Is my heat pump cycling after domestic hot water cycle?

Hi,

We’ve just had a heat pump (Samsung 16kW R290 Monobloc Heat Pump (AE160CXYDEK/EU)) installed in our house, and I’ve been monitoring it’s behaviour. I noticed that after a cycle of heating hot water tank, the heat pump seems to go into shorting cycling. I also noticed that the house seems to continue being heated during the hot water cycle, which I’m not sure should be the case. I am wondering if the community can have a look and let me know if something is wrong with the install?

Monitoring graphs attached. Hot water heating around 1435-1515. During the period the house heating mode is on. The period between 1515-1545 is when I think the cycling is happening. Water law temperature is the temperature I calculated the heat pump should be operating at for heating radiators (the measured temperature is usually lower than that a bit).



I’m very new to heat pumps, so any help would be much appreciated. Thank you very much!

best wishes,
yw

Welcome to the community @Yee_Whye_Teh :slightly_smiling_face:.

If the Gen 7s operate like Gen 6s, your graphs suggest that:

  1. Your DHW heating is not co-incident with your space heating - the reduced (but steady) heat pump flow during the DHW heating period suggests that the diversion valve has operated properly and you are seeing the effects of differing friction losses in the two circuits. Is your circulating pump PWM controlled? (It is behaving as I would expect a fixed speed pump to operate.)
  2. The cycling you see after DHW heating is probably normal, to the extent that your LWT is exceeding its water law target for a short period, so (if the compressor is already at minimum speed) it has no option but to stop, restarting when LWT falls below target by ~2degC (the normal Samsung hysteresis in Gen 6s). (In Gen 6s there is a timer which prevents restart for 5 minutes - I speculate that Gen 7s have this too, as your restarts look like ~5 minutes after stopping.)
  3. At this time of year (“shoulder months”) it is not unusual for the compressor to be running at minimum speed most of the time (if running at all) because heat demand is low.

Do you have a buffer or volumiser tank? I ask because the slug of water they contain can affect instantaneous RWT (and thus LWT) values when re-engaged at the end of DHW cycles (during which circulating fluid should bypass any tank).

Hope this doesn’t confuse you any further…
Sarah

Thanks Sarah, super helpful!

  1. PWM pump: I’m not sure. How do I tell if it’s a PWM pump?
  2. Thanks for looking into this, it’s good to hear it’s probably normal.
  3. Yes we have a buffer tank. I don’t know if buffer tank is engaged during DHW cycle.

cheers,
yw

Glad to be of help yw.

Have a look at https://midsummerwholesale.co.uk/pdfs/quick-start-guide-samsung-gen-6-kits.pdf - it answers both your questions, and I assume this will apply to Gen 7s too.

In the first diagram, you can see that the DHW circuit doesn’t go through the buffer tank (you should check that your piping is consistent with the diagram). And if you whip your controller cover off, you’ll be able to confirm whether you are PWM by comparing your wiring to the first wiring diagram (if PWM there will be a signal wire to the pump as well as power supply).

Sarah

Thanks SarahH!

I finally got time to look into the pipework. It seems that in my system the return from the DHW circuit actually merges with the return from the central heating, before they go into the buffer vessel, rather than straight back to the heat pump. This then makes sense of what I have been observing, as the water in buffer vessel would be too hot for the central heating so it goes into cycling until the heat is used up to heat the house. It also made sense of another thing I have been observing, which is that the central heating can heat the house even when DHW heating is on, because it’s using the hot water in the buffer vessel returned from DHW circuit to heat the house (and this uses the much hotter (and less efficiently heated) water).

It seems that this is an error in the plumbing system which is inefficient and can affect the operation of the heat pump in the long term due to the frequent cycling that results. Do you think that’s a correct assessment?

cheers,
-yw

Hi @Yee_Whye_Teh,
Yes, by the sound of it, you are absolutely correct. My installer (more precisely, his apprentice) made exactly the same mistake (though to fair he quickly fixed the problem at no cost once I had pointed it out).
It is worth checking the whole piping system if you can - I also discovered that my installer had exchanged the Outdoor Unit water inlet and outlet connections, which makes the heat pump condenser less thermally efficient. Again he fixed this quickly, but customers shouldn’t need to be checking for schoolboy errors :thinking:.
Sarah