp (before), R2T temp (after), and R4T (return), shows how 2C makes all the difference to the numbers! R1T never goes below R4T, but R2T drops quickly below until the HP compressor cuts in again. Basically, the HP/BUH/pipes are sucking heat out of the radiators, which has to be restored before the Target Flow Temp can be reached. This doesn’t affect the house temp, because the R2T is still above the indoor temperature, but the transfer of heat to the outside and the required extra energy to warm everything back up is hammering the COP.
I’m probably going to remove the panels and check the insulation, or lack of, on the BUH and piping. It’s got to be going somewhere! I’m quite impressed that it can lose that much heat in such a small area/volume, but then the ambient-flow dT is 20+C.