Another "is my intake too close to the ASHP" question

[Dunc]

There are several threads which discuss the loation of MVHR intake and exhaust ducts relative to eachother and/or ASHPs but I haven't found an example of a case similar to mine so would appreciate advice:

 

The ASHP and proposed MVHR intake/exhause are set in a small alcove, about 600mm deep. The ASHP will be stood off both walls at least 200mm.

MVHR intake is proposed at ~2000mm above ground level, so about 1000mm above the ASHP.

 

Any risk that the ASHP could create a cold microclimate due to the alcove whcih would adversely affect the MVHR?

 

 

[Andehh]

There is probably a reasonable risk, especially no wind days. I do find our ASHP really kicks out some "microclimate conditions" when working in the immediate vicinity of a few sqm. 

 

Possible angled vents on MVHR facing opposite direction to ashp? 

[Nickfromwales]

Do you have posi joists? If so you can redirect the intake and exhaust, or just the intake, to be further away. 

[saveasteading]

It's all rather close together, including the heat pump.

Will any of the exhaust air be drawn back into the ventilation? Probably, so not ideal, and a bit inefficient.

 

Likewise the ashp is in a recess, so draws in ambient air from a limited area, so is more likely to draw back its own expelled (cold) air, and thus not be totally efficient. Is there a reason it can't be further from the wall?

[jack]

14 hours ago, Dunc said:

The ASHP and proposed MVHR intake/exhause are set in a small alcove, about 600mm deep. The ASHP will be stood off both walls at least 200mm.

 

200 mm seems small. Have you checked your ASHP installation manual? I have a very low capacity ASHP (5 kW) and I believe the minimum spacing from the wall for that was 300 or 400 mm.

[JohnMo]

Mine is also 400mm from walls at back and side and 500mm for the maintenance access side. So that looks like it needs fixing 

 

The other thing I would change is the MVHR in out arrangements, would installed a combined flow in extract out grille and position as high up the wall as possible. Then both elements of the MVHR can have a good distance from the ASHP 

 

Grille such as this

https://www.paulheatrecovery.co.uk/product/200mm-directional-external-combi-grille-for-mvhr-black/

[Nick Laslett]

Hello @Dunc,  I feel that the short answer, is don’t worry too much about it. 
 

Does it follow best practice, no.

 

Will there be some additional inefficiencies in the MVHR and ASHP during the two short periods when we have more extreme weather, probably. 
 

When an ASHP is going full power to heat a house, when it is -2° outside, it does create a lot of very cold air. I imagine in winter the MVHR exhaust air will probably be warmer than the outside air. Will the extra moisture in the exhaust increase the freezing of the ASHP, probably. If your MVHR has a pre-heater, it will just work harder to heat up the cold intake air. There are numerous interactions between these two components, you’ve got yourself a long term experiment to find out the ramifications. 
 

I’ve made plenty of compromises/cock-ups on my build. Don’t beat yourself up too much on this one. 

*Edit. You should endeavour to put your best efforts into getting as efficient install of an ASHP and MVHR, within the constraints of your build. My curiosity got the better of me, too interested in the experiment results in 3 years time, to see how these two components interact in this situation. 

Edited Wednesday at 09:35 by Nick Laslett

[saveasteading]

7 minutes ago, Nick Laslett said:

Don’t beat yourself up too much on this one. 

The ashp could seriously underperform. Its purpose is to be efficient, and you are right to question the arrangement. 

Is it tucked in this recess for tidiness or due to space constraints?

Have you the opportunity to change this?

[Nick Laslett]

2 minutes ago, saveasteading said:

The ashp could seriously underperform. Its purpose is to be efficient, and you are right to question the arrangement. 

Is it tucked in this recess for tidiness or due to space constraints?

Have you the opportunity to change this?

 @saveasteading, we are in complete agreement.

 

I hope that any easy changes will be made as a consequence of his original post. 

[JamesP]

What is the distance between the supply and extract of the MVHR, mine are 2 metres apart with the extract above the supply.

Would move it to another wall or 2 - 3 metres above ASHP, if possible.

