I had a heat pump installed with open energy monitoring. (Axioma E4 DN25 QP3.5 heat meter on a 12kW Vaillant Aerotherm Plus) The monitoring started last week and I’ve now a few days of data for hot water heating. I’ve noticed that about half way through each cycle the heat power readings drop to zero and the electricity readings continue. I am trying to understand what might be going on. Images of the heat meter set up and an example trace showing what I mean.
Hi! Congratulations on your purchase, and welcome to the heat pump owners club! 
You’re seeing a classic case of trapped air in the pipework, which causes the flow meter to stop working above a certain temperature. Very common on new installs, and fixed by purging the air out of the system. This might take a few attempts to eradicate completely.
To clarify: the heat pump is working fine, it’s just the metering that’s a little off.
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Thanks Tim. That’s really helpful.