location of the ASHP outdoor unit

[chdb]

Hi all,
We're planning to retrofit our end-of-terrace Victorian house. We would like to ask for advice about the location of the ASHP outdoor unit.
I want to keep the garden for plants if possible. So, I want to place it on the flat roof (1st floor level) of our new extension at the rear, and position it to face the neighbours (Alternatively, it could be positioned 90 degrees to face the rear garden).  However the ASHP engineer has argued very strongly against a roof location and wants us to put it in the garden, facing across it, and our architect very much wants us to follow his advice. They have explained their reasons, but I don't find them entirely convincing. 
The reasons given and my sceptical reactions:
1. Neighbours:  they say it will annoy/infuriate the neighbours. The next door property is divided into a ground floor flat, and a separate maisonette upstairs.  The boundary is about 3.5 m away. So their wall would be 7m away (see the image attached).

• I think this is far enough away that they won't feel the cold air and will barely hear the noise (We went to visit a similar unit in a local property and it was surprisingly quiet.  Just a low hum.)
• The neighbours on the ground floor are friendly and I think probably won't mind it at all, because it will blow above their property. 
• The neighbours upstairs are on a shared short term rental.  I assume that it's usually home-owners, not so much short term renters, who complain about neighbours' heat-pumps.
• In any case I will consult them in advance and take their reactions into account.
   

2. Noise &  Vibration : We will hear some low-pitch noise (like humming) when the unit is in operation, if nearby windows are open. Also, they say that even if they are closed, the vibration could be transmitted into the building. The nearby windows are shower/bathroom - there are 2 bedrooms that have windows which could be affected (see the image attached). 

• I think this shouldn't be an issue if the unit is carefully positioned - EG mounted on a vibration-absorbing block and we could even mount the whole thing on a concrete block which would absorb the low frequencies, but under that it still would be supported by the timber roof structure.  Perhaps it could instead be fixed to the wall which is solid 9 inch brickwork with external insulation.
   

3. Servicing: Our engineer said Vaillant is said to be the best choice and they also have a good service network. However, he said their maintenance engineers have strict safety rules, and they will be unwilling to attend a roof installation.  Work by another company might invalidate our guarantee.  Furthermore current ASHP technology uses a Propane (R290) refrigerant which has some additional safety requirements - and he said this will also present problems for a roof location.
However I have checked this issue directly with Vaillant who said its ok if we have: 

• the required space around the unit - generally 1m to one side, 500mm to the other (yes, there's enough space).
• access to the unit not requiring use of moveable ladders (yes, we will have ladder-free access through the shower room windows).
• safety rail around the flat roof (yes, we can provide that).
• I believe that we will also be ok for propane regulations as we will have no windows or open drains within about a meter of the unit.
   

4 Aesthetics.  The architect is strongly of the view that a roof location will spoil the view from the garden of the rear elevation.  

• I find this rather odd - I think it will be barely noticeable, especially edge-on, but an ASHP in the garden will be certainly much more noticeable, front or rear.

So I think the Aesthetics and Servicing objections both look spurious. I guess 1st & 2nd points are the ones I really need to consider.  What are your thoughts? 
It seems to me that if we are careful, the risks of annoying noise and vibration are small, but they cannot be ruled out. On the other hand we would keep our garden space. So it is not clear which is best, and I would expect the engineer and architect to say this.  They do admit that it is ultimately up to us, but every time they say it, they also keep repeating that their professional advice is that a roof location would be a very big mistake. 

 

I am puzzled why they have such strong opinions, and feel the need to resort to spurious arguments.  What is your advise, opinion and experience ?

 

 

[Dillsue]

In your shoes I'd have no problem putting it on the roof as it's just like the 1000s of AC units on all manner of buildings. My only reservation would be having it bearing on the roof below with the risk of vibration being transmitted through the roof. As you suggested I think I would hang it off the wall or or on a pair beams spanning the roof and supported by the wall either end. At the distance it is from the neighbours, I don't think I'd even mention it to them

[Michael_S]

I have similar plans and have been strongly advised that roof mounting is likely to cause reverberation and that wall mounting is the safer option - I would also look into getting spring dampeners rather than just the standard rubber type mounts.  Also consider whether the roof (or wall) is strong enough - our roof only had 4 inch joists.

 

I assume you have stepped through the MCS permitted noise calcs with your set up and suggested unit noise power to 1m outside the nearest relevant neighbours window?

