COP temperature relationship

[MyForest (David Bowen)]

@moojuiceuk

Keeping flow temperatures as low as possible will keep the CoP as high as possible. Selecting the right size heat pump for the application will also help. Massively oversized heat pumps are a bad thing. Even on inverter driven systems, the inverter can only modululate the compressor output down to a certain low output. If a system is cycling on-off frequently as it cannot modulate low enough, every start up will mean the compressor is initially running at a higher output kW briefly for a couple of minutes before modulating back down to ensure oil moves to the right places inside the compressor. On fixed speed compressor installations, oversizing is even worse of an issue.

@moojuiceuk I have one of those “massively oversized” heat pumps and it does struggle to modulate down far enough.

I thought I’d mention that I have a reasonable workaround where turn the heat pump off pretty often:

Improve efficiency: Turn the heat pump off

As we all know, the most efficient way to run a heat pump is to keep it running all the time. However, I’d like to propose an alternative. I think there’s a case for turning them off. Here’s 2019-10-26 - the average temp was 9 Celsius. We can see in the evening the pumps we on all the time and my records show the system was flipping between heating and anti-freeze. This is a good example of why I split space heating and hot water in my efficiency analysis. The hot water usage is sporadic and …

Which of course brings us round to your point about the start-up being power-hungry.

Here’s the default Ecodan control algorithm which does use more power at start up and then settles down to something lower:

Below is my control algorithm attempting to hold it back a bit by starting with a target temp of 33.5 and walking it up to 38. It still runs away a bit at the start as you describe. I’ve tried various ways to hold it back but if I reduce the target too far the heat pump declares victory and shuts itself down which means it’s not amortizing the startup over a reasonable run. Most of my runs are 15-30 minutes specifically to help amortize the startup cost.

Of course I’ll claim my oversized heat pump is “for those bad days” and in fact it did cope really nicely in February 2021, but that type of cold is pretty rare. At least on the bright side my “gentle” control algorithm is stopping it cycling as much as it was on the “original” Ecodan algorithm which you can see in the chart below (which will make you fall off your chair):

Hopefully this’ll help people consider using more reasonably-sized heat pumps, but as Hugh is intimating, it’s hard to get a really accurate calculation without taking all the factors into consideration. I thought your analysis of the situation was pretty good moojuiceuk.