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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 dents the CoP in a way that distracts from an experiment like this. We can see space heating has a CoP of 2.58.

Contrast this with 2020-02-10 that also averaged 9 Celsius, but using an algorithm that turns off the heat pump off every so often. We can see it’s still £2.50 to do space heating but the CoP is 3.32. We got an extra 13kWh of space heating for free.

The obvious retort is that the residents will get cold when it’s off. However that’s a matter of perspective. I’ve surveyed them and they seem completely happy - they can’t tell the difference.

The over-sized radiators (cheap Stelrad K2) are acting as a heat store and gently releasing heat during the down time. Notice how we turn it back on when the “return” temperature of the heat pump is about 26 Celsius. At that point the radiators start to feel cold and the residents are sensitive to that. However, whilst they are “warmish” the residents don’t actually know if fresh hot water is being pumped around or not. The “return” of the heat pump monitor (even when not being pumped) is an approximate proxy for the radiator temperature.

My control algorithm uses a few heuristics to decide when to switch off the heat pump, such as “has it been at the desired flow for a few minutes” and “does the heat pump think it’s not worth putting any energy into the next cycle”. For turning back on it just looks at the “return” temperature.

One aspect of this is that the algorithm watches the trend of the cooling and predicts when it’ll come back on. On a number of occasions I’ve though to myself “ooh, it’s getting a bit cool now” only to go and look and see the prediction is for it to start in a couple of minutes or even that it has just started up. I’m the most sensitive person in the house to heat levels which is why my office is set to 23 Celsius - the radiator is 40cm from my chair.

The hysteresis includes the effective outdoor temperature in deciding what represents “cool” and how long it should delay turning on / off. In that way it’s a bit of an extension of the weather compensation.

The heat pump is of course turned on if the temperature gets really cold outside. We have glycol in the circuits though so 0Celsius isn’t a critical temperature for enabling it.

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Looking at the graphs, you seem to have some single data points for COP that are way out of range, hence the graph is basically flat, hiding the detail. These are surely anomalies and are maybe artefacts of the way the data is stored and processed? They only seem to occur when the pump is turning on or off.

@Bramco Hopefully the topic above will explain why it’s behaving that way and that it’s A Good Thing ™ :slight_smile:

I can see it’s a good thing to have the HP running more efficiently by not cycling. I have the heat bank set points set in such a way that our boiler when needed does long runs rather than cycling on and off.

However this still doesn’t answer why you suddenly get a COP value which can’t be realistic. You can’t put 1 unit of lekky in and get 300 times that out of a heat pump. So there’s something wrong somewhere. It has to be something to do with when and where you are measuring things.

Don’t get me wrong this isn’t a criticism, I think what you’ve shown us is great. But… something must be wrong in the measurements or we need to rewrite some physics.

I’d love to be proved wrong in which case someone will make a fortune by getting their HPs to run at a COP of 300 not 3.


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Yep, it does look wrong, but it’s because of the lag.

We’ve just heated the water and it’s still being pumped round so is providing “useful heat” in the way the MMSP measures it. On the last cycle we stop providing input power but there’s still warm water to pump around so it looks like we’ve done an amazing amount of work.

Here’s the actual stats coming from the MMSP. The last five columns are heat related - they measure in watt-hours.

You can see the “48” looking all lonely on one of the later rows. That’s the “useful heat produced” column and the “0” to the left of it is the amount of electricity we used in that cycle. FYI, the last column is the “how much electricity should we input on the next cycle” which the heat pump seems to take as a serving suggestion and not gospel.

Conversely, when starting up the CoP looks appalling because we’ve used electricity but the water is just starting to be pumped around.

This is why I usually average the CoP over 5 minutes - I just haven’t worked out how to do that in Emoncms yet.

Also, it looks as though you circulate your hot water, it gradually loses heat between operations of the HP.

@Bramco @borpin Hopefully the topic above explains how I’m using the sensors - notably as a proxy for the radiator temperature.

So “between operations” I don’t circulate the water.

Specifically, it was using 200W to pump the water round when the heating part was off. Seeing as most of the water was already in the radiators at that time there didn’t seem much point pumping the same water round and round. Especially because it also sent it outside to “anti-freeze” the heat pump. I didn’t buy a big lump of metal to sit outside so I could pay to keep it warm. There would be some water in the pipework radiating heat which would not be getting to the radiators. You could consider that “waste” or “poor man’s underfloor heating”, I’ll let you decide.

The other benefit has been that we don’t have to listen to the pump all the time. It’s ridiculously quiet where I live in the countryside and even the noise of a pump in the utility room is quite disruptive to the tranquillity. My brain thinks the noise is OK if the something is doing useful work (like the noisy tractors in the field next door) but can’t cope if there’s noise and no useful work.

Here’s where the sensors are…

Yes, I worked that out - as the water in the tank cools, the sensors are picking that up - hence the temp drop. The losses around the tank pipework are usually significant. I have double layered the pipes close to the tank where I can and next time I’ll make sure there is a lot more room around them to do it better!

Yep Brian, I’m losing heat in those pipes as you say. Notably it means they are roughly matching the radiators.

When I can get better monitoring of the rooms / radiators I could do as you’ve done and lag those pipes so they don’t lose heat.

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