A beginners question: when the heat pump is off but the pump is still running, the system is recording minus COP as, I presume to account for electricity still being used. Is this right?
The system has been in for a year and running well but wondered about whether this was right?
Thanks!
Hi Phil,
That’s a good question. The reported COP shouldn’t go negative just because the heat pump (compressor) is off. It should go close to zero, but not below zero unless heat is being taken from the house (which might be the case during a defrost cycle - or if the Room temperature is higher than the Flow & Return temperatures).
Based on your snapshot, it’s the fact that the Return Temperature (mean of 18.8) is fractionally higher than the Flow Temperature (18.7), hence the reported “HP heat output” of -20W. When that negative heat output is divided by the (positive) electricity input, you’re seeing a negative value for COP.
Assuming the Flow and Return temperature sensors are the right way around, I wonder it’s a minor issue with the accuracy of (one of) those sensors. You’d expect the Flow to be higher, if anything. Alternatively, since your mean electricity consumption in the snapshot is only 9W, that suggests to me it’s only the heat pump control board which is consuming power, not also any circulation pump - so perhaps the Return sensor is just in a slightly warmer location.
It’s entirely possible to have a negative COP: if water is pumped through the outside unit while the compressor is off, it will be cooled, as it is colder outside and the insulation is not perfect. So flow will be lower than return, you transport energy out of the house and you have a negative COP.
