Heat pump experiment review after two years

[borpin (Brian Orpin)]

I just checked and there is a MELCloud integration in HomeAssistant (MELCloud - Home Assistant). There are also a multitude of ways to control the demand, get weather info in etc. I don’t have a heatpump but use HA to control the temp of each room depending on day of week/ time of day & presence. I’ve not added any real weather compensation in other than gradually raising the target temp in the morning. I know that, by x o’clock, the room temp really needs to be Y for it to be comfortable when we get up.

Any folk who do not want to play with Python directly, may find this route useful.

I’m a fan of Homeassistant and you really can do anything you need either directly or in the Node-RED addon.

I am also hoping that companies wake up to the benefits of making their systems accessible to things like HomeAssistant. For me it is a key element of the purchase decision going forward.

[MyForest (David Bowen)]

@Timbones

(diffT * HeatLoss / HeatOutput + 0.117) / 0.0223 + T

Well that’s quite beautiful @Timbones

I love the scientific approach to diffT * HeatLoss / HeatOutput and your T makes sense. I think the bit I find hard is that I don’t have a T because there can be 5 °C difference between the rooms here. Notably HeatOutput is variable here because each TRV could be on / off depending on the local temp so I very rarely get full output.

I almost fell off my chair when I compared your 410W/°K to what I was seeing here. It’s roughly 20 °C in my house and the temp outside is 7 °C so the delta is 13 °K.

Here’s a typical run from today.

That 0.696kW at the top is the average power draw. So my total power draw is less than twice what yours is per degree!

Luckily I noticed that you are of course measuring heat loss after it’s come out of the heat pump

So in my case that’s 2.63kW on average => 200W/°K. As I mentioned, I’m near York so we’re seeing much the same weather. I’m right out in the countryside so we get blasts of wind hitting us. We’re in a 4-bed semi which is 200 years old and has lamentable insulation. I’m surprised you are seeing so much heat loss. According to BRE we might expect your heat loss to half of mine, not double - although having said that they expect my heat loss to be 640W/°K !

BTW I love this quote I tripped over:

In a typical British home up to one third of the heat produced by central heating systems is lost through the roof, walls, floor and windows.

Where does the other two-thirds go? Down the sink?

[mjr]

@MyForest

BTW I love this quote I tripped over:

In a typical British home up to one third of the heat produced by central heating systems is lost through the roof, walls, floor and windows.

Where does the other two-thirds go? Down the sink?

Maybe Brownian motion is great at cooling.

My science teacher used to tell us that we can only move energy around, and only God can create or destroy it. Maybe the author of the above lives with God?

[Timbones (Tim)]

@MyForest

I almost fell off my chair when I compared your 410W/°K to what I was seeing here.

Well, maybe my numbers are off, but I worked it out from the other direction: I entered the dimensions and materials for each room into a heat loss calculator. I found this one to be sufficiently sophisticated for my needs, using the default winter design temperature of -1°C and typical indoor temperatures. Adding them all up comes to a total of 9.5 kW. Divide by the delta (which could be different for each room) to get W/K. I actually got 473 out of my sums, but I found with trial and error that 410 just seemed to fit better with the curve. I think this is because there are other heat sources contributing to the home.

Using a calculator for radiators, I get a total output of 16.6 kW @ dT 50, or 9 kW @ dT 30 [see this conversion table]. That means for a room at 20 degrees, that would be for a flow temperature of 50°C, which would roughly match my heat loss when it’s freezing outside. My formula is working out the flow temperature needed for the radiators to replace the heat that is lost at a certain outside temperature.

I’m not sure if you can look at the heat pump output to judge the heat loss, but maybe you can?

Say it’s 10 degrees outside and 20 degrees inside, and my ASHP likes to hum along producing 4 kW of heat. Given that 10° delta, my calculated heat loss is also about 4 kW, and the house is staying a pretty constant temperature. [Massively simplified numbers, but it close enough for the illustration.]

Anyway, I think I’ve convinced myself that a) that heat loss figure is probably close, and b) I need to invest in a lot better insulation and draft proofing!

[MyForest (David Bowen)]

Your approach makes sense @Timbones

I’m doing roughly the same thing, but my heat pump is only running maybe a third of the time.

I’d credit all the non-heat-pump electricity as heating your home too …where else does it go? The inmates are 100W each.

[Timbones (Tim)]

@Timbones

I come up with the following formula for the weather compensation curve of my property:

(diffT * HeatLoss / HeatOutput + 0.117) / 0.0223 + T

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°.

@MyForest

I think the bit I find hard is that I don’t have a T because there can be 5 °C difference between the rooms here. Notably HeatOutput is variable here because each TRV could be on / off depending on the local temp so I very rarely get full output.

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}

[MyForest (David Bowen)]

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!

[Timbones (Tim)]

@MyForest

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”.

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.

[MyForest (David Bowen)]

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.

[Timbones (Tim)]

@MyForest

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.

[MyForest (David Bowen)]

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.

[Timbones (Tim)]

@MyForest

So my strategy of walking up the temperature means it’s running at very low temps for quite a while to start with, such as that 28 C for 10 mins at the beginning. That’s similar to what @johncantor and @TrystanLea see with their FTC2 slowly lifting the flow temp rather than my FTC5 and John’s FTC6 (and presumably yours) which are ramping up the flow temp really quickly.

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.

[mjr]

@Timbones

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.

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

[johncantor]

@Timbones

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.

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.

[Jimbo1 (Iain)]

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

[MyForest (David Bowen)]

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.

MyForest

[Timbones (Tim)]

@Jimbo1

One concern I have is the slow ramp up because of the low temperature output, so the HP has to be operating for longer …

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.

@Jimbo1

… leading to greater losses through the insulation

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.

[Jimbo1 (Iain)]

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?

[borpin (Brian Orpin)]

@Timbones

Heat loss is driven purely by the difference between the indoor and outdoor temperatures

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

[mjr]

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.