MVHR ducting through chimney stack? Should I move my boiler?

[Gene]

Hi,
 

First post here and with an incredibly stupid question, so please be kind.

I have bought a very cold, large, 1930s detached property in need of considerable love.  As part of this I'm intending to install MVHR; having little experience I'm probably going to employ a consultant to advise, but I'm some way from biting the bullet.

What I'm not far from is replacing my boiler as it is almost dead.  As the photos show the current boiler install is in the basement directly under one of the chimney stacks (please ignore the mess, I'm slowly removing the lath and plaster to check the state of the floorboards).

My very early ideas on installing the MVHR system is to run the ventilation ducting through the chimney stacks.  However I'm assuming that if I install a new boiler where the old one currently is that this will get in the way of the ducts.  Of course putting a new boiler in a new position is going to increase costs.

There are one of two answers here:

a.  Of course the natural place to put ducting is in the chimney.  You'll want to take this opportunity to move the boiler now or

b.  Installing ducts in a chimney stack?  What a nidicolous idea.  No one does that.  Put the boiler where you want.

I think the answer is a), but would love a second opinion.

 

[JohnMo]

7 minutes ago, Gene said:

very cold, large, 1930s detached property

 

Obvious first question, are you planning on getting the house pretty much airtight?

 

If yes crack on

If no, stop and have a think about it. What benefit will MVHR bring you? Would demand based MEV or dMEV be better alternatives?

[Nickfromwales]

Hi, and welcome!

As above, 

Firstly, what are you intentions with insulation and airtightness?
 

MVHR will be an expensive ornament if you’re not making the house airtight (and I don’t mean draught-proofing)  

 

Please give us more detail for the ‘bigger picture’ so the replies can be given to suit. 
 

Doors and windows swapped out and trickle vents being omitted?

 

Cavities being filled? 
 

EWI / IWI? 

[Mike]

Apart from considering the points above, yes, in principle you can put ducts through chimney stacks. However:

• The intake and exhaust terminals need to be separated by at least 1m horizontally, so you'd either need a large chimney, or two chimneys of similar height and exposure
• The intake would also need to be well away from any chimney that's still being used (or that might be reactivated as a chimney in the future), to avoid the obvious health risks

 

Edited April 11 by Mike

[Gene]

Thank you for the various welcomes and questions.  To get back to them.
 

On 11/04/2025 at 08:23, JohnMo said:

 

Obvious first question, are you planning on getting the house pretty much airtight?

 

So yes, I intend to improve the air tightness of my property.  It being a retrofit (and to a 1930s house as well) clearly I won't be able to go full PassivHaus; but I'd like to aim towards for one of the retrofit standards (e.g. EnerPHit or AECB).  I plan to hire a consultant to advise on some of this  (e.g. someone like https://www.21degrees.com/).  I must admit I was also quite taken by The Case for MVHR report by the PassivHaus Trust which suggested that MVHR is still useful even if you don't get all the way with air tightness....
 

 

On 11/04/2025 at 08:26, Nickfromwales said:

Hi, and welcome!

As above, 

Firstly, what are you intentions with insulation and airtightness?

...

Doors and windows swapped out and trickle vents being omitted?

 

Cavities being filled? 
 

EWI / IWI? 

As above, yes plan to look at more general insulation as well as air tightness.
 

 

On 11/04/2025 at 21:05, Mike said:

Apart from considering the points above, yes, in principle you can put ducts through chimney stacks. However:

• The intake and exhaust terminals need to be separated by at least 1m horizontally, so you'd either need a large chimney, or two chimneys of similar height and exposure
• The intake would also need to be well away from any chimney that's still being used (or that might be reactivated as a chimney in the future), to avoid the obvious health risks

 

Ah ok, this is a good point.  I must admit I wasn't thinking about the intake and exhaust terminals being via the chimneys.  I have a large half submerged (i.e. it has windows and light, but half is below ground level) basement - that's where the boiler in photo above currently is.  I plan to put the actual MVHR unit in the basement and my very initial thought was that the intake and exhaust would be at ground level - but I'd want to get professional advice on that.  I have three chimney stacks - two on the East wall and one on the West.  Two of the stacks have been completely covered over - i.e. you wouldn't know they were there apart from an air vent (and stack outline as part of the wall) - that's the situation of the stack directly above the boiler currently.  One of the stacks (South-East) has one of the original fireplaces at ground level with a (non-working) gas fire - very long term I'm thinking maybe a wood burning stove - with fully independent air supply and completely sealed for MVHR purposes - but that's mostly as the fireplace is genuinely too lovely to scrap.  Either that - or more likely when I run out of money - just put a nice bunch of flowers in there..... 

But anyway to get back to the point, I see the stacks as more for simply running ducting through as opposed to intake / exhaust.  Just kind of makes sense seen as I have large void spaces running through the house to put some of the ducting through there as opposed to punching new holes places there weren't previously holes.  But posting here in case just in case I'm being stupid about something.  Very obviously better to think about where to place the boiler now as opposed to later....

NB - Seen as you might ask, currently the basement is outside of the thermal envelope (though honestly using the term thermal envelope on any part of this house currently is something of a joke) - but which I mean it was deliberately intended to have lots of ventilation holes etc.  The plan would be to include the basement in the thermal envelope if I do go the MVHR route - though again will take advise when I get nearer that.

 

Edited April 13 by Gene

[Mike]

Your overall approach is good

 

4 hours ago, Gene said:

I see the stacks as more for simply running ducting through as opposed to intake / exhaust

Then unless you are choosing external wall insulation, using the chimney isn't a good idea as they will be outside your thermal envelope. You'd be loosing the heat before the air was delivered - so cold supply air, and little heat to recover from the extract. Unless, that is, your chimneys are really big - big enough to install really well insulated ducting.

 

4 hours ago, Gene said:

very long term I'm thinking maybe a wood burning stove ...  or more likely when I run out of money - just put a nice bunch of flowers in there..... 

I'd settle on the bunch of flowers. Or a TV screen the displays a roaring fire.

 

[Gene]

6 hours ago, Mike said:

 

Then unless you are choosing external wall insulation, using the chimney isn't a good idea as they will be outside your thermal envelope. You'd be loosing the heat before the air was delivered - so cold supply air, and little heat to recover from the extract. Unless, that is, your chimneys are really big - big enough to install really well insulated ducting.

 

That is a really good point actually, I hadn't really thought of the ducting itself in relation to the thermal envelope.  My plan was to put some sort of fill in the chimney too (eg. https://www.leca.co.uk/products/Chimney-Fill) just as a general insulation, but how well that would surround multiple ducting pipes I don't know....