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ASHP and hot water tank size confusion

  • September 14, 2025
  • 17 replies
  • 166 views

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Hi. I (we) come here because getting appropriate advice from manufacturer and/or installers is both conflicting and confusing. We are greener than grass with our first build. Trying to gain best knowledge as possible, but I'm this game,  20 people seem to have 20 opinions. 

Google led me to a very informative post from from 2 years ago  by ​@hydrosam. About a trial, pump size,  COP and LWT. 

I will do my best to summarise our needs etc and your responses would be most welcome. It's been a long build we started in 2011.  We have 4kw solar panels plus 11kw wind. The house is almost a passive build. 

Kingspan structurally insulated panels large new build detached 5 bedroom home 2 bathrooms and 2 ensuites plus downstairs shower.  Large entrance hall that has a void to the roof. Ground, first and second floors. (2nd floor is in the roof area).  3 beds on first floor, 2 ensuits and family bathroom. 2 beds plus bathroom on 2nd floor. 

There's only two of us!  Other bedrooms will be used when family or house guests stay or we decide to to B&B.  For the majority of time,  until we decide we're too old and move, there will be just us 2.  (Yes, bonkers, don't even go there). 

One installer has assessed an LG 11kw ASHP and UFH for all floors with 300L hot tank. Another installer has suggested that this tank would not meet hot water demand when the house is full.  Suggested a larger pump and 2 x 300L tanks. 

Our concern is that if 75/90% only us 2 are there would the bigger pump be totally inefficient due to constantly turning off and on.  

As heat rises do we also need UFH on 2nd/3rd floors.  We definitely don't want radiators. We can make provision for upstairs UFH and trial a winter with only ground floor installed. The bedrooms won't all be finished anyway.  I'm fed up with static caravan so prepared to rough it upstairs as long as downstairs is done. 

I know this all sounds rather muddled, but really hoping for help from users as opposed to installers 

Thank you for reading 

 

Best answer by BPLightlog

Updated on 26/09/25 by Ben_OVO

Hi ​@newbuild.janet , many who’ve been through various situations (including a new build) would say that each property is individual. Therefore it is not surprising , although unhelpful, if you get several differing opinions on heating needs.

You do really need to fully understand the heat loss calculations as a poor match to need will end up giving you a quite inefficient system. From your notes, it sounds like there are parts unfinished and therefore this will be a challenge to anyone trying to design.

There are several installers who will also have been trained to design to a maximum need of the property and therefore this will also add to the differing advice.

Having been involved in a farmhouse which sounds similar to your construction, I can understand why tank size and UFH have been suggested but I believe that the only way you will resolve need for your own peace of mind is to review various max and min options and compare with your general usage needs.

Sorry if that’s rather sketchy.

 

17 replies

juliamc
Carbon Catcher***
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  • Carbon Catcher***
  • September 14, 2025

From what I’ve read an 11kW hp would be too much in a house built to passive standards. Have you had heat loss calculations done by these installers ?

 


juliamc
Carbon Catcher***
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  • Carbon Catcher***
  • September 14, 2025

Further thoughts re your upstairs heating: have you considered MVHR ?


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  • Author
  • Carbon Cutter****
  • September 14, 2025

From what I’ve read an 11kW hp would be too much in a house built to passive standards. Have you had heat loss calculations done by these installers ?

 

They said they had


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  • Author
  • Carbon Cutter****
  • September 14, 2025

Further thoughts re your upstairs heating: have you considered MVHR ?

In passing. I'll look more into it?  Thank you.


BPLightlog
Plan Zero Hero
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  • Plan Zero Hero
  • Answer
  • September 14, 2025

Updated on 26/09/25 by Ben_OVO

Hi ​@newbuild.janet , many who’ve been through various situations (including a new build) would say that each property is individual. Therefore it is not surprising , although unhelpful, if you get several differing opinions on heating needs.

You do really need to fully understand the heat loss calculations as a poor match to need will end up giving you a quite inefficient system. From your notes, it sounds like there are parts unfinished and therefore this will be a challenge to anyone trying to design.

There are several installers who will also have been trained to design to a maximum need of the property and therefore this will also add to the differing advice.

Having been involved in a farmhouse which sounds similar to your construction, I can understand why tank size and UFH have been suggested but I believe that the only way you will resolve need for your own peace of mind is to review various max and min options and compare with your general usage needs.

Sorry if that’s rather sketchy.

 


Forum|alt.badge.img+1
  • Author
  • Carbon Cutter****
  • September 14, 2025

Hi ​@newbuild.janet , many who’ve been through various situations (including a new build) would say that each property is individual. Therefore it is not surprising , although unhelpful, if you get several differing opinions on heating needs.

You do really need to fully understand the heat loss calculations as a poor match to need will end up giving you a quite inefficient system. From your notes, it sounds like there are parts unfinished and therefore this will be a challenge to anyone trying to design.

There are several installers who will also have been trained to design to a maximum need of the property and therefore this will also add to the differing advice.

Having been involved in a farmhouse which sounds similar to your construction, I can understand why tank size and UFH have been suggested but I believe that the only way you will resolve need for your own peace of mind is to review various max and min options and compare with your general usage needs.

Sorry if that’s rather sketchy.

