Rank 2

Air Source Heat Pump: Electricity bills have tripled since having solar installed - any advice?

Forum|Forum|2 years ago
March 22, 2024
80 replies
3135 views

Help, we have had air source heat pump fitted with solar panels and back up battery. This is our first winter with this new system and our electricity bills have tripled, we are afraid to turn the heating on. Our bill for the last 3 months is £1150 when it hasn’t been exceptionally cold. How can this be?

Pinned Reply By juliamc

Updated on 12/05/25 by Ben_OVO

Without any more to go on this is what I suggest you do:

Turn the hot water down to 50 degrees C. Make sure you’re only heating it on a schedule, not on ‘reheat’. I heat our hot water at night during my cheap rate hours and again in the afternoon when the outside temperature is warmest. The space heating (ie radiators) won’t get any heat when the hot water cycle is running so do it at a time when you won’t notice it missing. Mine takes about 45 mins. If you have a ‘legionella’ or disinfectant’ cycle enabled you can disable it. It’s reckoned to be unnecessary in a domestic setting where you’re using and refreshing the contents of your hot water cylinder every day.

Next, rather than have the radiators on for a few hours at a time (generally an inefficient way of running a heat pump) run it at a lower temperature for longer hours. Without knowing what heat pump you have it’s a bit difficult to explain the changes you need to make. Mine is a Daikin Altherma 3 and I have a Madoka room sensor. I’ve set the controls to Weather Dependant, which means the radiators are cooler when it’s warm outside and warmer when it’s cool outside. You may have been set up with a fixed flow temperature which is not an efficient way to run it. Open all your radiator valves (the adjustable ones) fully so they all receive heat at the same time. The heat pump needs a good flow of water pumped around and there’s no benefit closing some off here and there. If that sounds extravagant then don’t worry, you’ll save by the hp running at tickover rather than having to burst into life every time a change of heat is called for, as it is currently. You can turn the overall temperature down to use less electricity rather than turn it off. This does all sound involved I know. With more info on your system I might be able to explain better how you can do all this.

Tips for Air Source Heat Pump efficiency can be found on the Energy Saving Trust website, and further discussion on the subject continues in these other Forum topics:

Show previous replies

S

serennuAuthor

Rank 2

Hi @serennu

Sorry, this is going to be a long message ! I’ve read up on a few things and hope to explain them here:

Heat pump size

Somewhere in your paperwork you should have a note of the heat loss for your property. That would determine the size of the heat pump installed. Mine is on a page headed MCS Compliance Certificate. On it is Design Information showing “design external temp” and “total building heat loss at design condition (kW)”.

For my 3 bed bungalow mine is 5.51 kW at -1.94 degC. I have a 6 kW heat pump, ie it produces 6 kW of heat at about -2 degC.

You said you have a Vaillant aroTHERM plus VWL 105 / 6 air source heat pump which is 10kW, suggesting your heat loss is considerable more than mine. It would be good to know your heat loss figure to check that your heat pump is the appropriate size.

Flow temperature and the influence of the weather

The same document shows the design flow temperature ie the radiator temperature at -2 degC. Mine is 50 degC. Generally the lower the flow temperature the more efficiently the heat pump runs, however the flow temperature has to be high enough to heat your rooms to balance out the heat loss. Running on a “Weather Curve” means the flow temperature is automatically lowered as the outside gets warmer, thus on milder days the efficiency is better. For example today it’s 12 degC here and my radiator temperature is 35 degC. The radiators are double panel mostly and at 35 degC they barely feel warm, but provide enough heat to get the rooms to 22 degC.

Getting the Weather Curve set correctly will give you the right flow temperature at any outside temperature.

Yield and COP

On your photo of the controller screen (the one with a block of 4 long buttons and the round reset button) the display shows 10,8 kWh/d. That number is what Vaillant call Yield, meaning (as I understand it) that when the photo was taken the heat pump had created 10.8 kWh of ‘energy’ since the start of the day.

You’ll hear a lot about COP, the Coefficient Of Performance. Just to be contrary Vaillant call this the “Working Figure” and you can look that up on the controller too.

I’ve copied this from a brilliant website called energy-stats uk https://energy-stats.uk/how-to-measure-vaillant-arotherm-cop/ where he explains how to get this on the screen:

So from the initial menu (press top left button) we want to dive into “Yield Indicator” where we can find the following: (press the + to scroll through the items)

Working Figure: Month (Heating)
Working Figure: Month (Domestic Hot Water)
Working Figure: Total (Heating)
Working Figure: Total (Domestic Hot Water)

All of these totals are cumulative for the time period stated.

