MVHR vs Air Con - if you had to pick one?

[Indy]

So our build is reaching the point where we need to make a call on whether the MVHR system goes in. Its a standard masonry build (block and block with 150mm cavity), silicone render on the outside and some attention paid to airtightness. 

 

We were all set on having MVHR, and have had a design done, posi joists laid, routes confirmed pending final design etc. However, in a conversation with the builder when I asked about Air con - his recommendation was to go with Air con over MVHR if there is a choice to be made. The reason is that the living areas and master bedroom are both South/West facing, and we have large vaulted ceilings in the master bedroom which may become quite hot and oppressive as the heat is trapped due to the solar gain. 

 

They will both cost in the region of £7k ish supply and install. I know people have done it cheaper when doing a self install and commissioning but that's not a realistic proposition for us so it would have to be done by the builder/subcontractor.

 

He's actually been quite helpful and given me a week to make the final call, but it's put me into a bit of a tailspin and I've tried to summarise my thoughts below:

 

MVHR (Pros and Cons)

• Can only do it when the walls and floor are open - so its now or never
• Avoid trickle vents in the windows (I don't like the look of them, but do I not like them £7k worth? Not sure!)
• Better airtightness
• No mould and condensation issues
• Some heat recovery though not enough to wash its face in terms of install cost (Capex) and running cost (Opex)
• So it's very much a 'luxury' good and a nice to heave.
• Expensive for something that most buyers wont put a value on (when it comes to resale time)

 

Air Con

• More immediate and noticeable impact as we have a South/West facing garden and master bedroom which will get a lot of solar gain
• Works for heating and cooling so a reduction in gas usage for some months when air cooling/heating in certain rooms would be sufficient
• Probably more of a noticeable feature and buyers may put a 'slight' premium on it at resale time - though this is not an imminent event!
• Will make the house much more liveable in the warm summer months - and running costs will be further subsidised by the Solar PV and Battery storage 

 

My current thought process: Install MVHR (£7k) as its virtually impossible to retrofit cleanly. Put the power and condensate pipework in for the Air Con (£1k-£2k now) and the actual units in a couple of years once we've lived in the property and know what rooms to install it in. 

 

If you are building a new house, is MVHR a must do and has anyone built without one? What was your experience?

 

TLDR: What would you put in, if you only had £7k to play with and were asked to choose between MVHR and Air Con?

[MikeGrahamT21]

You are comparing apples to strawberries, they both do entirely different things.

 

If I had to choose where to spend my £7k, it would be MVHR every time. The air quality and comfort you get from one is very hard to describe and quantify, but very real.

 

Besides, grants are coming in for A2A HP's soon so you might be able to get one retrofitted later under that if you want cooling/heating from an AC unit

[Thorfun]

agree with @MikeGrahamT21 and sounds like your builder doesn't actually know the difference or what MVHR is for! if the house doesn't need MVHR then the airtightness isn't good enough and if the airtightness is good enough then MVHR isn't a choice it's a requirement!

 

so sounds like your builder isn't aiming for a airtightness score of below 3ACH which i believe is the max for mechanical ventilation requirement.

 

so, really your choice is an airtight house or not. if you want AC and don't want MVHR then you need to design for a poor airtightness build. if you want airtight then you need MVHR and then the decision is whether you want to spend on AC.

 

in my opinion, you want both! it's not easy, in my opinion, to retrofit AC without a lot of hassle or having pipes up the side of the buildings from the unit. when it's an empty shell you can run them and cover them up.

Edited 9 hours ago by Thorfun

[Thorfun]

ps. have you considered external blinds on the windows that will have solar gain? our blinds keep those rooms cooler than the rest of the house but we also need AC as we've not got blinds everywhere and we like to actually look out the windows. 🤣

[Indy]

Yes, comparing apples to strawberries is an apt summary as is the fact that I do want both. In an ideal world, both would get installed at the same time!

 

The builder does fully understand the importance of airtightness and also what an MVHR system does and achieves, and how that compares against the Air Con. It was my pointed question at what he would choose for his own house when he said Aircon will give you the noticeable comfort, whereas MVHR is part of the fabric and not really that noticeable. Plus, our usage means we'll probably end up negating most of the advantages of the MVHR with the missus' need to crack open a window and let 'fresh air' in - blowing a hole through my airtightness layer!

 

I guess a better worded question would be - is putting in MVHR a must have?

[Iceverge]

You don't absolutely require MVHR, dMEV will give you adequate fresh air at less capex and slightly higher opex. 

 

However you'll have holes in the windows and you'll miss the chance to filter the incoming air for dust pollen etc. You will have more outdoor noise and drafts. 

 

I think your plan is a good one as you said. 

