MVHR ? Who needs it? I don't.

[Oz07]

Are these MEV just in the wet rooms and kitchen? I'm not really a fan of trickle vents

[JohnMo]

1 hour ago, Oz07 said:

Are these MEV just in the wet rooms and kitchen? I'm not really a fan of trickle vents

Yes. A ducted single fan unit like MVHR but without the supply.

[Oz07]

Any heat exchanger or just extract?

[JohnMo]

MEV mechanical extract ventilation - so just extraction, but ONLY when needed. A simpler system with dMEV could be done with no ducts 

[Bonner]

23 hours ago, Oz07 said:

what levels do you get up to in bedrooms at night? Do you have windows cracked? Think winter is where it saves the money. I generally agree that the benefits are oversold, probably because when we have spent thousands on it we are not always honest with ourselves. That being said I would always put one in another new build.

Assume you are referring to @ToughButterCup’s CO2 monitor? My bedroom is typically 1200ppm at night, over 2000ppm without MVHR running.

[Mike]

10 hours ago, Bonner said:

My bedroom is typically 1200ppm at night

If I was recording that frequently, I'd be increasing the ventilation rate to cap it around 900ppm. I've not come across extensive research on the topic, but the 2016 study 'The effects of bedroom air quality on sleep and next-day performance' by Strøm-Tejsen, Zukowska-Tejsen et. al. is one that uses it as a benchmark.

[SteamyTea]

13 minutes ago, Mike said:

I've not come across extensive research on the topic,

There seems to be a lot.

https://scholar.google.co.uk/scholar?q=effects+of+elevated+co2+concentration+on+sleep+cycle&hl=en&as_sdt=0&as_vis=1&oi=scholart

[JohnMo]

10 hours ago, Bonner said:

My bedroom is typically 1200ppm at night

When we first had the MVHR running we were getting similar, so tweaked up flow rate in bedroom and lowered elsewhere, it never goes above 1000ppm (window closed and door closed). Now with window open most nights it's closer to 600ppm.

[Oz07]

Yeh i'm getting below 600 at the minute with windows on latch but its not heating season!

[JohnMo]

22 minutes ago, Oz07 said:

Yeh i'm getting below 600 at the minute with windows on latch but its not heating season!

Wish it wasn't the heating season for us, we nearly got to the point of thinking about cooling and temperature took a dive. Average temp here the last week has been between 6 and 8 degs.

[Iceverge]

I had no interest in MVHR initially. 

 

It was only when I sat to do some sums on the energy lost through ventilation that I was convinced. 

 

 

 

[G and J]

18 minutes ago, Iceverge said:

I had no interest in MVHR initially. 

 

It was only when I sat to do some sums on the energy lost through ventilation that I was convinced. 

 

 

 

When I started reading about airtight houses I instantly took against the idea.  Soon after I twigged the heat exchange concept and I had a Gru style ‘light bulb’ moment.  Precise, controlled, energy efficient ventilation.  What’s not to love?  
 

2 hours ago, JohnMo said:

Wish it wasn't the heating season for us, we nearly got to the point of thinking about cooling and temperature took a dive. Average temp here the last week has been between 6 and 8 degs.

I’m actually hoping for a cold spell to encourage peeps to want my free (but a bit naily) firewood.

[ToughButterCup]

On 19/04/2025 at 07:16, Oz07 said:

... what levels do you get up to in bedrooms at night? Do you have windows cracked? 

 

450 ppm or less (I think). Window always open on tilt : better put - rarely if ever shut. The wind direction and open-window/door configuration is very important. The chimney effect is very noticeable

Sensor positioning appears to be important - I have found 'dead spots' in our house. As I write this at my desk (a dead spot)  its 775 - with a 3m window wide open 4 meters away. Bedroom seems to be 'OK' to the extent that I can tell. The CO2 meter needs an a/c socket nearby, so the test hasn't been as extensive as I would like (flex length).

 

The only time I've noticed a 'red' alert ( on the meter)  is when over Christmas the house was packed with friends and family, cooking on full blast and some people were vaping like it was going out of fashion.

 

I think that MVHR might well lessen the number and extent of dead spots. It'll be interesting to do a comparison.

 

The question is how to do a fair comparison?

Because for six or seven months of the year we have at least two windows open. Maybe @SteamyTea or @TerryE could help my thinking process with a 'fair' design ? Please?

 

The main point is I realise that our approach is not for everyone: most want to switch MVHR on - forget it - and pay the bill. But until recently we couldn't afford MVHR, and so have been forced to experiment and 'feel' what the house will do for itself.

To me, it seems to do better than I expected : slow temperature change up or down, and to an extent sauerkraut resistant. It is very clear though we have to open a combination of doors and windows every day. Whereas with MVHR we wouldn't need to think about it.

[SteamyTea]

4 hours ago, ToughButterCup said:

 

The question is how to do a fair comparison

Easy, smoke tests.

I shall buy 200 Marlboro and come around and test every room.

 

[TerryE]

We had to make all sorts of design trade-offs and TBH sometimes "shooting from the hip" decisions during our build.  Sometimes these were driven by Local Planner push-back and sometimes to avoid exacerbating relationships with neighbours that were getting fed up with the new build going on next door.  Good examples here are (i) PV: we wanted to have cassette in-roof PV (similar to JSH) but the planners really didn't like the visual impact on the street scene, so we gave up on this one;  (ii) ASHP: my modelling showed that we would be hard pushed to get a 10-year payback for installing it and it would have to be installed quite close to a sensitive neighbour's boundary, so I took the easy way out an put in the pipework and electricity supply but installed our Willis Heater, and deferred the decision on the ASHP unit "until we had a enough data to make an informed decision" -- 9 years later and I still can't make the investment case.

