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Comparing COP (coefficient of performance)

Mars · 2021-03-11T14:43:55Z

This is probably the most contentious subject when it comes to air source heat pumps in particular, so I figured it would be a great place to have a forum where we can compare COPs for the various pumps we have across this website and forum. I'll get the ball rolling. I submitted an RHI reading today, and our lifetime COP (two years now) for our 18kW Caernarfon air source heat pump stands at 2.71. But I need to take this point forward, as this is the bit that confuses me. I've been told (and I've not verified this) that the heat pump industry's standard for calculating COP is done at a flow rate temperature of 35C. When we took a look at our ASHP, there is a sticker that confirms that (see below). I'd be interested to know if other ASHP brands have also calculated this at 35C. If this is the industry standard, then it's a tad misleading because only well insulated houses are going to be running at 35C. We tried to run at 40C in late February, and had to switch back to 45C. It's only in April that our pump will automatically scale back flow temperatures as it gets warmer. So our COP will inevitably always be the product of a 45C flow temperature. So when people say our COP could be better, there isn't an element of comparing apples with apples if their pumps are running at 35C. Or am I misunderstanding this? This topic is now open to the floor.

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Air Source Heat Pumps (ASHPs)

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Mars

(@editor)

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Topic starter 27/03/2022 8:24 am

I ran an experiment this week to see what our COP and electricity consumption would be like by varying flow rates (45C and 40C): https://myhomefarm.co.uk/air-source-heat-pump-cop

As the winter comes to an end, please share your COP figures for your systems. It should make for some interesting reading.

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Derek M

(@derek-m)

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27/03/2022 1:47 pm

@editor

Hi Mars,

I suspect that the method being used to calculate COP in your system is not that accurate. I do believe that the MMSP system, even when correctly installed, is of the order of +/- 5% accurate. I don't think that any heat pump manufacturers system will come close to that.

Whilst electrical energy consumption can be measured with reasonable accuracy, to accurately measure the heat energy produced would require not only accurate temperature measurements, but also reasonably accurate mass flow measurement, which is not that easy to do.

Your tests proved what I think most people already know, that lowering the water flow temperature improves efficiency and reduces running costs. By using weather compensation this should be achieved automatically, without continual user intervention.

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Mars

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Topic starter 27/03/2022 1:59 pm

@derek-m, good points. Because we have a bivalent system, we've had to install two separate metres that are exclusively connected to our heat pump so that we can submit these readings to RHI on a quarterly basis to get our payments. I would therefore assume that the two metres provide us with two accurate data points: ASHP electricity consumption and ASHP heat generated. If it's of interest, I'll provide the names of both metres later this evening.

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Derek M

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27/03/2022 2:37 pm

Posted by: @editor

@derek-m, good points. Because we have a bivalent system, we've had to install two separate metres that are exclusively connected to our heat pump so that we can submit these readings to RHI on a quarterly basis to get our payments. I would therefore assume that the two metres provide us with two accurate data points: ASHP electricity consumption and ASHP heat generated. If it's of interest, I'll provide the names of both metres later this evening.

Hi Mars,

I am assuming that an electrical energy meter was installed along with a heat energy meter. Did the installers provide any form of calibration certificates?

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Mars

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Topic starter 27/03/2022 8:36 pm

@derek-m, you always give me some very useful homework to do 😀. I have no idea whether they were calibrated and I don't think I have any certificates to this effect, but I'll rummage through the paperwork tomorrow. Failing that, I'll contact the installers and enquire. I'll also post some photos in the morning of the two metres. The one that registers heat generated is extremely helpful as it also keeps tabs on physical flow of water through the pipes, which I use regularly. We're now averaging 30 litres/minute on the ASHP which is good. When that drops, it's time to give the mesh filter a clean, but thankfully we've been running very clean of late.

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