Edited August 3 by Michael_S

[nod]

Our neighbor sited there’s on there flat roof some  ten years ago 

and it’s been fine up and till this year 

Home serve came out to service it and it was deemed a Health and safety risk Three vans later they buggered off with looking at it 

They got someone local who had a ladder 😁

[sharpener]

6 hours ago, chdb said:

3. Servicing: Our engineer said Vaillant is said to be the best choice and they also have a good service network. However, he said their maintenance engineers have strict safety rules, and they will be unwilling to attend a roof installation.  Work by another company might invalidate our guarantee.  Furthermore current ASHP technology uses a Propane (R290) refrigerant which has some additional safety requirements - and he said this will also present problems for a roof location.
However I have checked this issue directly with Vaillant who said its ok if we have: 

• the required space around the unit - generally 1m to one side, 500mm to the other (yes, there's enough space).
• access to the unit not requiring use of moveable ladders (yes, we will have ladder-free access through the shower room windows).
• safety rail around the flat roof (yes, we can provide that).
• I believe that we will also be ok for propane regulations as we will have no windows or open drains within about a meter of the unit.

 

If you install i.a.w. Vaillant's spec as above I don't think you will have a problem. Work by a third party will not invalidate the warranty if they are Vaillant approved installers - in practice most of their MCS installers are. 

 

I would go for wall bracket mounting though and I am not sure this is advisable for the 10 and 12 kW twin fan units as although very thin front-to-back they are quite tall and rock a bit on the isolating mounts. But they are extremely quiet.

Edited August 3 by sharpener

[Kelvin]

Your reasons for doing it are less convincing than theirs for not doing it which seem reasonable rather than spurious. They take up very little space. They are also right about the aesthetics, look at the eyesore Nod has posted. 
 

ASHPs are relatively quiet but obviously not silent and are noisier the harder they work. The neighbour 7m away will easily hear it if they have a window open. They might still hear it if it’s ground mounted of course depending on where it’s situated but fences etc tend to attenuate the noise. 

[jack]

8 hours ago, Kelvin said:

Your reasons for doing it are less convincing than theirs for not doing it which seem reasonable rather than spurious. They take up very little space. They are also right about the aesthetics, look at the eyesore Nod has posted. 
 

ASHPs are relatively quiet but obviously not silent and are noisier the harder they work. The neighbour 7m away will easily hear it if they have a window open. They might still hear it if it’s ground mounted of course depending on where it’s situated but fences etc tend to attenuate the noise. 

 

100% this. 

 

It's trivial to hide an ASHP at ground level in a garden. The right shrub or an open screen a short distance from the unit will completely hide it.

[Dillsue]

26 minutes ago, jack said:

It's trivial to hide an ASHP at ground level in a garden. The right shrub or an open screen a short distance from the unit will completely hide it.

Our therma v needs a minimum area around it of 2100mm x 1300mm for operation and that includes 700mm at the front for maintenace access. 700mm might be OK for a Korean midget to work on it but I used at least a meter to wire it. That would give an area of 2100mm x 1600mm before shrubs/fence/screen. That's quite a chunk of space in a small garden. 

 

It's a fact that some people find them intrusive in small gardens as our HP came from a plumber that had removed it and refitted a gas boiler for the very reason the new house owner didn't want it in their garden!!

[JamesPa]

Mine is 50cm from a picture window in a room we use a lot.  It's side on, and since no air comes out of or goes into the sides or top, I built a smart wooden structure with a smart ceramic top that hugs it on two sides and on top.  It makes it look like a smart piece of furniture and the wood is styled to blend in with the wooden noise screen I also built.  Twice a year it's simply lifted off for maintenance.

 

You can also have ashps vinyl wrapped with a design of your choice.

 

Obviously it depends on situation, but disguise doesn't always have to take up lots of space or be a long way from the unit, depending on it's orientation.

 

Edited August 4 by JamesPa

[JohnMo]

54 minutes ago, Dillsue said:

Our therma v needs a minimum area around it of 2100mm x 1300mm for operation and that includes 700mm at the front for maintenace access. 700mm might be OK for a Korean midget to work on it but I used at least a meter to wire it. That would give an area of 2100mm x 1600mm before shrubs/fence/screen. That's quite a chunk of space in a small garden. 

 

It's a fact that some people find them intrusive in small gardens as our HP came from a plumber that had removed it and refitted a gas boiler for the very reason the new house owner didn't want it in their garden!!

