Heat pump experiment review after two years

I’ve since refined my formula into something simpler, while still being rooted in some science I made up:

DeltaTemp × HeatLoss ÷ HeatGain + DesiredTemp
  • DeltaTemp = difference between outside temperature and the desired indoor temperature (°K)
  • HeatLoss = how many watts of heat are lost per degree of delta temperature - 450 for me (W/°K)
  • HeatGain = total output of all heating emitters per degree of flow temp - mine is 333 (W/°K)
  • DesiredTemp = the desired target temperature for the whole house (°C)

To explain how this works: deltaTemp × HeatLoss will be the total watts of heat escaping the house. Dividing these watts by HeatGain gives how many degrees hotter the radiators need to be. Add the desiredTemp to get the ideal flow temperature for the given outside temperature.

The HeatGain can be calculated from the total wattage from all the emitters, divided by the dT (i.e. 50). Or, take the “Peak Heat Load” from the heating system design spec, divided by the design temperature which is typically -3° outside vs. 21° inside (i.e. 24°).

What’s nice about this is that I can configure the set-back temperature to be the 2° lower overnight, which reduces the flow temperatures by about 5°.

I’ve taken the opposite approach, which is to set the TRVs to be relatively high and use the weather comp to keep the temperature balanced. I additionally boost the flow temp if the house is too cold, and suppress it if it’s too warm. I also aim to have all the rooms at the same temperature as many of them are in use throughout the day, and inmates keep leaving doors open anyway, so trying to “micro-zone” just isn’t working out for me. {Heat Geek have a good video about this}


Thanks @Timbones that’s interesting.

There’s certainly something weird going on out in other people’s houses. Ours is really badly insulated, but leaving the heating off for six hours overnight only causes it to drop by three degrees. That is about what people would call “setback”. In our house that’s just “sitting peacefully”.

I still can’t work out why people are heating their houses when they are snuggled under a duvet.

You can see our house got warmer again in the morning, but not by much. It’s more about the radiant heat the occupants are receiving than a boost in air temperature. You and @mjr can probably see now why I don’t care about the Inside temperature, it doesn’t really help understand what the occupants are feeling when they are eating their breakfast at the table next to the vast radiator in the dining room.

BTW we have some rooms with no radiators at all (!) and they do get pretty cold even though they are surrounded on three sides by rooms with radiators. In fact, that’s why it’s a good place for the larder. It does need people to keep the door shut though, which they mostly do. In fact I just heard them shutting it as I was writing that!

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Is your chart from last night? If so, here’s what mine was doing over the same period for comparison:

The internal temperature is dropping from 18° at about 11pm when heating is “set back”, down to 16° at about 3am. The room stat calls for 6 short bursts of heat (at lower flow temps) which keeps the internal temp level until it switches up a gear at 6am, running continuously for 3 hours at 1.7 kW. By 9am it’s back up to 18° and goes back to intermittent cycles.

That 60% more heat put into my house compared to yours (assuming its the same period), though it looks like the outside temperature here was lower (measurements from local weather station).

Maybe I need to worry less about letting the house go “cold” over night. I know that I haven’t quite figured out the optimum heating strategy yet, so will continue to try out other ideas. Only got a properly working heat meter yesterday, so can now get some decent experiments going.

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Hi @Timbones here’s mine from the same times as yours last night. They look quite a bit like each other, mine averaged about 0.9kW like yours.

I’m paying 15p a kWh at the moment which probably explains why your 13.1kWh were so much more than my 12.3kWh.

I’m working through the night a lot at the moment, so this is a good example of a human kicking it off about 01:30 to be a bit warmer. If I was under the duvet it wouldn’t have come on. That would have meant it was down from 22:30 until 06:00. As you can see, the extra heating didn’t do much for the air temp, but it did make me feel warmer because I literally have my arm rested on the radiator in my office. I do have a trigger to keep it on if my monitor is on, but that got boring because it was coming on when I was just about to go to bed!

That extra burst in the night used 2.9kWh = 0.42GBP. When it was whirring away it was producing about 7kW. It also had a CoP of just 3.08.

It’s a smidge warmer over here than where you are even though you are about 10 miles away. We are sheltered from the wind which helps.

It looks like you are doing OK from that chart, but I suspect your daily costs are troubling you a bit.

Thanks for sharing your chart; it’s good to see a comparison. I’m paying an extra 10p a unit right now, so I avoid looking at the costs too closely, but the solar PV from sunnier days is beginning to offset that. I also take solace in the fact I’m no longer burning any gas. :slight_smile:

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Here’s last night. As you’ll remember, it finished heating the DHW at 23:00 and then just sat around until the crontab kicked it off at 08:30.

The “Inside” temp went from 22 °C down to 18 °C. It was fairly linear but did start to get more shallow. I believe it settles at about 15 °C if the heating is left off for a long time. The house consumes about 650W of electricity when it’s just sat doing nothing so that heats it up a smidge.

