My Heat pump is isolated from the central heating circuit by a heat exchanger
Where should i place thermometer probes for the heat meter to give realistic COP measurements
If I measure heat loss between the send and return for the heat pump it ignores heat losses at the heat exchanger but gives a COP for the heat pump
If I measure heat transferred between the heat exchanger secondary output (into the house heating system) and the heat pump return it will reflect losses for the entire system, which is naturally a bit lower
That’s a good question - not one that is asked often enough, in my opinion.
The topic of what should be included in (or excluded from) the measurements of Electricity Input and Heat Output relates to the “System Boundary” being assessed; searching for that term will turn up some previous discussions on related questions, though I don’t recall your specific question being asked before.
For HeatpumpMonitor we have a strong preference to report the performance against the “H4” System Boundary as defined by the SEPEMO research project; there’s a relatively readable summary of H4 versus H3, H2 etc. in Section 2.1 of this UK Government Publication.
The key point is that we’re assessing the performance of the heat delivery system, not just the heat pump unit. That’s why things like the electricity consumption of secondary circulation pumps (e.g. for UFH manifolds) should be included in the electricity metering, if you want to be comparable to other systems reporting against the H4 System Boundary.
My interpretation of that document’s guidance is that you should ideally be measuring ‘downstream’ of the heat exchanger, i.e. measuring the heat being transferred into what you referred to as the ‘house heating system’.
In practice, the plumbing of the DHW circuit tends to act as a constraint on heat meter placement - if you want one heat meter to measure both DHW and CH heat delivery it needs to be ‘upstream’ of the diverter valve. If you’re willing to install two heat meters (or if DHW isn’t being heated by the heat pump) that’s less of an issue.
Note that the primary circulation pump will slightly heat the water passing through it. Since the electricity consumed by that circulation pump is (almost always) included in the electricity consumption measurements, it’s legitimate to include that small heating effect as part of the heat pump’s heat output. The temperature probes should therefore be located on the ‘house’ side of the primary circulation pump.
I would say you want to put the heat meter on the primary pipework i.e before the heat exchanger since you want to calculate the efficiency of the whole system, therefore losses across the exchanger should be included.
The heat meter is deriving its heat reading from two measured parameters:
The Flowrate past its flow sensor
The Temperature Difference between its two temperature probes
(together with some assumptions about the characteristics of the fluid).
If the heat meter is on the Primary pipework (i.e. between the Heat Pump and the Heat Exchanger), it’s going to report all the heat added by the Heat Pump, so it won’t reflect any losses from the Heat Exchanger - it will consider the Heat Exchanger as just another component of the heating circuit, like a radiator.
It the heat meter is on the Secondary pipework (i.e. ‘downstream’ of the Heat Exchanger), it’s going to report a smaller temperature difference - because the Flow temperature coming out of the Heat Exchanger will be lower than the Flow temperature from the Heat Pump - so it will report a smaller amount of heat, accounting for the losses from the Heat Exchanger. However, in that configuration the Flow sensor will want to be on the Secondary pipework too, since the flow there will be different than on the Primary.
Ideally you want rid of the plate - it only reduces performance.
Failing that I would put the heat meter on the secondary side, then either add a pair of DS18B20s to the primary side OR try to read the heat pump internal temperature sensors/flows sensor to see what impact the plate is having. (see how much higher temperature the heat pump is having to work at due to the presence of the plate)
I suggest this because the heat pump is “much of a muchness” (there’s nothing that you can do to modify this) but the heating system you can make changes to (balancing etc) to improve it’s performance. There should also be negligible dirt in filters etc on the heat pump side to worry about measuring the flowrate there.
If you weren’t interested in making improvements and just want to monitor the heat pump itself then put it on the primary side.