Our ASHP has an open space in front. Early in the year tidings things up, I extended a fence about 1m in front of heat pump, fence was mostly open, but it caused enough recycling of airflow to promote frosting very frequently from about 5 degs outside and down. Removed after a couple of cold days and everything returned to normal.

[Dillsue]

So even working with manufactures guidelines doesn't guarantee flawless operation. Interesting observation

[jack]

52 minutes ago, Dillsue said:

Our therma v needs a minimum area around it of 2100mm x 1300mm for operation and that includes 700mm at the front for maintenace access. 700mm might be OK for a Korean midget to work on it but I used at least a meter to wire it. That would give an area of 2100mm x 1600mm before shrubs/fence/screen. That's quite a chunk of space in a small garden. 

 

It's a fact that some people find them intrusive in small gardens as our HP came from a plumber that had removed it and refitted a gas boiler for the very reason the new house owner didn't want it in their garden!!

 

I don't disagree with any of that, but it's a matter of personal preference which option is riskier or less pleasant to look at.

 

My personal opinion is that I wouldn't mount one in the rooftop position being proposed. I'd be concerned about the noise impact in the bedrooms. I think it looks horrible on the roof (the drawing view from the garden places it against the wall - it needs to be moved a long way out to give a more accurate impression). And I think the risk of noise to the upstairs neighbour shouldn't be ignored. Yes it's 7m and a decent unit won't make a lot of noise, but there's brick wall behind the unit, a brick wall to the side, and the same on the neighbour's side of the fence, without a fence or any other ground-level obstacles to absorb and disperse sound.

 

I could live with it being in the garden if that were the only alternative. Face the outlet down the garden. Screen the garden side parallel to the fence. That will leave the unit in a corridor without any obstructions to the front or rear, so there's no risk of recycling cold air.

[chdb]

Thanks for all the very helpful and interesting responses.

 

Quote

Your reasons for doing it are less convincing than theirs for not doing it which seem reasonable rather than spurious. They take up very little space. They are also right about the aesthetics, look at the eyesore Nod has posted. 

 

I said the access and aesthetics reasons are spurious because -

1. I checked with Vaillant about access for maintenance, and in our case there is no problem because there will be enough space around it, and we will have ladder-free access to the roof in question, and a safety rail at the edges.
2. As for aesthetics, they are of course mostly subjective, but it certainly will be much less noticeable (to us) on the roof than at ground level, because we wont go onto the roof very often, but we use the garden a lot. And we wont have much space for hiding it in our relatively narrow 7m garden. (Its a typical Victorian terraced inner city house, not a detatched suburban one like in Nod's photo.)   

The architect and engineer both know or should know all these details - thats why I dont really understanding their strong positions on those two points.

 

Admittedly a roof position is more noticeable to neighbours. (The one in Nods photo is huge - probably double the size that we will have. And while not beautiful, its not really offensive.  I would not call it an eyesore!)  I think the issue for neighbours is mostly about noise, but if they hate the noise, and can easily see it, they will inevitably hate the sight of it too. Hmm. That is something I had not considered before.

[JamesPa]

35 minutes ago, chdb said:

Admittedly a roof position is more noticeable to neighbours. (The one in Nods photo is huge - probably double the size that we will have. And while not beautiful, its not really offensive.  I would not call it an eyesore!)  I think the issue for neighbours is mostly about noise, but if they hate the noise, and can easily see it, they will inevitably hate the sight of it too. Hmm. That is something I had not considered before.

The converse is also true 

 

You don't say which model this is but given your comments about physical size im presuming it's either the 5kW or the 7.  The 7 (which I have) will be inaudible even outside at 7m most of the time, but when the compressor ramps up to max, which for me only occurs during recovery from defrost and at very low temps like -4 or below, it will be audible outside, although with double glazing it will be barely audible (if at all) inside even at max.  If you have a little spare capacity you can have invoke noise reduction mode at night to calm it down, obviously at the expense of output.  Is there any possibility of a noise (and visual) barrier maybe disguised as something decorative.

 

 

[HughF]

On 04/08/2025 at 10:05, Dillsue said:

So even working with manufactures guidelines doesn't guarantee flawless operation. Interesting observation

Manufacturers guidelines are often wrong/poor design choices.

[HughF]

Put it in the garden. It’ll reverberate something terrible on the flat roof.

[Kelvin]

You sound like you’ve made up your mind so good luck with the choice.