Flow Rate Discrepancy affecting COP?

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 :sob:

Sorry this is long and rambling but I thought the context might be helpful

I think this must be the issue, if the flow rate is lower relative to the value reported by the vaillant and that’s changed in recent days then that seems very likely.

Sontex meters are susceptible to dirt in the system, it’s not something we’ve experienced widely as far as we’re aware but it is known to be a potential issue

I would see if the warranty on the HP is valid. If it is out on some of the readings, it may well be out on others. A small change in one of the critical parameters can have a large impact on efficiency.

Assuming the flow meter and the HP are in series, which they should be, the flow through the two should be identical and the readings within manufacturers tolerances (not familiar with the flow meter in the HP, but assume it is a meter, not just an approximated or averaged value).

Our unit seems to run at just over 14 & just over 8 l/m when set to Eco and cycling (as measured by the Sontex.

It would be worth having a system flush and also cleaning the sontex as best you can I think. A system flush is a common thing to do when servicing heating systems.

thanks Trystan

As I am loathe to call out my plumber for the system to be flushed for the 3rd time in 5 months (not sure how it isn’t working!!!) I decided to have a go myself using the system pump, with the options available to me in the installer menu. First I ran the purge programme, then the high pressure programme (no idea what this actually does…!) and then forced the circulation pump to 100%

I then closed all the radiators down and opened them up 1 at a time to force water through each one, shut the system down again, cleaned out the filter and opened everything up again. The filter did have crud in (seems to mostly be the jointing compound they use to seal joints).

I then set the pump to 100% again. This was giving me a flow rate of ~1030-1060 l/h / 17.2-17.5 l/m and the flow rate being measured by the heat meter does tally pretty well with this at ~16.8 l/m and also tallies well against historical flow rates.

red = earlier DHW cycle, pink = running pump at 100% and opening and closing rads, blue = pump running at 100% with system fully open

~16.8 l/m

compared to historical

So… :crossed_fingers:

I was getting decreasing flow rates on my Sontex which necessitated cleaning it several times.

Cured by fitting a decent mesh filter which I’m cleaning about once a month.

It could be much simpler. I had an issue with flow rate on my Ecodan 6kW machine which, after much investigation, turned out to be a faulty isolation valve next to the circulation pump. Although the handle made it look as if the valve was fully open, the paddle inside had worked loose and was in fact partially closed. Low flow rate destroyed the COP. Once replaced, flow rate back to 16L/min and COP back to normal (sadly, 3 at best, but anway back where it was).

I’ve had issue with crud in sontex heat meter. They are more susceptible to this when then are mounted vertically apparently, and it can be worse if they are installed before the filter.

See the thread I started on this some time ago: Sontex heat meter accuracy over time