[SteamyTea]

1 tonne of air dropped in temperature by 3.6°C, had diminished in energy by 1 kWh.

This is a power of 1 kW.

Air has a density of around 1.2 kg/m³. So a tonne has a volume of around 125 m³.

Assuming an air mixing ratio of 10:1 between the ASHP outlet and the MVHR, that will be 100W difference (2.4 kWh/day).

In reality, I suspect that there is so much turbulent mixing from the relatively fast moving air  that the difference is closer to 100:1, so 10W (0.24 kWh/day).

So not worth worrying about, fit one less downlighter.

 

Those that have both MVHR and an ASHP could help clear this up by taking some temperature readings, one I  the MVHR inlet, one for OAT  and the important one, physically halfway between the two.

[jack]

7 hours ago, Nick Laslett said:

I imagine in winter the MVHR exhaust air will probably be warmer than the outside air. Will the extra moisture in the exhaust increase the freezing of the ASHP, probably. If your MVHR has a pre-heater, it will just work harder to heat up the cold intake air. There are numerous interactions between these two components, you’ve got yourself a long term experiment to find out the ramifications. 

 

I remember reading about someone (maybe the guy who did the Denby Dale Passivhaus) putting the outlet of his MVHR right behind the ASHP, the thinking being that the ASHP would recover some of the additional heat from the outlet air being slightly warmer than the ambient air. Personally I'd be worried about differences in pressure (between the ASHP being on and off) unbalancing the MVHR.

 

The moisture point is an interesting one. When it's very cold, at least some of the outlet air moisture condenses on the heat exchanger and goes down the condensate drain. I don't know what proportion of the moisture exits the MVHR outlet, but if it's [corrected typo:] not a lot, then the impact on icing might not be that significant.

 

Also, when it isn't quite so cold as to cause increased icing (and hence more defrost cycles), moisture condensing on the ASHP fins will improve efficiency, because of the large amount of energy released due to the change of phase.

I suspect the whole thing is too complex to easily model.

 

The one thing I would try to do is to direct the MVHR outlet air towards the far side of the ASHP. With a bit of luck, the outward airflow from the ASHP might entrain some of the air from the MVHR outlet and push it away from the house.

 

Some sort of baffling or screening could also help encourage the "right" sort of air movement overall.  

[Iceverge]

The numbers will tell you like @SteamyTea has done. 

 

ASHPs move vast amounts of external air and drop it by such a small amount that there MVHR and ASHP won't notice eachother. 

 

I suspect the microclimate effect people are feeling is very much due to the wind chill from the movement of the air when standing nearby rather than an explicit drop in the air temperature . 

[Iceverge]

I would be much much more concerned with the heat loss due to the external length of water pipes to and from the ASHP.

[Dunc]

Hi all, thanks for the thoughts! Lots to ponder. 

Just to try to address some of the thoughtful comments:

 

Nothing is fully fixed in position yet but I need to cast the slab for the ASHP soon and the MVHR needs to be installed soon too (so it is a bit too late to make larger chages). The original ASHP position was architect driven and I suspect just because it looks tidy on the plan.

 

ASHP ideally should stay on that elevation as the plant room with DHW cylinder and UFH manifold are just the other side of the wall so giving shortest pipe runs (as @Iceverge just hinted at). The ASHP can certainly move outwards from the alcove; my plumber/installer was the one who said minimum 100mm off all adjacent walls...I upped that to 200mm coz I'm paranoid. But sounds like others may feel more gap is better. Will check Mitsubishi instructions. Another option may be to rotate the ASHP 90 degrees? Bit ugly perhaps.

 

Similarly MVHR box is in the same plant room so shortest runs for intake/exhaust pipes is into that alcove. That wall is single storey so intake can't move any further up. One proposal from CVC is to put the intake on the wall left of the alcove, same elevation. But really this only moves things ~900mm and introduces a world of pain installing through the garage/house dividing wall which of course must be fire resistant and airtight. And the timber claddign on the external wall of the garage is already in place . Interestingly CVC have only suggested moving the intake and don't seem to be concerned about the exhaust. The combigrille is interesting but if we are arguing the ASHP will move a lot of air around having intake and exhaust closer may increase rather than decrease the problem?