Month being the current month and Total being since installation.

Ideally the Working Figure for heating should be at least 3.5, it’s likely to be less than that as your bills are so high, but there are several things we can address to improve it.

Heating v hot water

When the heat pump heats your hot water it heats to a much higher temperature than for the radiators, that means the COP or Working Figure for hot water will be low, possibly around 2.5. If you don’t use much hot water then heating it once a day is sufficient, also heating it to 50 degC is plenty. I’m afraid I don’t know how to set that but your installers should be able to change it from the 60 degC you mentioned previously. It can be heated once a week to 60 degC for a legionella disinfection but really that’s not necessary. There are many heat pump owners who don’t do this, it’s an unnecessary waste of electricity, and there’s less chance of scalding yourself if it’s kept to 50 degC.

Controlling the radiators - don’t !

Your heat pump doesn’t need any other controls to adjust the radiator temperatures in different rooms.

It will work most efficiently when the valves on all the radiators are open and the water flowing around them is allowed to move unrestricted. You mentioned previously that you have Honeywell controls to have different rooms heated for only a few hours in the day. These will be pushing up the bills. Also the heat pump uses most electricity when starting up, so constantly stopping and starting will be costly.

Although it sounds counterintuitive there is no benefit in turning off the heating in some rooms (unless you close down a whole wing of a mansion maybe!!). Each cold room will be drawing heat from the warmer rooms.

With the low flow temperatures your rooms will take longer to warm up, so it’s important not to let them cool down too much. In my house I have the day temperature set to 22 degC until 10pm, when I set it back to 21 degC. It heats the hot water at 2.30 am, then starts the heating again to 22 degC at 3.30am.

If you are hardier than me you could have the room temperature at 19 degC during the day and set back to 18 degC at night or even less ! To stop the Honeywell having an effect I understand you should set them to a high temperature so they never close and therefore they won’t restrict the flow. Just use the Weather Curve to set the appropriate flow temperature.

For more info on setting the weather curve see https://energy-stats.uk/vaillant-arotherm-weather-curve-information/

Thank you so much for all that, your kindness has reduced me to tears, I so appreciate you taking time to help me. It is a lot of information and I will work my way through it all. The lady from SevernWye is coming tomorrow and I will show her your suggestions. She admitted that she is not an expert on solar/HSAP but I am hoping that she can take away these suggestions and information to pass to someone in the organisation who knows more about it. I will let you know the outcome of the meeting tomorrow.It will take me a couple of days to work through all this info but I will get back to you asap.

Summary!

So to summarise it would be useful to know your heat loss and design flow temperature, and determine if the heat pump is the correct size. Also checking the Working Figure (COP) to see how it’s performing. Only heat your hot water enough for your requirements, and heat every room in the house but at a lower temperature if that sounds extravagant.

Obviously I haven’t explained how to do this as I don’t know the controls, but when you get the installers back in, and when SevernWye visit, I hope you can explain what’s needed.

Please report back how you get on and we can improve this all for you.

Best of luck! Julia

Thank you so much for all that, your kindness has reduced me to tears, I so appreciate you taking time to help me. It is a lot of information and I will work my way through it all. The lady from SevernWye is coming tomorrow and I will show her your suggestions. She admitted that she is not an expert on solar/HSAP but I am hoping that she can take away these suggestions and information to pass to someone in the organisation who knows more about it. I will let you know the outcome of the meeting tomorrow.It will take me a couple of days to work through all this info but I will get back to you asap.

juliamc

Rank 20

You are very welcome!! I hate to think your setup isn’t performing for you as it’s a very good machine.
I’ll let you get on with your homework now 😊.

S

serennuAuthor

Rank 2

Hi @serennu

Sorry, this is going to be a long message ! I’ve read up on a few things and hope to explain them here:

Heat pump size

Somewhere in your paperwork you should have a note of the heat loss for your property. That would determine the size of the heat pump installed. Mine is on a page headed MCS Compliance Certificate. On it is Design Information showing “design external temp” and “total building heat loss at design condition (kW)”.

For my 3 bed bungalow mine is 5.51 kW at -1.94 degC. I have a 6 kW heat pump, ie it produces 6 kW of heat at about -2 degC.

You said you have a Vaillant aroTHERM plus VWL 105 / 6 air source heat pump which is 10kW, suggesting your heat loss is considerable more than mine. It would be good to know your heat loss figure to check that your heat pump is the appropriate size.

