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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

There should also be a web based monitor .. not sure if this was set up for you

https://www.suns

There should also be a web based monitor .. not sure if this was set up for you

https://www.sunsynk.net/login

I can find something under the inverter that looks like that wifi thingy, it has a flashing red light on it but I can’t see anything that looks like a ct clamp. The only wifi that was set up was to connect the rads to the control hub which controls the temperature.

Yes the flashing red light sounds like a WiFi connection (although they’re often green when connected).

You should be able to set up an account either on the app or web to look at the inverter set up. This shouldn’t need much adjustment but it would show how things have been set up.

OK, so I have set up an account but the pages are blank, no information recorded

https://www.sunsynk.net/plants

ynk.net/login

I can find something under the inverter that looks like that wifi thingy, it has a flashing red light on it but I can’t see anything that looks like a ct clamp. The only wifi that was set up was to connect the rads to the control hub which controls the temperature.

Yes the flashing red light sounds like a WiFi connection (although they’re often green when connected).

You should be able to set up an account either on the app or web to look at the inverter set up. This shouldn’t need much adjustment but it would show how things have been set up.

S

serennuAuthor

Rank 2

There should also be a web based monitor .. not sure if this was set up for you

https://www.suns

There should also be a web based monitor .. not sure if this was set up for you

https://www.sunsynk.net/login

I can find something under the inverter that looks like that wifi thingy, it has a flashing red light on it but I can’t see anything that looks like a ct clamp. The only wifi that was set up was to connect the rads to the control hub which controls the temperature.

Yes the flashing red light sounds like a WiFi connection (although they’re often green when connected).

You should be able to set up an account either on the app or web to look at the inverter set up. This shouldn’t need much adjustment but it would show how things have been set up.

OK, so I have set up an account but the pages are blank, no information recorded

https://www.sunsynk.net/plants

ynk.net/login

I can find something under the inverter that looks like that wifi thingy, it has a flashing red light on it but I can’t see anything that looks like a ct clamp. The only wifi that was set up was to connect the rads to the control hub which controls the temperature.

Yes the flashing red light sounds like a WiFi connection (although they’re often green when connected).

You should be able to set up an account either on the app or web to look at the inverter set up. This shouldn’t need much adjustment but it would show how things have been set up.

OK, so I have set up an account but the pages are blank, no information recorded

https://www.sunsynk.net/plants

BPLightlog

Super User

I don’t have this system exactly unfortunately but there are a series of videos which might help

https://m.youtube.com/@sunsynk

Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP and Home Assistant automation}

S

serennuAuthor

Rank 2

I don’t have this system exactly unfortunately but there are a series of videos which might help

https://m.youtube.com/@sunsynk

Thank you but doesn’t really help, I just don’t understand it.

BPLightlog

Super User

Unfortunately it’s not a simple “press this button” system .. it can be difficult to explain everything, especially remotely.
The best route would be to have someone walk you through the whole system. The installer should have done that and because these things work in quite different ways to older technology, old ‘solutions’ (like turning off the radiators) doesn’t help.
We’ve asked for any other advice from others in case they can explain better.
The videos on the link above walk through the solar system but I understand they might not look straight forward

Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP and Home Assistant automation}

BPLightlog

Super User

I’ve read a little more about the Sun Synk inverter and there should be a CT coil

The image shows where the CT should be, which is where the mains electricity enters the premises.
Here’s the manual although I realise it might not be simple to follow

Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP and Home Assistant automation}

juliamc

Rank 20

@serennu

Not to give you information overload I’ll come back tomorrow and suggest what you could look at regarding the heat pump.

I think you have a good system there, it just needs to be set up better, which will give you lower bills and a warmer house.

S

serennuAuthor

Rank 2

@serennu

Not to give you information overload I’ll come back tomorrow and suggest what you could look at regarding the heat pump.

I think you have a good system there, it just needs to be set up better, which will give you lower bills and a warmer house.

Thank you juliamc, I will be grateful for any suggestions you might have.

juliamc

Rank 20

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/

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

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

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

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