Is retrofitting MVHR worth it?

[smart51]

We're going to put an offer in on a 1930s detached house tomorrow.  The house is going to best and final offer this week.  The plan is to insulate and extend.  2 years of looking for a self build plot has been 100% unsuccessful and this is the next best thing.

 

We have MHRV on our current house and it does seem to work, certainly when it is on boost.  The think is it is noisy.  Even though it is in a cupboard in the converted loft, you can hear the hum upstairs when everything is quiet at night.  In the loft bedroom you can hear it in the day time too.

 

We're going to fit new windows throughout, external wall insulation, underfloor insulation.  We're going to have an air source heat pump and underfloor heating.  We want the house to be as low energy as we can practically do.  But we're not going to strip off the plaster or seal up every crack.  There will be no taped up plastic sheets in the walls keeping the house airtight.

 

Given the 'good but not perfect' fabric of the building, is MHRV going to have a useful effect on energy consumption that is worth the effort of putting the ducting in?

[JohnMo]

If your leakage rate is greater than 3m3/m2 @50Pa, keep your money in your pocket.  As cost to run v savings made will not be a benefit to you. 

 

I would instead (irrespective of your leakage rate), look at a demand based MEV or dMEV, more cost effective to purchase, simpler to install, run and maintain and only ventilates at the rate required.

[MarkyG82]

I'm thinking the same thing and seen your comment @JohnMo a few times.  How can we know/predict the air tightness of a building without doing a test?  Can we make a broad statement that doing the stuff in the list below will seal a house enough to qualify it under your stated leak rate?  The jobs below are all relatively normal/simple tasks so should be in the realms of most who are making improvements to their homes.

 

- Second front door (i.e. porch) with no letterbox on internal door.

- Blocked up chimney.

- External breaks in the shell (pipe entry points) sealed.

- Light fittings sealed.

[JohnMo]

Seal loft hatch,

ensure all windows are draft sealed, if they have trickle vents these would need to be sealed.

No cat flap 

Seal all wall sockets and light switches 

 

Even with all that, it could still leak like a sieve, due to the gap left at behind the skirting 

[MarkyG82]

58 minutes ago, JohnMo said:

...

 

Even with all that, it could still leak like a sieve, due to the gap left at behind the skirting 

 

Appreciate that.  Also not all houses are built the same.  At what point would you suggest a system be installed?  Do the benefits fall off a cliff or are they on a sliding scale as the air tightness reduces?

[JohnMo]

Not sure, but they say at over 3m3/m2 leakage rate the benifit of MVHR is small.  You have to consider the energy use to drive the MVHR unit compared to recovery of heat.  If air is coming into the house in an uncontrolled manner, but at an acceptable rate for ventilation, you are just adding additional ventilation, which you have to pay electric for.

[MarkyG82]

Thanks @JohnMo. Makes more sense. We have an order in for PV +battery so running costs would be reduced. We aren't doing it for a purely financial gain. Air quality is a bit one for us. We have the windows open year round when sleeping. Hoping a system like this would mean we can keep them shut. Also the house is quite stuffy when getting home each day (no trickle vents) so again this would help. Our kids are also quite noisy so being able to keep the racket from going to the neighbours would be good. Also need to redo the wet room ventilation anyway so makes sense to have a central system.

 

yes I have convinced myself. I just want to make sure I am covering as many based as I can without having to knock the whole house down.

[mathraki]

i am installing a MHRV into a mid-terraced Victorian house. Over and above the running costs and effort, there are many counter benefits such as you have outlined....

 

- offsetting the sealing of openings (new windows/doors)

- off-setting any airtightness measures added to rooms, floors etc. to ensure there is adequate fresh air ventilation.

- allow for windows to remain shut

- obvs. preheat incoming supply

- reduce heat losses

- expel humidity, mitigate damp, mould, condensation etc.

- Improve air quality

- increase comfort levels

 

Controversially I am not even doing all rooms - just middle and top floors (4 bedrooms, 2 shower/WC rooms); this approach will further decrease the efficiency of the system. I would summarise as follows, I think it is worth it, as I want to reap some of the benefits above and I am limited in what I can do (listed building) and I acknowledge it is far from a model-installation. 'Routinely opening windows' is trite suggestion and not an option for a large house and/or modern family (2 working parents ) that are trying to reduce heat losses and improve comfort and I think a MHRV retro-fit addresses many issues.

 

Best of luck with your installation if you proceed.

 

PS - it helps to try and pick up a good condition 2nd hand unit, I did (Vent Axia sentinel kinetic FHL) and it completely enhances the justification and negates commercial downsides.

 

 

[Adsibob]

If you are buying new windows anyway, and extending the back, you have a good opportunity to make most of the house fairly airtight, as the main points of weakness are junctions with windows and doors. If you are replacing all windows, you can make sure those junctions are taped properly and that the windows don’t have any trickle vents. New rear extension suggests new rear door (sliders/bifold if/French doesn’t matter, so long as they are airtight). If you are asking EwI that will also improve air-tightness. So I would go for MVHR. You just need to make sure the front door is draught proof.

[MikeGrahamT21]

Do it… it’s still the best thing I’ve ever added to my 70s bungalow!

[larry]

Another vote for doing this in a house that remains unquestionably leaky - we've been very happy.  I can understand the argument above in that in a leaky house you are getting background ventilation anyway, but in our case we were also routinely pulling in a lot more cold air from outside through bathroom/kitchen extract fans every time I had a shower etc. For me I suspect the energy savings from being able to take all the extract fans out and not send warm air out the window is the key saving, the fact that energy then remains useful is just a small added bonus. 

Also love the fact that the smell after a roast dinner just disappears...