Air tightness (gues)stimation

[timhowes]

Noob here - please be patient!

 

We are renovating an already-modernised 1949 rendered brick construction bungalow, adding 4 bedrooms and a bathroom upstairs. Timber construction first floor. Cold tiled roof. New windows. Major job.

 

We had originally hoped to install MVHR, but I doubt we will be better than 3m³/m².h - however, I have no idea really and this is purely guesswork. We aren't enveloping in a membrane and don't have PH ambitions (though will be generally attentive to minimising leaks).

 

My latest plan is dMEV with humidity-actuated wall inlets, which will save money and, I imagine, perform adequately given the likely level of air-tightness. Our builder has suggested we can't assess this until the build has got to a point at which it can actually be measured - however, at this point I imagine it would be very difficult/impossible to re-design for MVHR if we found we were better than expected.

 

I suspect dMEV is the best option, but please contribute if you think I've got this wrong!

 

Many thanks

Edited yesterday at 09:10 by timhowes

[JohnMo]

If you don't need to airtest I wouldn't, you end up boxing your self into a corner. Especially if you live in Scotland?

 

I would progress the way you are planning. Tried a couple of dMEV fans in our summer house - Greenwood were head and shoulders better than other fans I tried. And cheap enough on eBay.

 

Remember you need to have all internal door under cut at the bottom for dMEV to work correctly. I would add a co2 sensor to main bedroom as well so you can fine tune your system.

[Night Owl]

Plus 1 for the Greenwood fan.  I'm pretty sure they can be installed in either a wall or ceiling.  Fitted mine in a bathroom ceiling and vented it out through the roof tiles.  A reasonably easy job even with the addition of a condensate type drain as the ducting was vertical.  Only issue I had with the fan and I'm probably being over cautious was that there are 3 or 4 very thin sensor wires that are taped to the body of the fan that's inserted into your ducting.  I found this fit to be quite tight and was concerned these wires would potentially get damaged during fitment / removal despite the insulating tape on them.  In the end, I cut a small channel in the ducting where these wires would sit and sealed the channel up.  No risk now of wire damage.  Fan looks nice on the ceiling too.  Not powered it up yet so can't comment on it's performance but will hopefully save me all of the hassle and expense of a MVHR install.

[timhowes]

Thanks both. I shall continue in the same vein.

 

@JohnMo I have read your many supportive posts regarding the Greenwood (made by Zehnder) fans and have earmarked these, plus some inlets you've recommended. Extremely helpful!

[JohnMo]

I have also found these through wall inlets look a bit nicer.

 

https://store.beamcentralsystems.com/collections/mechanical-ventilation/products/dcv-intake-air-valves

[Mike]

6 hours ago, timhowes said:

I suspect dMEV is the best option, but please contribute if you think I've got this wrong!

In a deep renovation that didn't warrant MVHR, I'd choose central MEV over dMEV. That is, one central fan that extracts via ducts from kitchen, utility bathrooms, WC (and potentially the hallway & elsewhere, depending on layout). The advantages include that the unit can be positioned somewhere noise-insensitive, there's only 1 exhaust duct to the outside (so no risk of a wind-tunnel effect from holes on opposite sides of the building), in refurb the disruption of installing the ducts likely isn't a problem, and a ceiling / wall vent is the only part you see in the rooms. You also have a single central control - which could be a pro or a con depending on your view point.

 

6 hours ago, timhowes said:

with humidity-actuated wall inlets

This is where it's trickier. MEV has been common in France since rules brought in in 1982, and humidity-activated wall / window inlets were normally the go-to solution (a set-up known in France as VMC hygro B). However, now that new homes are more airtight (even those that are not class-leading), there there are reports that humidity-activated air entry is not providing enough ventilation for good indoor air quality - sometimes not enough to keep mould at bay either. Consequently some installers no longer fit them (e.g. see here on YouTube, in French). Instead, they favour permanently open trickle vents (VMC hygro A), so I'd suggest choosing that option. If not, be sure to get the CO₂ sensor suggested by @JohnMo; it is, in any case, a good idea.

 

7 hours ago, timhowes said:

We had originally hoped to install MVHR, but I doubt we will be better than 3m³/m².h - however, I have no idea really and this is purely guesswork. We aren't enveloping in a membrane and don't have PH ambitions (though will be generally attentive to minimising leaks).

The 3m³/m².h is a guideline that originated from a paper published in 2000 that used SAP to consider the economics and CO₂ emissions of ventilation strategies. Both the economics and the carbon intensity of electricity generation have changed considerably in the 25 years since then, SAP has been updated multiple times, and I doubt that it took into account the possibility of using heat pumps, so it's unlikely that an updated study would come up with the exact same figure today. I've seen 5m³/m².h suggested in several places as the cut-off point, including on page 14 of this 2019 paper.

[JohnMo]

An English video by same person, but doesn't mention anything other than humidity activated?

 

 

You do need to watch the mm² area, no use putting a small vent in each room, follow building regs for sizing. In Scotland they are pretty huge.

 

 

[Mike]

1 hour ago, JohnMo said:

An English video by same person, but doesn't mention anything other than humidity activated?

Yes, the translation's not very good (it's tricky translating technical language, for AI as well as humans). The original French version of the video is here; the ones 'that consist of a simple grid' are regular trickle vents, the 'adjustable air inlets' are humidity-controlled - two different types that sound like one in the English version.

 

Edited 14 hours ago by Mike

[MikeGrahamT21]

Isn’t the 3m³/m².h number related to this…

 

 

It’s talking about carbon emissions rather than overall efficiency, of course the lower the ACH the better it will work but also doesn’t mean it won’t work at all.

 

https://www.passivhaustrust.org.uk/UserFiles/File/research papers/MVHR/2020.04.27-The Case for MVHR-v7.pdf
 

it is expensive though, I’ll give it that!

[Mike]

1 hour ago, MikeGrahamT21 said:

Isn’t the 3m³/m².h number related to this… It’s talking about carbon emissions rather than overall efficiency, of course the lower the ACH the better it will work but also doesn’t mean it won’t work at all.

Yes, that's the same 3m³/m².h (and the same Lowe who first came up with the figure in 2000); it crops up in many places. The conclusion to that paper is interesting (though I know that it too has received some criticism):

 

This paper has shown that previous analyses indicating that MVHR systems should only be installed in
properties with an air permeability of 3 m³/m².hour @50Pa or less are based on outdated information and
flawed assumptions using a modelling system which was never intended to be used to compare ventilation
systems.

 

A modern MVHR system will result in significantly lower CO2 emissions at any reasonable level of air
permeability.

 

However, an MVHR system’s primary function is ventilation and, in this capacity, it is far more effective at
providing a good quality indoor air environment – regardless of external conditions – than natural
ventilation.

 

There is no clear up-to-date evidence behind the rule of thumb that says MVHR is inappropriate for
dwellings with air permeability above 3m³/m².hour @50Pa.  This analysis has shown that MVHR systems
result in improved ventilation and lower carbon emissions for all levels of airtightness.  There is a compelling
case for MVHR systems to be fitted in all new dwellings and to be strongly encouraged in retrofits where
significant reductions in energy demand are being targeted.

 

It does underline, though, that the 3m³/m².h 'guideline' is far from universally accepted and that it shouldn't be taken as definitive when deciding whether or not to install MVHR. Particularly if there are other reasons why MVHR might be desirable - to filter the incoming air or avoid external noise infiltration, for example.

 

Edited 12 hours ago by Mike