JM97

Thank you, that makes perfect sense. Are there also any mechanical considerations? I know with boilers you need minimum flow rates to prevent damage to the heat exchanger. There was a brief reference in something I read to "pressure" problems if the delta T on an ASHP goes above 7C. And that ASHPs will quickly trigger a fault if the delta T strays.

I am looking into an ASHP (air to water) for the first time and have been reading widely around the subject. In various places, I have found lots of information about needing a delta T of about 5C to 7C. So far, though, I have not been able to find a simple explanation as to why this sort of delta T is required, or why a heat pump works best like this. I would be delighted if anyone could explain the basics, or point me in the right direction. Thanks.