 

lots to ponder!

[JohnMo]

4 hours ago, Iceverge said:

I would be much much more concerned with the heat loss due to the external length of water pipes to and from the ASHP.

Not really that big an issue. Mine is a fair way from the house, behind a couple of sheds. Don't see any issues even when it's well below freezing.

 

Micro climate effect of the heat pump and MVHR. Damp laden air being ejected from the house should be ejected out far enough that it's mixed pretty well with normal air especially if there is any wind. So wouldn't worry unduly, just keep the extract air as high up as possible to allow the most mixing.

8 hours ago, jack said:

outlet of his MVHR right behind the ASHP

Think that's a good way to encourage defrosts to occur sooner and more often than needed, humid air from house though ASHP condenser.

[Dunc]

Well thanks for the recommendation to check MI @jack. Intreaguingly my installer was half- right. >=100mm from adjacent side walls but >=200 (or 300mm for the 5kW unit - why more for the smaller pump?) from a back wall (https://library.mitsubishielectric.co.uk/pdf/book/EcodanPUZ-WM50-60-85-112VH-AA_Installation_Manual_BH79D849H02UK#page-6-7). So it can sit in snug, but I think we'll bring it forward a good 500mm anyway.

 

What I handn't realised until I read that, is that the big grill and fan are the outlet side of the pump. AFAICT warm air is drawn in the "back" of the unit. In my ignorance I assumed that was intake. So the ASHP exhaust air will be expelled outside the alcove area. Given we're in northern scotland and it's windy 99% of the time, especially in winter, I'm reassured that it'll mix with amient quite rapidly and the MVHR should be fine. I'd love to have had it modelled but as others have said, seems too complex to achieve. Fairly unconcerened about MVHR outlet condensate - it'll be minor compared to atmosphere most of the time (coz it's always raining up here  )

[JohnMo]

8 minutes ago, Dunc said:

5kW unit

A slight alarm bell, the way you said 5kW was as if the ASHP installing was way bigger - new build with MVHR, so airtight well insulated, what size unit are you installing - would have expected 5kW even if your house is 300m².

[jack]

1 hour ago, JohnMo said:

Think that's a good way to encourage defrosts to occur sooner and more often than needed, humid air from house though ASHP condenser.

 

Any or all of what I said might be wrong, but that's exactly the point I was discussing:

 

 

 

 

[Dunc]

@JohnMo

~150m2. MBC passive kit, but not slab, so insulation and airtightness are passive-ish. Themal bridging not perfect though.

Heat load calc from ufh supplier gives 4.7kW at -9. Plumber recommended the 8.5kW pump.

Have to confess my calcs using the J. Harris spreadsheet give 1.7kW so there is a decent discrepancy....

If the 4.7kW value is correct then a 5kW pump would struggle to provide DHW in the depths of winter?

 

[JohnMo]

3 hours ago, Dunc said:

@JohnMo

~150m2. MBC passive kit, but not slab, so insulation and airtightness are passive-ish. Themal bridging not perfect though.

Heat load calc from ufh supplier gives 4.7kW at -9. Plumber recommended the 8.5kW pump.

Have to confess my calcs using the J. Harris spreadsheet give 1.7kW so there is a decent discrepancy....

If the 4.7kW value is correct then a 5kW pump would struggle to provide DHW in the depths of winter?

 

Unlikely to be that high, our actually and design at -9 is just over 3kW for 192m² living area plus another 20m² within the heated envelope not actually heated. We aren't passivhaus by a big margin, due to form factor being atrocious (in passivhaus terms).

 

So I believe your figure, wouldn't believe UFH supplier. 

 

Do not go for the Mitsubishi 8.5kW, if you insist on going that big, get the Mitsubishi 8kW. It has two refrigerator compressors, a 6kW inverter driven and a 2kW fixed speed unit. So has very good modulation.

 

We have a 6kW ASHP but it's massive for our house, as it doesn't modulate well. Zero issue experienced doing DHW. But at -10 it still kicks out nearly 6kW.

