When it is cold and my 12 kW Samsung Gen 6 ASHP is not running, the water pump periodically runs for 5 minutes to keep the circuit from freezing and takes heat from the DHW circuit. This is standard behaviour and apart from reducing the temperature of our DHW, it usually doesn’t cause any problems. However, I have noticed that if the space heating is scheduled to come on during one of these freeze prevention episodes, then the space heating behaves very abnormally as in the graphic below.
The compressor starts while the 3 way valve is still switched to the DHW circuit making it appear on the Emoncms graphic as if it is heating DHW. The valve then switches to space heating, but electricity consumption rises to very high levels with an instantaneous COP falling to 1.1. The first time this happened, the compressor stopped by itself and then restarted a few minutes later and space heating returned to normal. On this occasion, I stopped it and restarted it.
Has anyone seen this behaviour before and has anyone any suggestions how I might avoid future occurrences?
Just to clarify that this issue has nothing to do with defrosts. I was referring to the uncommon situation where a call for space heating occurs during a freeze protection cycle. In the graphic below I have highlighted two normal freeze protection cycles where the water pump circulates water around the DHW circuit. The compressor is off and the 3 way valve is open to the DHW circuit.
When there is a call for space heating during a freeze protection cycle, ideally the compressor should not start until the cycle has finished and the 3 way valve returns to being open to the emitter circuit. Unfortunately, the compressor starts while the 3 way valve is open to the DHW circuit and only after a few minutes does the 3 way valve return to being open to the emitter circuit. The subsequent space heating is very inefficient and consumes huge amounts of electricity. Stopping the heat pump and restarting it then restores normality. Note the defrost during the normal space heating cycle - totally different to a freeze protection cycle.
Heating has started, presumably with the water in the heating circuit at a low temperature. A lot of energy will be needed to bring this up to operating temperature (depending on the volume of water) and that’s what it looks like to me.
The maximum input power is 6.3kW with an outside temperature of 0C which seems in line with expectations for that unit.
I think the switch from DHW to heating is a red herring.
The graphic below shows a normal cold start at 1 C with maximum consumption of 3.9 kW and heat output of 8.8 kW prior to the defrost. This is very different to the second example.
In the graphic below, you can see the RWT rises above the LWT, which is typical of freeze protection when the compressor is off. The subsequent space heating is very different to the first example. I never see 6.2 kW electrical consumption producing 6.7 kW heat output at any other time.
Hi Mike, do you have a backup heater fitted? Maybe this scenario is turning the backup heater on. This might explain the extra 3kw of electricity and very low cop.
I’ve just looked back to the start of Dec and I’ve never had space heating start during a frost protection cycle (lucky fluke?) but I will keep checking and if they do clash will post the results.
