@shadowterrace, welcome to the forum.  Very good question. Like a lot of others, my ASHP is in a detached house with plenty space.  The two links below are terraced houses. 

https://tomkiss.net/life/going_fossil_free_in_a_terraced_victorian_house

https://www.youtube.com/watch?v=m2-_x0XZUSM

As you say the ASHP won't be doing much in the warmer months when you are likely to be sitting out.  And, you'll get a nice cold air blast if it does come on!!  How far is it from the wall to the fence?  It looks like you could fit one in the space you have; it needs to be at least 1m from the boundary to be classed as permitted development (I assume Scotland is the same as England but don't know) and also needs space for air to circulate.  It'll have to be 300mm from the wall and they tend to be 3-400mm deep. 

They really aren't noisy when compared with all the other outdoor sounds but it's a matter of opinion and you'd have to hear one yourself if you're concerned. When you and your neighbours are sat at your tables you're not going to hear much.

@shadowterrace, @kev-m is correct. Over late spring, summer and early autumn, the heat pump is not on very often, and when it is, it's not running on max. The thing, is if it's cold enough for the pump to be on, you probably won't be sitting in the garden. Having said that, the noise can be annoying if you are outside and braving chilly weather to enjoy your time outside, so it's best to try and find a pump that operates at less than 40dB. Our heat pump, at max speed, is over 60dB, and is clearly audible at 5-10 metres away if you're facing the fan.

Having looked at your garden, I would suggest you identity a pump you think would be good for your property and see what the published noise levels are - then find something that emits a sound/noise at around that level (a hair dryer at a lower speed, perhaps) and see if you can "live" with it. 

Hi @kev-m thanks for the welcome! Good to know the space requirements. We're planning to put a narrow storage shed on the left of that space, so we'll need to ensure there is room for passage through - and airflow of course. It's a 3m wide space, the shed will take up 1m of that, and from your estimates about 0.7m required for the ASHP unit, so the remaining 1.3m will hopefully be a reasonable gap for air flow.

Thanks for sharing the other resources too, the case studies at the blog and video have tonnes of relevant detail for us! Lot of homework for the weekend there.

Regarding the noise, as you say it is subjective. I'm quite sensitive to noise and I notice when our gas boiler comes on, but don't mind as it's short lived. If it was that level, throughout summer days, that would be too much, although based on what Mars has said it may be quiet enough then. 

Ideally I'll be able to hear a representative model of ASHP in action before taking the plunge, I'm hesitant to even proceed with quotes (as there will be a lot of integration work needed) before I know if noise would rule it out anyway.

In the mean time, Mars @editor I like your suggestion of finding a suitable reference noise. A hairdryer is a good idea. I'll see about some experiments that can simulate how it might sound.

ShadowTerrace

We’ve just had the Daikin Altherma 3H HT unit fitted. It is quiet. I will try and record. But only have a iPhone microphone 🎤 

As with any ASHP, there is an icy blast when the unit is running. But as stated elsewhere, not really running it in the summer, when you are sat outside. 

But you need a home assessment. An EPC rating. And a room by room heat loss and heat demand assessment before you decide whether an ASHP is right for you. And then which solution. 

My Daikin is less than the planning permission 0.6m3 unit volume. And 18kW. So powerful. But your building assessment will define the size of system you require to keep your property warm in the depth of winter.

You also need to be 1m from your neighbours property boundary which could impact your design. Check local planning regulation. 

See a description of my system Worcestershire ASHP on this site 

Hi ShadowTerrace,

One thing that you need to look into seriously is the airflow around an ASHP. If there is any possibility of the cold air coming out of the ASHP being drawn back into the inlet, then you could have serious problems with both efficiency and noise levels.

Totally agree Derek. The ASHP needs to “breath” and be free

Hi @julianc - great to get your assessment of quietness while running a larger unit (18 kW). I think it's difficult to produce an accurate recording, as so much depends on your recording process - and software can helpfully filter out parts of background sound without even telling you. But perhaps it's possible, if you have a go I'll listen of course 🙂 and will check out your other posts on the Daikin too, thanks! 

I think it'll be useful for me to figure out what a typical running pattern might be - if I'll be able to run the unit for 3 hours in the morning and then off for rest of day, for example. Or if it just runs very low speed (and very quietly) all day... With a break in the afternoon? But then, if that's also when air temp is warmest, this could impact efficiency. Perhaps a lower power unit is better, for having a lower minimum running speed. A quick chat with a possible supplier indicated that based on our EPC, we might need something of 17 kW although I'm hopeful that since we'd need to upgrade the insulation first, we might be able to get something smaller. 

We'll definitely be positioning it at least 2 m from boundary with neighbours, and I believe Scotland regulations are also 1 m minimum, but will check. Using our EPC from when we bought the house 2 years ago, we have begun researching the other requirements (posts and recommendations there are extremely helpful) so getting a sense of what's needed, estimated costs, and funding available - both RHI, and Home Energy Scotland loans. Due to complexity, accurate costing will be a fair bit of work, so I'm checking other constraints that are possible dealbreakers.

@derek-m that makes sense - so efficiency affects noise, and in turn, airflow affects efficiency. Perhaps this is an argument for mounting the unit higher up on the wall; although that makes it less accessible for maintenance. 

The garden is sort of boxed in by the higher walls, and of course the two storey house. There's no wall space to mount the unit on the end of the building (because windows, and sloping roof) so the only possible location is on the side of the kitchen/bathroom block shown in the photo.

Higher up on the wall, above the height of the walled area, might be breezier than down below.

I'm also starting to wonder what effect the fan will have on nearby plants and bird-feeding stations; so a higher up location could interfere less with the stability of the ground climate. 

Hi ShadowTerrace,

The  units are very heavy, so you would need to check about wall mounting.

Your starting point should be to improve the insulation and air-tightness of your property, since this will impact both the size of unit required and running costs. It will also have an effect on efficiency, since you should be able to operate at lower water flow temperatures.

Since it was installed Wednesday (Earth Day 😊) it’s been warm and sunny, so the Daikin hasn’t really run. I have a decibel meter on my iPhone so will try and take a reading at 1m and 3m. In front. And to the side. When it was running, I would describe it as a low hum. I can’t hear the compressor - 56 years young. So will ask my 21 year old daughter if she can hear any high pitched noises. 

@julianc absolutely agree however the certified installer will be aware of the correct space required surrounding the ASHP. Having an ASHP fitted as we speak.