 

To throw a spanner in the works you could have an air driven heating system and allow the A2A units do everything and omit other central heating. There's drawback there too but it is a option. 

[JohnMo]

MVHR is ventilation. 

Aircon is heating and cooling, it's not ventilation.

 

You need both ventilation and heating. MVHR doesn't solve the heating and Aircon doesn't solve your ventilation requirements.

 

Your builder is talking out of his ....

 

MVHR or another ventilation system is a must have. It's not something a heating appliance will replace 

[Nickfromwales]

2 hours ago, JohnMo said:

MVHR is ventilation. 

Aircon is heating and cooling, it's not ventilation.

 

You need both ventilation and heating. MVHR doesn't solve the heating and Aircon doesn't solve your ventilation requirements.

 

Your builder is talking out of his ....

 

MVHR or another ventilation system is a must have. It's not something a heating appliance will replace 

Just about to write this, the builder is clueless.

 

For my current 2 clients new builds I’ve recommended A2A AC for the 4 FF bedrooms, and ASHP + wet UFH in-slab (both insulated rafts) and MVHR in both; airtightness will be <0.6ACH for each (I’ll likely get these below 0.4 as I’m a freak when it comes to airtightness). 
 

If you go for airtight, then add another £2.5k and just get AeroBarrier in to treat the place, and it’ll be the best money you spend. Airtightness trumps insulation every time, as ventilation heat loss is huge compared to fabric heat loss. Stop the air infiltration, recover the heat, and enjoy an extremely comfortable, and cheap to run home.

 

AC is the luxury, MVHR is a necessity (as you’ve said you’re going airtight you don’t have a choice!!!!”); hence your builder is giving you shoddy advice that he’s picked up in the pub. 👎

[Mike]

3 hours ago, Indy said:

if you only had £7k to play with and were asked to choose between MVHR and Air Con?

You can probably add cooling to your heating system for much less than £7K, or to add it later, if you design it in now. In the meantime, MVHR is essential.

 

3 hours ago, Indy said:

we have large vaulted ceilings in the master bedroom which may become quite hot and oppressive as the heat is trapped due to the solar gain

Then change the design and avoid the problem. Upgrade your roof insulation using a product with greater decrement delay, add a brise soleil to the window, etc.

 

[Mr Punter]

Do the MVHR and just put AC in the rooms that need it.  If you have lots of PV on the roof the AC will cost nothing to run during the day.

[Nickfromwales]

3 minutes ago, Mr Punter said:

Do the MVHR and just put AC in the rooms that need it.  If you have lots of PV on the roof the AC will cost nothing to run during the day.

Nor will the MVHR.

[Mr Punter]

11 minutes ago, Nickfromwales said:

Nor will the MVHR.

 

True but the AC could be over 10x the MVHR

[Nickfromwales]

29 minutes ago, Mr Punter said:

 

True but the AC could be over 10x the MVHR

It’s Friday. Let’s settle this at the pub!

[Nestor]

Ideally both.

Airtight here.

I fitted MVHR and plumbed for AC in 1st floor bedrooms and main kitchen / dining during the build.

 

 

 

[Oz07]

5 hours ago, MikeGrahamT21 said:

If I had to choose where to spend my £7k, it would be MVHR every time. The air quality and comfort you get from one is very hard to describe and quantify, but very real.

The problem i have with this is everyone who says it has spent thousands on an mvhr system. I had one in our last place, the air was a lot drier (too dry really) and it was nice cleaning the filters and seeing all the muck its taking out the air. I'm not sure it was worth the investment. @joe90 used to turn his off in summer, @ToughButterCup hadn't bothered with one last time I heard and serial self builder @nod never bothers, but then his motives might be different! They are a nice to have and I will probably have one again in next place but I think you'd enjoy the comfort an aircon provides a lot more. How much would it be to first fix the pipes for the mvhr then make the decision later, the unit is usually the big ticket item but probably not if your having to pay labour on the install. 

[Nickfromwales]

1 hour ago, Nestor said:

Ideally both.

Airtight here.

I fitted MVHR and plumbed for AC in 1st floor bedrooms and main kitchen / dining during the build.

 

 

 

Are you cooling the GF slab with the ASHP?

I guess you intend to retro-fit the downstairs ones if you deem that necessary?

[Nickfromwales]

1 minute ago, Oz07 said:

They are a nice to have and I will probably have one again in next place but I think you'd enjoy the comfort an aircon provides a lot more. How much would it be to first fix the pipes for the mvhr then make the decision later, the unit is usually the big ticket item but probably not if your having to pay labour on the install. 

Sorry, but this is barking mad.

 

They're not "nice to have", they are a B Regs mandated requirement in an airtight house.