 

In our case MVHR was an easy decision. We had ecoJoist floor voids and this was before boarding out, so this was one of the jobs we could do ourselves, so the cost was in the few £K range and at that time we could self certify for the BInsp. The modelling showed that we had a clear case for putting it, so for us it was a no-brainer. And we've never regretted that decision.  The entire house is always fresh smelling and mould-free despite it being pretty airtight (about 0.5 ACH).  The system does need maintenance but that's one of Jan's jobs 🤭. And when it comes to the warmer period (maybe 6 months a year), we just open doors and windows if we want direct external ventilation; I don't turn off the system because even if the kitchen patio doors are open, say, you still notice the street noise etc. even in what is basically a quiet village, and it can get stuffy in the bedrooms on the street-side with the windows closed overnight.  

 

But in your case Ian, this might be your equivalent of my ASHP decision.  The fact is that you now live in the house and have over-wintered without it, so your heating can cope with the heat losses from non-heat recovery air exchange.  It is more an issue of CO2 build-up and moisture control.

 

In your OP, you mentioned that you haven't got BCont sign-off. The reality is that there are two design modes for ventilation: the leaky model where all of your windows have get weep vents etc, and an airtight one where none of fenestration includes vents. In this second case you won't realistically get sign-off without an MVHR system installed, and this is a big problem for you.  You need sign-off, so unfortunately MVHR isn't really negotiable.  My BInsp went through my report in some detail and asked some supplementary Qs on it.  Your inspector might be a bit more lax, or perhaps you could just fabricate a piece of fiction, but IMO that's a dangerous path to try.  The least risk root is to self-install properly and go through the self-certification exercise (if Bregs haven't closed that path now).  

 

Edited April 21 by TerryE

[ToughButterCup]

Thanks very much Terry. 

 

This thread is about cashflow, not so much about MVHR. I know I HAVE TO have it. Now that I have some, I  begrudge the money especially when the house appears to perform really quite well on its own.

[JohnMo]

1 hour ago, ToughButterCup said:

thread is about cashflow, not so much about MVHR. I know I HAVE TO have it

It doesn't have to be expensive, extract from each wet room and then depending on house layout one or two locations to have supply too. Then the rest of the ventilation is via transfer through different areas, as the air finds it way back to extract points.

 

My MVHR came from ebay

[Iceverge]

Just what I was going to say. No need to make it expensive. Just rig this up to your wet rooms. 

 

 

 

 

All those stoners growing weed in their lofts had to come in for something useful in the end. 

 

 

[TerryE]

6 hours ago, ToughButterCup said:

This thread is about cashflow, not so much about MVHR. I know I HAVE TO have it. Now that I have some, I  begrudge the money especially when the house appears to perform really quite well on its own.

 

Surely this is more about getting to an end-point in such a way that your BInsp accepts your Test Report as demonstrating your builds compliance (without you abusing the truth to the point where you get caught out).  

 

[ToughButterCup]

15 hours ago, TerryE said:

without you abusing the truth to the point where you get caught out

 

I've dedicated my life to that skill....

[SteamyTea]

3 hours ago, ToughButterCup said:

 

I've dedicated my life to that skill....

What, getting caught out.

[Mike]

On 20/04/2025 at 18:36, SteamyTea said:

On 20/04/2025 at 18:22, Mike said:

I've not come across extensive research on the topic,

There seems to be a lot.

https://scholar.google.co.uk/scholar?q=effects+of+elevated+co2+concentration+on+sleep+cycle&hl=en&as_sdt=0&as_vis=1&oi=scholart

Loads. This one seems particularly insightful: Effects on breathing in awake and sleeping goats of focal acidosis in the medullary raphe :)

[ToughButterCup]

On 26/04/2025 at 00:13, Mike said:

Loads. This one seems particularly insightful: Effects on breathing in awake and sleeping goats of focal acidosis in the medullary raphe

 

Cor blimey mate: nerdish this place innit? 

 

I mean '... focal acidosis ...'   I read fecal acidosis. 😔 Can't help it....

[JohnMo]

22 minutes ago, ToughButterCup said:

 

Cor blimey mate: nerdish this place innit? 

 

I mean '... focal acidosis ...'   I read fecal acidosis. 😔 Can't help it....

I opened the document and closed it again, when I couldn't be bothered. If they can't do an introduction in plain English, not worth the time.

[MikeSharp01]

58 minutes ago, JohnMo said:

I opened the document and closed it again, when I couldn't be bothered. If they can't do an introduction in plain English, not worth the time.

I thought the abstract said it all really - it says that someone just used Google to find some connection between C0₂ and brain activity and then just posted it to sharpen up the tone of the conversation / make their tribulations with MVHR appear somewhat technical  - so probably a bit of wind up.

 

In fact what these researchers did was to use a process called microdialysis which involved pushing tiny tubes into the brains of Goats and then injecting mock brain fluid mediated with CO₂ , hence Google found it, to see what effect this had depending on what the goats were doing at the time - awake or sleeping. They found a number of effects during waking which were not there during sleeping and the common effect was that the goats brains worked really hard at keeping the brains chemistry balanced and that they were quite good at it. It is not obvious that the same effects might be connected to the concentrating of CO2 in the breathing atmosphere of the goats and they may or may not have developed insights into a number of other things but then I only read the abstract.

 

There is, of course, an alternative explanation for the OP's highlighting this piece of research and that is that they are a retired vetenary brain surgeon masquerading as a self builder.