Flow temperature and the influence of the weather

The same document shows the design flow temperature ie the radiator temperature at -2 degC. Mine is 50 degC. Generally the lower the flow temperature the more efficiently the heat pump runs, however the flow temperature has to be high enough to heat your rooms to balance out the heat loss. Running on a “Weather Curve” means the flow temperature is automatically lowered as the outside gets warmer, thus on milder days the efficiency is better. For example today it’s 12 degC here and my radiator temperature is 35 degC. The radiators are double panel mostly and at 35 degC they barely feel warm, but provide enough heat to get the rooms to 22 degC.

Getting the Weather Curve set correctly will give you the right flow temperature at any outside temperature.

Yield and COP

On your photo of the controller screen (the one with a block of 4 long buttons and the round reset button) the display shows 10,8 kWh/d. That number is what Vaillant call Yield, meaning (as I understand it) that when the photo was taken the heat pump had created 10.8 kWh of ‘energy’ since the start of the day.

You’ll hear a lot about COP, the Coefficient Of Performance. Just to be contrary Vaillant call this the “Working Figure” and you can look that up on the controller too.

I’ve copied this from a brilliant website called energy-stats uk https://energy-stats.uk/how-to-measure-vaillant-arotherm-cop/ where he explains how to get this on the screen:

So from the initial menu (press top left button) we want to dive into “Yield Indicator” where we can find the following: (press the + to scroll through the items)

Working Figure: Month (Heating)
Working Figure: Month (Domestic Hot Water)
Working Figure: Total (Heating)
Working Figure: Total (Domestic Hot Water)

All of these totals are cumulative for the time period stated.

Month being the current month and Total being since installation.

Ideally the Working Figure for heating should be at least 3.5, it’s likely to be less than that as your bills are so high, but there are several things we can address to improve it.

Heating v hot water

When the heat pump heats your hot water it heats to a much higher temperature than for the radiators, that means the COP or Working Figure for hot water will be low, possibly around 2.5. If you don’t use much hot water then heating it once a day is sufficient, also heating it to 50 degC is plenty. I’m afraid I don’t know how to set that but your installers should be able to change it from the 60 degC you mentioned previously. It can be heated once a week to 60 degC for a legionella disinfection but really that’s not necessary. There are many heat pump owners who don’t do this, it’s an unnecessary waste of electricity, and there’s less chance of scalding yourself if it’s kept to 50 degC.

Controlling the radiators - don’t !

Your heat pump doesn’t need any other controls to adjust the radiator temperatures in different rooms.

It will work most efficiently when the valves on all the radiators are open and the water flowing around them is allowed to move unrestricted. You mentioned previously that you have Honeywell controls to have different rooms heated for only a few hours in the day. These will be pushing up the bills. Also the heat pump uses most electricity when starting up, so constantly stopping and starting will be costly.

Although it sounds counterintuitive there is no benefit in turning off the heating in some rooms (unless you close down a whole wing of a mansion maybe!!). Each cold room will be drawing heat from the warmer rooms.

With the low flow temperatures your rooms will take longer to warm up, so it’s important not to let them cool down too much. In my house I have the day temperature set to 22 degC until 10pm, when I set it back to 21 degC. It heats the hot water at 2.30 am, then starts the heating again to 22 degC at 3.30am.

If you are hardier than me you could have the room temperature at 19 degC during the day and set back to 18 degC at night or even less ! To stop the Honeywell having an effect I understand you should set them to a high temperature so they never close and therefore they won’t restrict the flow. Just use the Weather Curve to set the appropriate flow temperature.

For more info on setting the weather curve see https://energy-stats.uk/vaillant-arotherm-weather-curve-information/

Thank you so much for all that, your kindness has reduced me to tears, I so appreciate you taking time to help me. It is a lot of information and I will work my way through it all. The lady from SevernWye is coming tomorrow and I will show her your suggestions. She admitted that she is not an expert on solar/HSAP but I am hoping that she can take away these suggestions and information to pass to someone in the organisation who knows more about it. I will let you know the outcome of the meeting tomorrow.It will take me a couple of days to work through all this info but I will get back to you asap.

Summary!

So to summarise it would be useful to know your heat loss and design flow temperature, and determine if the heat pump is the correct size. Also checking the Working Figure (COP) to see how it’s performing. Only heat your hot water enough for your requirements, and heat every room in the house but at a lower temperature if that sounds extravagant.

