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

[saveasteading]

Just now, JohnMo said:

Are you a big building firm massaging the results.

Good point. No I'm being an individual who would design and supervise the process. Ie talking to buildhub not the industry.

An airtight construction but with some flaws, with window vents and a 4" hole in every toilet wall and the kitchen, will get about 3.5. For a better figure, close the vents and covet the fan outlets.

In real life these will all vent the house with or without mvhr.

 

The mass industry will cheat, but the real houses will have plenty of air movement.

[SBMS]

8 minutes ago, Dave Jones said:

As long as you have UFH AND fancoils in all the upstairs rooms you have free aircon.

This was my reasoning. 
 

And I have loxoned everything to death so can automate the immersion (a timer would probably have sufficed like JohnMo said - but does mean I can integrate PV production to top up when it’s generating most). 
 

To respond re eco credentials - it’s a balancing act right? Building a house isn’t an eco statement for me. But where I can I have tried to minimise heat loss with good quality triple glazing, 200mm cavity, airtightness (meaning I need MVHr) etc. Then the home automation and using ASHP for cooling is for comfort and because I like the hobby of tinkering with software and programming. I guess it’s more eco friendly to only have one unit to save resources on building, shipping etc, but wasn’t primary motivator. 
 

the ultimate eco statement is not to build a new house at all probably and buy existing stock - but then most of us wouldn’t be here would we?

[saveasteading]

4 minutes ago, JohnMo said:

 it starts tomorrow

I'm prevaricating too. Just one more coffee and BH post.

[saveasteading]

1 minute ago, SBMS said:

ultimate eco statement is not to build a new house at all probably and buy existing stock -

 Between us and the progeny we are on upgrade/ conversion 8. It is very sustainable/ what we could afford.

My business  was all new build and it is very much easier except that you are juggling client's desires with realities (ye canna change the laws of physics) and their budget.

[Dave Jones]

existing is more of a challenge where there is none/not enough cavity insulation. Compromise is EWI with doggo render or smaller rooms and internal. 

 

Damp being major side effect of poor install would defiantly have networked decentralised MVHR units to take care of any potential issues where there is no room for ducting a normal system.

[Nickfromwales]

1 hour ago, saveasteading said:

The mass industry will cheat, but the real houses will have plenty of air movement.

In winter they’ll have masses of cold air infiltration and bump their heating bills up.

[Mike]

11 hours ago, Gus Potter said:

But if you really want to make a contribution then I would love to hear about eco friendly ideas that are going to work for 50  years plus that are easily and cheep to maintain,  by someone else, say a young couple buying you home.

Evidently not an original idea, but build to the Passivhaus standard (or close to it as feasible) and minimise the need for high-tech solutions. Though I'd now evaluate summer overheating against forecast climate data for the 2070s, now that it's available in granular detail for the UK.

 

As for long term maintenance costs & durability, the 25-year evaluation of the original Darmstadt passive houses (occupied since October 1991), available here, is interesting. TLDR:

• Heating requirement: average for the 4 homes remains stable at 8.4 kWh/m²/year (compared to the 2016 German average of 138 kWh/m²/year)
• Glazing: gas losses from the triple low-e glazing units so low that these can be used for at least two additional decades
• Airtightness: after replacing the seals of the opening lights, airtightness returned to the as-built value (0.21, from 0.26 pre-replacement)
• MVHR: no significant contamination despite never having been cleaned, expected to last >50 years apart from filter changes and fan replacement (1 of the 4 MVHR units had 2 fans replaced in 2002 for about €500; all others were still original)

 

[Nickfromwales]

The mechanicals will always need replacing, that’s just acceptable as things that move wear out. 
 

The backbone (such as UFH pipes etc cast into screed or slabs, plus MVHR ducting set into the frame and fabric of the dwelling) is where you should focus concerns of “ultimate longevity”.

 

Most build systems have to withstand 50-60 years of occupancy and ‘use’ but that’s mostly for sub and superstructure; the pretty stuff can be changed retrospectively, and one can assume a full cosmetic makeover (x2) in a 50 year span, to allow for end of life or keeping up with modern taste / style etc. 

[MikeGrahamT21]

On 10/04/2026 at 17:57, Oz07 said:

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. 

I think I only spent around £1.8k on my entire system, got the unit off eBay cheap and did all the install myself, but if the cost was £7k that’s what I’d be getting out the two 

[Nickfromwales]

On 11/04/2026 at 18:03, MikeGrahamT21 said:

I think I only spent around £1.8k on my entire system, got the unit off eBay cheap and did all the install myself, but if the cost was £7k that’s what I’d be getting out the two 

I pay >£1.8k for a decent (PH certifiable) MVHR unit, or more if it’s above 300/325 size.

[Indy]

Thank you all for the wonderful replies and lots of interesting information. Some people picked up on the fact that I do want the best of both worlds, which is accurate.

 

I guess what I was looking for is justification to help me pull the trigger on MVHR based on actual experiences. I will be blunt here and everyone I've spoken to including friends and family who've built new, done major renovations and retrofits - none of them have anything good or bad to say about the MVHR apart from the price. But they do at least say that they wouldn't change their decision to put the system in, so I guess it is one of those things that once you experience it - hard to go back to being without it.

 

So on that track, I think I will stick with my original decision and go with the MVHR. And then plumb in for the Aircon system for the actual internal/external units to be added in a couple of years time.

[SteamyTea]

If you double the diameter of a circle, you quadruple the area.

I have often wondered, if cooling (or heating) are marginally small most of the year, if enough flow can be passed though larger MVHR pipework.

Just a case of doing the sums and seeing if larger pipework can be fitted in.

[joth]

1 hour ago, SteamyTea said:

If you double the diameter of a circle, you quadruple the area.

I have often wondered, if cooling (or heating) are marginally small most of the year, if enough flow can be passed though larger MVHR pipework.

Just a case of doing the sums and seeing if larger pipework can be fitted in.

In my experience larger pipework and some degree of zoning would be needed.

Most of the year I'd like additional cooling ij the upstairs rooms, even though the ground floor is in heating mode. 

I guess getting one mvhr per floor would be simplest, although adding yet more cost onto already expensive system 

On the plus side, at least in my house redoing the FF pipework with larger diameter would be simple enough. (Indeed I've essentially already done that piecemeal, by adding ducted Fancoils and then forcing air from outside directly into those ducts at night time. My own mvhr uber bypass).

 

All that said those are specific challenges hit in our passivhaus retrofit, and regardless of the improvement opportunities I'd fit MVHR again every single time. The system efficiency and comfort while avoiding and  moisture/mould risk is incredible. And the reduced dust build up a real plus.