Installing a HRV system in a 1940s home

RaineyK

Looking for advice from those in the know...

We're doing a major refurb job on a 1940s 3 bedroom house including external insulation, underfloor heating on the ground floor and basically a tonne of insulation everywhere else.

We're considering installing a HRV system as the plan is to make the house airtight.

Looking at forums online, opinions seem to be mixed on whether HRVs are effective in older buildings.

We'd love to hear from anyone who has installed one in an old building and what their verdict is.

Any advice would be much appreciated!

Metric Tensor

The effectiveness of HRV is directly related to air-tightness. If your upgraded air-tightness is better than 3ACH* at 50Pa then MHRV is the way to go.

If it's higher than that then it's a waste of your money and some form of wall vent + DCV set up would probably be a better option.

One thing to be aware of is the large amount of timber that is likely to be present in a 1940s building. It needs to be effectively ventilated and this often gets ignored when people use modern retrofit techniques on old buildings. The exact same applies to lime mortars if any exist in the building fabric.



*This figure may vary based on who you ask.

BryanF

OP, You didn’t mention air-tightness?

RaineyK

Cheers for that! So basically the only way of knowing if the house will be adequately airtight is by measuring it after the builder does the necessary to seal areas around windows, doors, roof etc? Does a DVC system require some/any airtightness?

Metric Tensor

A MHRV system is primarily a ventilation system - it just has the added bonus of rescuing some heat as it is ventilating the house. So there is no point designing a system to deliver X amount of air to a particular room if twice that is coming through the gaps in the building fabric (or five times it on a windy day, etc. etc.).

Making an old building suitably air tight to make a mechanical ventilation system effective is very difficult and it's impossible to tell if it was done well without a test. However, a contractor who has done this before should have a fair idea of what results they got on the last job and you could use that, or slightly better or worse, as a "target" for your job. You could use this guesstimated value as a means of making preliminary decisions regarding ventilation options.

DCV = Demand Controlled Ventilation - and can be turned on and off either manually or automatically depending on need and the type of system chosen. If it's based on internal air quality then it should only kick in when needed and it should not be needed at all if the draughts are keeping the air fresh! - N.B. This is a vast simplification based on your particular needs because DCV can be a lot of things to a lot of people.

BryanF

RaineyK wrote: »

Cheers for that! So basically the only way of knowing if the house will be adequately airtight is by measuring it after the builder does the necessary to seal areas around windows, doors, roof etc? Does a DVC system require some/any airtightness?

Yes
(DCV )

RaineyK

Thanks MetricTensor, that’s really clear. The DCV might be enough for my needs alright as I’m primarily looking for something to remove humidity from the kitchen and bathrooms - the extra heat recovery element just seemed to be a win win. I will talk to builders to see what they think based on previous projects they’ve done.
Really appreciate your advice, cheers ðŸ™

Dudda

RaineyK wrote: »

Thanks MetricTensor, that’s really clear. The DCV might be enough for my needs alright as I’m primarily looking for something to remove humidity from the kitchen and bathrooms - the extra heat recovery element just seemed to be a win win. I will talk to builders to see what they think based on previous projects they’ve done.
Really appreciate your advice, cheers ðŸ™

One last tip. Get it written into the contract or specification that the builder has to achieve 5ACH at a minimum. This will get rid of cowboys and also set a standard. If you just say you want it airtight and don't have a target they'll just tape around windows quickly and then an electrician or plumber will drill a large hole for an outside light or waste pipe later without anyone taping it. You need a realistic target to focus them and ensure corners aren't cut.

Lumen

Why does DCV require air tightness? It's typically just pulling air under the doors into wet rooms and out through the attic.

I guess if the structure is leaky the air will not be pulled from where you expect it to be (passive vents in dry rooms) but does that matter?

BryanF

Yes. For one, as per the reason you describe. Have a look at the government retrofit guidance.

Calahonda52

RaineyK wrote: »

Cheers for that! So basically the only way of knowing if the house will be adequately airtight is by measuring it after the builder does the necessary to seal areas around windows, doors, roof etc? Does a DVC system require some/any airtightness?

A/T is much more than that, unless its all in the etc: the entire perimeter needs to be airtight