Obviously I haven’t explained how to do this as I don’t know the controls, but when you get the installers back in, and when SevernWye visit, I hope you can explain what’s needed.

Please report back how you get on and we can improve this all for you.

Best of luck! Julia

Hi Julia

An update on what has happened so far. Yesterday I turned the water down to 55, I took a meter reading today and we used 13kw less yesterday so today I have turned the water down to 50. I will take another reading tomorrow. The lady from SevernWye so totally, bloody useless. I told her I had turned the water down and she said ‘no,no you will get legionaires, you must keep it at 60. I nearly showed her the door then, she was clueless, so much for an energy advisor. She said it is wrong to turn on the heating and keep it running and couldn’t understand when I told her this is a completely different sort of heating system. She wasted an hour of my time when I could have been doing my homework. So tomorrow, am going to turn the heating on at between 18-20 and turn it down to about 15-16 overnight, does that sound ok to you? We have 9 rads so do you think I should have them all running? I am going through all the paperwork to find the info at the moment (a lot of paperwork). I also emailed Warm Wales as they are running this heating scheme and they recommended SevernWye to tell them how useless SevernWye are and they are going to contact the installers about this, so hopefully there will be some movement there. So that is what I have done so far, now going to click the link To Vaillant website you gave me. More updates to follow, thank you so much Julia.

Thank you so much for all that, your kindness has reduced me to tears, I so appreciate you taking time to help me. It is a lot of information and I will work my way through it all. The lady from SevernWye is coming tomorrow and I will show her your suggestions. She admitted that she is not an expert on solar/HSAP but I am hoping that she can take away these suggestions and information to pass to someone in the organisation who knows more about it. I will let you know the outcome of the meeting tomorrow.It will take me a couple of days to work through all this info but I will get back to you asap.

juliamc

Rank 20

That is disappointing about the SevernWye lady, but not really surprising as heat pumps do seem to be a dark art compared to ‘normal’ high temperature heating that we’ve all been used to. They need to get up to speed if they’re going to give energy advice nowadays !!
You certainly don’t need to have the hot water at 60 degC all the time. Stick to 50 - even the cautious only heat to 60 once a week.

juliamc

Rank 20

@serennu Regarding the heating, it would be good to know if your weather curve is set correctly. If we had a Vaillant expert to hand they could see straight away how it’s set !!

I think for now there’s nothing to lose by having it at 18-20 as you propose, and setting back overnight. As it’s quite mild at the moment it shouldn’t send the electricity use through the roof, and it would be useful to see that all 9 radiators are working ok.

How are you setting that temperature ? is it with the radiator valves or on the Vaillant controller ?

juliamc

Rank 20

@serennu A couple more questions !! You mentioned earlier that you have been taking readings - are these from your electricity meter ? Are you able to copy them here ? They could help to explain what’s going on !!

S

serennuAuthor

Rank 2

@serennu A couple more questions !! You mentioned earlier that you have been taking readings - are these from your electricity meter ? Are you able to copy them here ? They could help to explain what’s going on !!

Yes sure Julia, I started on 21/03/24 when we got that huge bill

21/03 60899

24/03 60963

25/03 60986

26/03 61015

27/03 61038

28/03 61063

29/03 61089

30/03 61110

31/03 61125

01/04 61155

02/04 61176

03/04 61198

04/04 61218

05/04 61229

06/04 61246

07/04 61263

08/04 61281

09/04 61310

10/04 61323

11/04 61344

I have turned the heating on today, all rads are set at 18 in the day and 15 through the night. The reading tomorrow will be interesting.

juliamc

Rank 20

Thanks. You probably have had enough with tech at the moment but if you had a smart meter it could give half hourly readings automatically. That really helps to see what’s going on…

S

serennuAuthor

Rank 2

Thanks. You probably have had enough with tech at the moment but if you had a smart meter it could give half hourly readings automatically. That really helps to see what’s going on…

We can’t have a smart meter here, too rural and very poor signal hence no mobile phones. I think you are right about leaving the heating on, I have noticed there hasn’t been the noise of the rad heating up in the lounge and house is nice and warm. As I said tomorrows reading will be very interesting

juliamc

Rank 20

Ah. Shame about the meter but good that the heating’s on. As it’s mild now it shouldn’t be too expensive to try this. I think heating the hot water to 60 might have been pushing the bills up. It’s best not to change too many things in one go. I’m going to do a graph with your electricity v mine… back later.

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Air Source Heat Pump: Electricity bills have tripled since having solar installed - any advice?

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