No not sketchy at all. Helpful. I'm so overwhelmed by it all my thoughts are very jumbled (not helped by ADHD which is like living in a nightmare when too much to deal with)


Chris_OVO
Community Manager
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  • Community Manager
  • September 15, 2025

Hey ​@newbuild.janet

 

It’s easy to feel overwhelmed in these situations, especially when decisions can have a significant impact on your home and you can’t find a clear answer to your questions. If your thoughts feel jumbled, I recommend giving yourself a break and focusing on something else for a day or two. This can help reset your mind and ease decision-making anxiety.

 

I’m glad to see that ​@BPLightlog and ​@juliamc, two of our most knowledgeable forum members, have already participated in this thread. They are always willing to share their personal opinions and offer helpful advice.

 

Feel free to use the forum as a sounding board for your thoughts and ideas. Together, we can support each other on this journey!

 

You can read about ​@BPLightlog’s experience with having an ASHP installed below:

 

I’ve added a couple of badges to your profile but just to confirm when you said “We have 4kw solar panels plus 11kw wind.” do you have a turbine at home? I know you’ve added it to your profile but i’ve applied the badge anyway 😃


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  • Author
  • Carbon Cutter****
  • September 15, 2025

@Chris_OVO Thank you for the kind words and advice. Yup we do have both wind and solar energy.  In fact between us we have 2 x Gaia 11kw wind and 2 x 4kw Shot solar panels. Installed in 2012. I did a lot of research at the time choosing best performing turbines. The same with solar panel brands. Compared to ashp and ufh they were a doddle to decide. The panels are on a barn roof.  Turbines in our field. One supplies our temp accommodation the other hours to the new build.  We have export meters for all of it.   If we had a stream we would have preferred a hydro pump for performance. 

I tried to tag ​@hydrosam in the first comment but it failed. 


Chris_OVO
Community Manager
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  • Community Manager
  • September 15, 2025

Hey ​@newbuild.janet,

 

You’re welcome! That sounds amazing, and you're always welcome to upload photos if you'd like! We’re always eager to see fascinating setups like yours.

 

I also noticed a rogue comment that you wanted deleted and the tag, so I took care of that for you.


BPLightlog
Plan Zero Hero
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  • Plan Zero Hero
  • September 15, 2025

Just on the subject of DHW ​@newbuild.janet , we are a 4 adult household; 3 use the main bathroom (shower, no bath) and 1 uses a second bathroom with an electric shower (preference).

Our tank is 180 l and lasts the day for basic needs (washing, kitchen etc). It has a heat cycle early in the day and a boost ( heat if needed) mid afternoon.

Perhaps not as demanding as your set up but I thought it useful for comparison.


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  • Author
  • Carbon Cutter****
  • September 15, 2025

Hey ​@newbuild.janet,

 

You’re welcome! That sounds amazing, and you're always welcome to upload photos if you'd like! We’re always eager to see fascinating setups like yours.

 

I also noticed a rogue comment that you wanted deleted and the tag, so I took care of that for you.

Brilliant.  Thank you for deleting the comment. I'll look out some pics 


Forum|alt.badge.img+1
  • Author
  • Carbon Cutter****
  • September 15, 2025

Hey ​@newbuild.janet,

 

You’re welcome! That sounds amazing, and you're always welcome to upload photos if you'd like! We’re always eager to see fascinating setups like yours.

 

I also noticed a rogue comment that you wanted deleted and the tag, so I took care of that for you.

 

 


Forum|alt.badge.img+1
  • Author
  • Carbon Cutter****
  • September 15, 2025

Hey ​@newbuild.janet,

 

You’re welcome! That sounds amazing, and you're always welcome to upload photos if you'd like! We’re always eager to see fascinating setups like yours.

 

I also noticed a rogue comment that you wanted deleted and the tag, so I took care of that for you.

Brilliant.  Thank you for deleting the comment. I'll look out some pics 

 


Forum|alt.badge.img+1
  • Author
  • Carbon Cutter****
  • September 15, 2025

Hey ​@newbuild.janet,

 

You’re welcome! That sounds amazing, and you're always welcome to upload photos if you'd like! We’re always eager to see fascinating setups like yours.

 

I also noticed a rogue comment that you wanted deleted and the tag, so I took care of that for you.

Brilliant.  Thank you for deleting the comment. I'll look out some pics 

 


Chris_OVO
Community Manager
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  • Community Manager
  • September 16, 2025

Morning ​@newbuild.janet,

 

Wow, thank you for sharing those stunning photos! The view is absolutely amazing!


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  • Author
  • Carbon Cutter****
  • September 16, 2025

Morning ​@newbuild.janet,

 

Wow, thank you for sharing those stunning photos! The view is absolutely amazing!

Tthank you.  The engineer took it from the top of the turbine during the annual service. They're 18m high. Huge, worrying investment at the time.  Thankfully the FiT repaid it after 6 years (including what we saved on our electricity bill). We hope that our running costs in the new build home will be seriously reduced by having the renewables and battery storage.  Goodness knows how long it will take for battery storage cost to return. 


Chris_OVO
Community Manager
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  • Community Manager
  • September 16, 2025

@newbuild.janet Interestingly, my profile picture was taken inside a turbine while I was helping to service it a couple of years ago! Kudos to the engineers who work on turbines; it’s a tough job, but it comes with the reward of stunning views, just like the one in your photo.