I was awake until about 05:00 and wasn’t cold. The min effective temp was -1 °C.

You can see it got going again just fine at 08:30 and it’s nice and toasty now.

The rather crazy 24 °C is from that experiment of leaving it running! It had got down to a more normal 22 °C inside by the time it shut down so last night wasn’t really influenced by the experiment.

So between 23:00 and 08:30 my CoP was 1. It used 0.1kWh / 0.02GBP just doing monitoring.

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I’m curious: What’s the rationale behind “walking up the temperature”? Why is that seen beneficial?
I’ve be experimenting with the same strategy, but can’t quite see any difference in overall efficiency.

During the day I do like to keep the HP running at minimum power to keep within the solar PV budget, but it seems a shame to not use the full power when it’s needed (i.e. first thing in the morning before the sun’s up).

I can also understand why the manufacturer may have tweaked FTC5/6 work harder to reach the set flow temp, for the average consumer that expects the heating to come on fully when asked to.

There is a previous short discussion at Improve efficiency: Hold down the flow temperature as long as possible


Yes, it seems as if that is there concern. Clearly there will be situations where a system has been on set-back, and the occupant wants heat quickly. However, if you want to set up your system more regularly, there seems no way around it. e.g. when I know I want about 3kW max going to my house early morning, unfortunately my 6kW ASHP ‘revs’ up to 8kW, but 1hr later its cycling inefficiently. This is quite a challenge for manufacturers, but I tend to feel that they could do much better. A ‘response time’ setting would be very useful, but manufacturers probably try to keep things as simple as possible, and many would say they are already too complicated. I fear it will be may years before software becomes clever enough to optimise efficiency properly.
For interest, this graph shows a floor slab getting an early morning injection of heat. If only the input could be limited (as per the red line). My best guess is that a slow ramp-up of the flow set-point could achieve this.


Thanks for posting such a detailed study

I’m considering a heat pump for my newbuild
One concern I have is the slow ramp up because of the low temperature output, so the HP has to be operating for longer, leading to greater losses through the insulation

How many kWh of oil per year were used in the years before fitting the HP? To compare with how many kWh post HP fitting. That would give a closer to realworld COP

Hi @Jimbo1

Depending on how frugal we felt we used between 1,400 and 3,000L of oil so that’s 14,000 - 30,000kWh but with our 70% efficient boiler that meant the house got 10,000 - 21,000 kWh.

Our house has high thermal mass so takes ages to get cold. We usually turn the heat pump off between 22:00 and 06:00. It only drops about 3 degrees during that time and the large radiator in the kitchen-diner gives some nice warmth in the morning. It does take until about 08:00 to get back up to 21C. To be honest people are moving around so much they don’t really notice it’s cooler. Also, the temperature varies quite a bit around the house - the bathroom radiators are set to 24 for example.

As you can infer, we’re not suffering with heat-up time.

You may find your house is more variable if it has low thermal mass of course.

Hope that helps.


While the “ramp up” may be slower, and the heating may be on for longer, the benefit of the heat pump is that it provides a gentle heat throughout the day rather than bursts of heat from a traditional boiler. At the end of the day, it’s the same amount of heat being produced, but with different efficiencies and cost.

The ideal scenario is for the heat source to provide a constant output that matches the heat loss, for optimal comfort (this is what ‘weather compensation’ does). However, it’s normal to have a “set-back” temperature at night, as people generally prefer cooler temperatures when sleeping. There’s some discussion about this further up this topic.

Heat loss is driven purely by the difference between the indoor and outdoor temperatures, and the U-value of the insulation - I can’t see that the heat source makes a difference.

I would expect that a modern new build would be very well insulated, and a heat pump should be well suited, especially if the heating system is properly designed from the outset. I’ve heard that underfloor heating can work particularly well with heat pumps, otherwise some suitably sized radiators will be able to do the job.

There’s also the expectation that price of electricity compared to gas will eventually come down, so cost of running a heat pump will become more favourable.

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Just to be clear on the units

Energy is in Joules, 1 Joule is lifting 1 Newton of force (weight) up 1 metre
1kg weighs 9.81 Newtons on earth, on the moon it’s still 1 kg, but weighs only 1.63 N
Power is I Watts, 1 Joule per second

The kWH is an odd unit to use, but we are stuck with it
1000 watts of power for 1 hour
So it’s power x time (P x t)
Expands to energy per time times time (E/t x t)
The time terms cancel out, leaving Energy

It’s energy we pay for, not power

“Heat loss is driven purely by the difference between the indoor and outdoor temperatures, and the U-value of the insulation “

Heat loss rate is driven purely by the difference between the indoor and outdoor temperatures, and the U-value of the insulation – agreed, but the heat loss energy to be paid for depends also on the duration. That’s why a slow ramp up is a downside, the house is warmer than it needs to be for longer (all in bed / out to work/school)

So the losses during the longer ramp up times become significant, the oversized HP is a wise choice – faster ramp up

So for HPs, a COP of 4 is a plus, but in normal times, elec is 3x to 4x the price of gas, so it’s close on cost savings, to pay back the cost of the HP

“Depending on how frugal we felt we used between 1,400 and 3,000L of oil so that’s 14,000 - 30,000kWh but with our 70% efficient boiler that meant the house got 10,000 - 21,000 kWh.”

