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 4 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.