Heat Pump Truth or Myth#2 - Cycling uses a lot of power and ruins your COP

Kev M · 2022-12-29T05:52:32Z

Heat pump cycling is one of these topics where a lot of people seem to have an opinion but no actual data to back it up. Cycling is blamed a lot for poor ASHP performance and it seems to be generally accepted that cycling or even <gasp> short-cycling is a “Really Bad Thing”. I do have data so I decided to have a look at it. This is a graph of my 14kW Ecodan over a few hours on a recent mild winter’s day. After the morning startup, it runs for an hour or so without cycling. As the ambient rises it starts to cycle. And because cycling is a Really Bad Thing, it's now going to use more power and my COP will be really poor, right? Let’s see. This is a comparison of two periods after the initial warm up; not cycling (5-6am) and cycling (6am-1pm) time energy consumed Wh energy delivered Wh Ave Ambient deg average power consumed W average power delivered W ave flow temp COP 5-6am 1082 4246 8.9 1082 4246 36.1 3.9 6am-5pm 6112 27578 10.3 777 3506 35.9 4.5 During the cycling period, the COP is better and the power consumed is less. Surely that can’t be right? That’s not meant to happen; the power used should be much more and the COP should have plummeted. So what happened? Well what you’d expect really. The COP is better because it’s warmer outside and the average flow temp is nearly the same. The average power consumed is less because the heat demand is less and the COP is better. The bit that might be surprising, is that cycling hasn’t made a big difference to consumption or efficiency. Why not? I think it’s because the widely touted theory that starting up an ASHP and warming up the flow a bit uses an excessive amount of energy is a load of cobblers. There are no large peaks in power. And, at the risk of stating the bleeding obvious, between peaks there are troughs, empty ones. I am sure the cycling does make some difference and a smaller ASHP delivering 3.5kW without cycling will do it a bit more efficiently. If I look at the next size of ASHP down from mine, it can modulate down to around 3.5kW and it looks to have a 10-15% better COP at this output than I got with my cycling. Let’s say 15%. That means that I’d be saving around 120W of power, or about £1 per day over 24 hours at today’s prices. I know there is more to it than this and there are a lot more extreme examples of cycling out there but I think cycling sometimes gets a bit of a bum rap for being the cause of all heat pumps’ ills. And I’d rather have a heat pump that cycles a bit when it’s warmer and cheaper to run than be inside shivering when it’s freezing outside, the ASHP is defrosting every hour and running flat out trying to match the number on its badge. I’d be interested if anyone else has done any similar analyses. Or even just has an opinion. 😉

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Derek M

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06/05/2024 7:34 pm

Posted by: @jamespa

↑

I understand that A2A heat pumps have better turndown than A2W thus sizing is less critical. I haven't seen the proof that this is true however.

I cannot state that I have noticed that our A2A heat pump can modulate to a lower output, but it can certainly respond much faster than an A2W heat pump and also may be more efficient, since it is cutting out at least one stage in the process.

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