 

AC just recirculates the same air in the same space, it doesn't dump heat or moisture or stink to outdoors.

 

If someone has made an airtight home and doesn't install MVHR, or worse that they have installed it but don't use it, then they must be nuts.

[SteamyTea]

If you build to a standard that does not need MVHR (3ACH), then your heating bill will go up considerably.

7 minutes ago, Oz07 said:

not sure it was worth the investment. @joe90 used to turn his off in summer

The Cornish summers are not that hot, it is extremely rare that we get air temperatures over 24°C, the Atlantic Ocean is our AC.

He did open his windows, mainly as his ex wife, who was quite mental, insisted on it. She also refused to use an induction hob.

[Oz07]

19 minutes ago, Nickfromwales said:

Sorry, but this is barking mad.

 

They're not "nice to have", they are a B Regs mandated requirement in an airtight house.

 

AC just recirculates the same air in the same space, it doesn't dump heat or moisture or stink to outdoors.

 

If someone has made an airtight home and doesn't install MVHR, or worse that they have installed it but don't use it, then they must be nuts.

I don't think they are mandated. I've had air scores lower than 3 in houses before and nobody has ever came along and forced me to have one. There's no mechanism to, unless the bco is very switched on. Besides isnt it mechanical ventilation which is in the regs not nessecarily mvhr? What's all this Dmev stuff you hear about?

 

You're in the m&e business can we trust your impartiality here 🤣  I'm just saying I've lived with one in an airtight house and without one in a house with a score less than 3. Its just my personal opinion, nice to have but windows also work. Are @ToughButterCup and @nod nuts too?

[Oz07]

18 minutes ago, SteamyTea said:

If you build to a standard that does not need MVHR (3ACH), then your heating bill will go up considerably.

The Cornish summers are not that hot, it is extremely rare that we get air temperatures over 24°C, the Atlantic Ocean is our AC.

He did open his windows, mainly as his ex wife, who was quite mental, insisted on it. She also refused to use an induction hob.

Where is OP Cornwall?

[tuftythesquirrel]

In addition to whatever @Nickfromwales said, I would recommend reading the CIBSE Domestic Heating Design Guide, which was significantly updated in early 2026. This document, along with CIBSE TM59 (the industry standard for assessing overheating), takes a very firm stance on the "Passive First" hierarch.  I would hope they know what they are talking about, perhaps more than your builder (with all due respect).

In the previous version, it said that if a client expressed a desire to fit AC that they should strongly recommend against it. In the document, the engineering guidance (aka “Ye Cannae Change the Laws of Physic”) is specifically framed to prevent a "cooling death spiral"—where we install air conditioning (AC) to solve overheating caused by poor design, which in turn uses more energy and pumps more heat into the external environment.

We have a SE (18m) and a SW (22m) facing aspects and don’t need AC. If we know that it going to be 30C+ we actually keep the windows closed and use blinds to limit the solar gain. We very rarely get above 23C. So it can be done.

[Nickfromwales]

20 minutes ago, Oz07 said:

I don't think they are mandated. I've had air scores lower than 3 in houses before and nobody has ever came along and forced me to have one. There's no mechanism to, unless the bco is very switched on. Besides isnt it mechanical ventilation which is in the regs not nessecarily mvhr? What's all this Dmev stuff you hear about?

 

You're in the m&e business can we trust your impartiality here 🤣  I'm just saying I've lived with one in an airtight house and without one in a house with a score less than 3. Its just my personal opinion, nice to have but windows also work. Are @ToughButterCup and @nod nuts too?

Depends on whether the one without had trickle vents?

 

You know me and my impartiality lol. “Resistance is futile” 🤣

[Oz07]

@Indy

https://forum.buildhub.org.uk/topic/43530-mvhr-who-needs-it-i-dont/#comment-611326

[Nickfromwales]

32 minutes ago, tuftythesquirrel said:

If we know that it going to be 30C+ we actually keep the windows closed and use blinds to limit the solar gain. We very rarely get above 23C. So it can be done.

Agreed, but it’s a little like being imprisoned by the sun, and also that’ll be on the handful of nice days where you want to be able to look out of those lovely (and very expensive) glazed elevations, to the views outdoors.

 

In the villa we stayed at in Faro (Portugal) there were a few days where dropping all the external shutters was necessary, as well as AC running, but it was horrible to sacrifice the views.

 

For those current projects I’m on, we’re using solar reflective glazing, and the blinds etc will only be needed for privacy.

 

The AC on these will only react to a slight rise in temps, and will have very little to do, so it won’t be a cure it’ll be a prevention. 

[saveasteading]

27 minutes ago, Nickfromwales said:

being imprisoned by the sun

Like all those miserable people in the Mediterranean climate. It's OK.