I have had my Vaillant Arotherm heat pump since November.
Initially I was running it in continuous operation, with setback at night and when out the house.
In early March I decided to trial the eco setback setting, which rather than reducing flow temp when in setback mode, will turn the heat pump off if the current room temp is already met.
To some people this will be sacrilegious but I am a firm believer that it’s not all about SCOP, but total energy consumption.
As expected my COP reduced a bit but it was still running pretty efficiently (around 4) but most importantly my total energy consumption also reduced so I was happy with that. (I have not yet done the analysis taking into account the average external temperature to see how much of a difference was weather related).
In March I went on holiday for a week and set the heat pump to absence for heating (which in hindsight didn’t result in it doing what I wanted to do, but that’s a different story). I kept DHW on so my cylinder didn’t drop too low and need ages to warm back up.
When I got back from holiday I ran the legionella cycle, mostly because I was interested in how efficiently the heat pump could do it rather than any fears of legionella. Answer: not very efficiently.
I also noticed my heating flow rate was down on its normal value. This has happened before and occurs when sludge in the system blocks my filter. The system has been flushed and dosed twice but there is still crud in there it seems. But cleaning the filter sorted the flow rate as measured by the heat pump interface, e.g. I took out the filter, removed the crud and the flow rate instantly went back up to its standard ~860 l/h.
However, in subsequent days, I have noticed my COP really dropping off in the past few days, despite the warmer weather. At first I thought this was solely due to the heat pump not running for very long, so spending more time in start up, but on closer inspection the flow rate which I am seeing through Emoncms (and therefore, being measured by the heat meter and used to calculate COP) is well down on the flow rate being measured by the heat pump itself.
e.g. just now whilst producing DHW, the unit itself said it was doing approx 1000 l/h (~16 l/m) but the flow meter says it is more like 8 l/m. The flow meter is the sontex unit supplied as part of the OEM kit.
[[Unfortunately I cannot use the kWh comparison between my heat pump and the heat meter, because due to a previous issue with the heat pump, its self monitoring of power (and therefore energy) is inaccurate (e.g. it’s a 5 kW heat pump but often says its doing 10 kW of heat output). However, I have no reason to believe the flow rate reading from the heat pump is incorrect.]]
Some of my working hypotheses:
- there is some crud in the heat meter, affecting its flow measurement
- running the legionella cycle screwed something up (unlikely?!)
- there is some crud in the circulation pump, affecting its flow production (I don’t think this is the case)
blue = continuous operation, orange = eco setback, pink = holiday mode, red = running legionella cycle, green = eco setback past few weeks
heat meter measuring around 8 l/m
heat pump measuring 1000 l/h = 16 l/m
COP diving off a cliff
Sorry this is long and rambling but I thought the context might be helpful
