Is Cycling bad on a heat pump?

Don't shoot me, I dont like Elon, it think he is a dick (I'm sure he doesn't rate me either) but despite my dislike of the man I've got a Powerwall, my solar guy told me it was the best kit for my house. I have no idea if its any good, I'm not qualified to comment , but I have found it has a hidden heat pump monitoring feature.

If I look at the energy screen I can see how much my heat pump is cycling. Our house uses 8kWhrs of energy a day plus the heat pump. When the heating is off there is a fairly steady load. Those peaks in the graph above are my heat pump starting , and the troughs are it stopping. As you can see its cycling.

Its not reaching room temperature then stopping, its just running as slowly as it can but the heat isn't getting out of the radiators as quickly as the compressor (the bit that does the heating ) is putting heat in. So it stops to avoid overheating. The big bump at 4am is the car and the hot water. This graph above was on a day when its 10 degrees C outside so the heating is on but the unit cant go slow enough to run continuously. Hence the cycling.

Now if I was a traditionalist and I would advise that putting a buffer (a bloody great water tank) in the heating circuit to reduce this cycling. I'm not a traditionalist, I hate buffers, they have no place in a heating system and are yet another hurdle to installing heat pumps. There is no where to put the cylinder in the house FFS so where are we putting the buffer?

So how can I reduce this cycling and do I need to?

First is cycling bad? The real answer is no one knows, no one has measured it in a blind trail, all the evidence ive seen is based on best guess, but word on the street is the compressor doesn't like cycling. I'm sure if your system is cycling every 5 minutes that's not good, but is 2 x an hour too much?

But lets play devils advocate and say cycling should be reduced if possible. How can we easily reduce it?.

Cycling happens because when your heating system just cant go slowly enough. if you only need 2kW of heating and your heat source can only ramp down to 4kW then there is no choice the system has to cycle on and off. The idea of a buffer is that you heat loads of water up to temperature, turn off the heat source and let the buffer heat the house for a while. Its a really crap mechanical solution, there has to be a cleverer way.

So I decided to experiment on my system. I spoke to the legend Fergal O'Driscoll at Ideal (I have an ideal heat pump) and ran my idea past him, turns out its not that revolutionary, Hes been doing it on his system at home too.

My plan was to tell the unit it cannot start the compressor for at least 15 minutes after it stops, its called a guard timer (normally its set for 3 minutes). The plan is to let the heat pump run and settle at minimum capacity, let it get to temperature and then stop.

Once the compressor stops we are going to keep the water pump running all the time with the compressor off and let the water in the rads slowly fall in temperature for at least 15 minutes. I chose 15 minutes because my house doesn't cool significantly (you cant notice its off) in this time as the house is moderately well insulated. The better the insulation the longer you can make the timer period.

After 15 minutes, if required the compressor can start again and has a load of work to do to bring all of the water in the circuit back up to temperature , its like a virtual buffer, its free and its small.

So does it work?

I adjusted my machine mid morning on the 11th November. Fortunately the weather on the 10-12th November was the same every day, mild, dull British weather. Perfect to test the idea as the unit cycles a lot in this weather.

Note how the unit cycled 60 times on the 10th November, but after adjustment it cycled only 33 times on the 12th November. I've reduced my cycling by 50%. I'm now starting the compressor 1.5 times an hour. Note how it uses more power on each cycle. But also note it cost me roughly the same to run. I know its not scientific, and there are a load of variables but its a live house not a lab.

So I've reduced my cycling, it made no difference to my run cost but i didn't have to buy a buffer.

Some who are really geeky will say a perfect system will not cycle at all. If the unit is very clever it will modulate the compressor down to meet the load and alter the water speed to suit. In this way the compressor will never go off.

I agree, its the dream. Its also very rare to see this in heat pumps. I last saw this on RED heat pump 4 years ago. See below, 7 starts a day, epic. Sadly the unit no longer exists.

Some modern heat pumps do this level of control as standard, most dont. Mine (the older version of the Ideal unit) currently does not alter the water pump speed with load, (the new R290 unit does) After xmas my PWM function (more on this next week) will be switched on, my pump will run in concert with the compressor, it will cycle even less, I will update you then.