Typically how many kWh of electricity do you use for the HP in a year?

I’d contest that :slight_smile: . There are lots of other factors; wind speed & direction, air-tightness, solar gain etc.

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Don’t things like wind and solar gain mainly change the outdoor temperature at the surface of the building? But we account for them separately because our outdoor temperature sensors are on the back of the pump or on a weather station, not dotted over the surface of our buildings.

Solar gain is the heat generated inside the house through glazing (AIUI). So heat loss as an absolute will be different on sunny days to dull days from the same starting point. A wall on the sunny side will be warmer than a wall in the shade for the same air temperature so affecting the heat loss.

It is true that in a perfect environment, heat loss is the difference, but in the real world, there are many other dynamics as well as many different materials between those 2 temperature which will also be different where in the building you are.


With a heat pump it isn’t necessarily the case that it’s more economic to have an oversized unit to reduce ramp up times. The CoP of the heat pump is the biggest factor in energy use, and that depends on a whole host of interacting variables. In a well controlled ASHP system, you would expect that the CoP would be higher when re-heating in the evening for example, because, typically, the ambient will be slightly higher than re-heating in the morning. However, in either case the heat pump is often operating at a lower CoP at full load than at part load (with inverter drive units) and the room temperature will also affect the return temperature from the radiators which impacts on heat delta T etc etc etc… In practice the ASHP will often deliver a higher SCoP when it’s operated almost continuously or with only a small overnight temperature set-back. I’ve seen this in data from other people, but have not had a full winter with mine yet, so yet to determine the optimum operating mode.


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As @Rachel says, we’ve seen some good results from people running their sensibly-sized pumps smoothly.

Mine is probably the most oversized one we’ve got on the forum and that leads me to:

So I have the luxury of mine always being able to deliver whatever heat I ask of it, but it only runs smoothly when it’s about -3 °C which is pretty rare in the UK. I simply replaced my oil boiler with an ASHP with the same output.

Here is 2022-01-15 and it wasn’t very impressive at all - that’s a lot of kWh:

For comparison here it is when it’s warmer outside and it modulated down, but the house was getting too warm with 6kW input. Similar situation even with 4.67kW delivered.

So if I were looking at a “too big” heat pump I’d want one that could modulate down much further (in the case of my house, to about 2kW delivered) so I could run it more smoothly.

Having said all that, I feel @Jimbo1 is saying something that I feel too. Having the heat pump on all the time may be unnecessary and leads to you spending money running it overnight when you could potentially let the house just cool down gently if you have enough thermal mass. The occupants are in bed, under duvets, and the house doesn’t need to be kept warm for it’s own benefit. In one of my friend’s passivhaus new-builds they purposefully added a core of a large mass of concrete to stop the temperature oscillating too wildly. I haven’t looked into the research enough to have an informed opinion though.


@MyForest I’m just starting out monitoring our Ecodan ASHP and have found this thread extremely useful - thank you!

I’m struggling with the energy consumed / produced figures and have read about using the data from the device vs the reports in MEL Cloud from various posts at various times.

What have you settled on? And how is this set up in Emoncms?

And am I right that you store all the data in files before posting it to Emoncms?



Ooh, hi Chris, I responded a bit on the other thread, but I’ll expand a bit more here.

My setup is all in Docker using Python scripts. I’ve not published the MELCloud interaction because I don’t want to upset Mitsubishi and have them block our access (as we’ve seen BMW do this week).

So my stuff goes like this:

MELCloud → JSON files → EmonCMS

Particularly this allows me to go back in history and replay old data when I realise there is something interesting in there that I hadn’t realised.

Similarly for my weather station:

pywws → files → EmonCMS

I have a “post to EmonCMS” script for each thing I do. It allows me to choose other ways to process my data in the future and also means my EmonCMS instance is re-buildable if I ever stupidly destroy it.

It also means I have resilience to problems. For example, I actually broke my script when re-formatting it to answer your other thread and that simply meant the data wasn’t uploaded to EmonCMS. I just fixed the script and it raced through all the missing data and uploaded it.

Here is what my feeds look like:

You can see how it’s all built by looking at the device I created (with @Timbones’s improvements):

You can even import that into your EmonCMS if you want to.

All those sad looking “inactive” ones are the things that broke in November that I haven’t been able to repair.

If you want lots of data that you control you’re going to need to hook up to the physical device. If you’re willing to just have what you are given then you can use MELCloud. I’m not touching the physical device because of the MMSP rebate and warranty I have which I don’t want to jeopardize.

Let us know how you get along.