[Iceverge]

I really wouldn't get concerned about inlet and exhaust spacing either. 

 

Even if they're exactly beside eachother the mixing will be minimal assuming it's an open wall they're going. 

 

 

Think about how to keep snow off the ASHP. Lots of damp snow getting sucked into it doesn't do much for performance right when you need it most. 

 

[saveasteading]

I cant do sums on this, but my take is this.

I  studied hydraulics and flow and designed pipelines and channels, a very long time ago. I long ago forgot the maths of it, so this is based on observation and 'feeling'.

And I have stood in front of units and observed that the airflow can be considerable.

.

The outlet is at the front and the chilled air is thrust into open space, and away. Meanwhile it needs to draw in exactly as much air through the back. Your positioning constrains air flow to the back, so it will reduce, the fan will try to work harder, and you don't get the full amount of air through. so it less efficient, and quite a lot I feel.

Just increasing from 100mm to 200mm at rear and side should make a big difference...twice as much air flow. It will also be less likely to clog with leaves and webs.

 

If the manufacturer says otherwise, then do what they say.

[Gus Potter]

On 23/09/2025 at 18:57, Dunc said:

There are several threads which discuss the loation of MVHR intake and exhaust ducts relative to eachother and/or ASHPs but I haven't found an example of a case similar to mine so would appreciate advice:

 

The ASHP and proposed MVHR intake/exhause are set in a small alcove, about 600mm deep. The ASHP will be stood off both walls at least 200mm.

MVHR intake is proposed at ~2000mm above ground level, so about 1000mm above the ASHP.

 

Any risk that the ASHP could create a cold microclimate due to the alcove whcih would adversely affect the MVHR?

Just as a comment as a philistine. 

 

Why on gods earth are you sticking you ASHP in a recess. It's retarded and I think that is what you have clocked! Do you smell shite?

 

Oh yes your ASHP installer says it's ok! .. how much have they evaluated the wind in the winter (when you need it most) and the geometry of the building and how the wind flows around it locally? As an SE I am interested in local wind loads and often see ASHP insatallers are talking pish. 

 

Look ASHP's needs in the winter a bit of fresh air and free flow of air around the building is essential.

 

Next is why are you locating it near a window that let's sound in, more retard. 

 

If I was you I would think.. I'm spending a lot on an ASHP so let's think about where the prevailing wind comes from in the winter/ defrosting and see how we can get the best air flow. If you can't get to this level of thinking then you are just chucking your money away and when it gets proper cold your heating won't likely work as well as you expect.

 

Advice.. go back to basics, recognise that if you want an ASHP there might be an Architecutral compromise. So get the thing out the recess and place it where you get some wind in the middle of winter. Or you buy some electric heaters. 

 

|I made another post about wind uplift on roofs. But here is a diagram about how how wind work on walls. Ideally you want to place an ASHP where it gets some residual wind even on a cold daydue to the geometry of the house.

 

 

The best wind flow Zone A is near the corners of the building. 

 

Another way, my sister has two tandem ASHP's on the island of Tiree which faces into the prevailing wind. But it is not always mega windy. The ASHP's are placed about 1/3 from the corner walls. When you do a wind analysis you'll find that 90% of the time you get a flow over the ASHP.  That is clever design.. understand how the winnd flows around the building and postition your ASHP in the best place. 

 

If there are any ASHP designers out there let me know. 

 

@Nickfromwales Just to name check you, would welcome your professionall comment, am I being to practical or not? 

 

They cost a shit load, so why tie your hands at the outset by putting them in the wrong place? 

 

On 23/09/2025 at 18:57, Dunc said:

There are several threads which discuss the loation of MVHR intake and exhaust ducts relative to eachother and/or ASHPs but I haven't found an example of a case similar to mine so would appreciate advice:

I've given pelters here but it would help a lot if you posted a plan view of your house so we can see the orientation and size of the house with dimensions. My gut feeling is that you may be paying for something thta may not perform as well as you expect. 

 

 

 

 

 

 

 

 

Edited Thursday at 00:31 by Gus Potter
Typos due to being left handed