Air supply issues/thoughts/plan

[HughF]

We’ve removed an open fire (in one room) and a gas fired back boiler in another. My wife wants a wood burner installed in each opening (I’ve tried to win the ‘take down chimney’ argument, and lost)…. All fine you would think.

 

However, I went on an air tightness drive when we did the knock through rear extension (one of the fireplaces is now in this larger room) and sealed as much as a I could up with tape and fm330.

 

The plan was always to buy stoves with external air input and draw from under the timber floor (we maintained ventilation to this floor void even though we built on the back with the extension) but I was working away when the builders did the floor and hearth, and there’s no pipework/vent through the hearth into the void.

 

1. Am I worrying about this for nothing and will the general air infiltration into the property (trickle vents on the windows, 1946 block/block rendered construction, triple glazed) be sufficient for 2x <5kW stoves?

 

2. If I’ve screwed this up, is there a concentric flu liner that I can install to provide combustion air from outside?

 

3. Failing 2, is it possible to draw combustion air from the cavity around the flue liner via a second opening in the register plate?

 

I don’t anticipate these stoves ever being lit, as we have a perfectly functional central heating system that I installed new about 18months ago that does a fine job of heating the place to 22-23 in the depths of winter.

 

But if they do (we have a power cut or a heat pump problem) then I’d like them to work safely.

Edited Thursday at 08:31 by HughF

[Mike]

1 hour ago, HughF said:

1. Am I worrying about this for nothing and will the general air infiltration into the property (trickle vents on the windows, 1946 block/block rendered construction, triple glazed) be sufficient for 2x <5kW stoves?

Well it would be against the Building Regulations - though you'd only be prosecuted if you survived the carbon monoxide poisoning.

 

You'd need a 'room sealed' stove installed in according to the manufacturer's guidance - I've only seen horizontal of under-floor supply pipes. The alternative is to knock a hole in the wall and install a permanently-open vent, in accordance with the dimensions in Part J of the Building Regs.

 

If it's only a decorative effect you want, block the flues, install a TV screen inside an old stove and set it to show a flame video.

[HughF]

The stoves are in the middle of the property, no way to knock through to an outside wall.

 

Stoves <5kW are assumed to be able to use the air in the room, no? That’s what I though the regs said.

 

Wife does want a backup heat source…. So they need to work. 
 

I guess the only option is a grill in the floor in front of the hearth, or core drill through the slate hearths and the concrete underneath, at an angle, to intercept the floor void 🤦‍♂️

Edited Thursday at 09:59 by HughF

[Temp]

I'm sure there are wood burning stoves on legs that are designed to draw external air in at the bottom/rear so perhaps you could drill through the slab at the back?

 

Try Clearview stoves.

[Tosh]

50 minutes ago, HughF said:

I guess the only option is a grill in the floor in front of the hearth, or core drill through the slate hearths and the concrete underneath, at an angle, to intercept the floor void 🤦‍♂️

tend to agree, my experience with wood burners is trickle events just don't allow enough supply air. Just opening the window slightly usually provides a near instant combustion improvement and kindoff proves the point. Something to think about if you're not having the fire lit much is whether there's a risk of mice in the floor void deciding to nest in the pipe? If so how will you provide a louvre or the like to prevent that?

[HughF]

1 hour ago, Temp said:

I'm sure there are wood burning stoves on legs that are designed to draw external air in at the bottom/rear so perhaps you could drill through the slab at the back?

 

Try Clearview stoves.

I’ve already got the stove with an external air supply inlet on the back, the ductwork up/down/left/right is no issue. The issue is where to take the external air supply from now that there’s no pipe installed in the hearth.

[HughF]

As I think it is unrealistic to core down through the hearth retrospectively and  get a duct into the floor void, I think we’ll have to settle for a grill/grating in front of the hearth.

 

It’s a real shame a concentric flue liner system isn’t an option.

[Alan Ambrose]

If you don’t have passivehouse levels of air tightness, I think you’ll have ample air supply as it is. If you want, have a blower test done - maybe with and without the trickle vents open.

 

We have a barn conversion done 7 year’s ago (not by me) with double glazing, trickle vents and (I assume) minimum BC standard insulation and air tightness. I put in a woodburner as I wanted it done right (I didn’t like the look of the standard flue hangers and had a custom SS plate bent up to fit). (BTW DIY is fine with BC involvement.) No external air supply - I leave one trickle vent open near the stove and that’s it. We have a nest combined smoke & CO alarm and, as I have asthma, I have a Davis particulate detector as a check. We use the stove a lot and everything is good.

 

Yeah in the new place we’ll be passivehouse-like insulation and air tightness … and therefore have external air supply for it.

Edited yesterday at 09:41 by Alan Ambrose

[Mike]

On 23/01/2025 at 10:57, HughF said:

Stoves <5kW are assumed to be able to use the air in the room, no? That’s what I though the regs said.

If you have a stove <5kW, and if it doesn't have a flue draft stabiliser, and if the design air permeability of your house is >5.0m3/{h.m2), and if the manufacturer doesn't say otherwise, then the Building Regs say you don't need a permanent vent. Though why they use the design air permeability instead of the actual air permeability, I don't know.

 

However you are planning to have more than one stove, so logic dictates that you'd need to add the outputs together and still be <5kW - which seems unlikely.

[G and J]

51 minutes ago, Mike said:

If you have a stove <5kW, and if it doesn't have a flue draft stabiliser, and if the design air permeability of your house is >5.0m3/{h.m2), and if the manufacturer doesn't say otherwise, then the Building Regs say you don't need a permanent vent. Though why they use the design air permeability instead of the actual air permeability, I don't know.

 

However you are planning to have more than one stove, so logic dictates that you'd need to add the outputs together and still be <5kW - which seems unlikely.

Dangerous thing, logic.  As is assuming that regulations are sensible.  I think you’ll find each stove in each room is viewed individually. 
 

On 23/01/2025 at 11:42, HughF said:

I’ve already got the stove with an external air supply inlet on the back, the ductwork up/down/left/right is no issue. The issue is where to take the external air supply from now that there’s no pipe installed in the hearth.

As you have said stove already why not hitch it up and see?  You can leave the air supply pipe open at the back. My guess is that you are likely to discover that it works just fine.  Mid century house with trickle vents I would guess won’t be that airtight.  
 

Alternatively, as a precursor to trying it you could test the draft on your flue.  Shut everything, everywhere, including all other flues/chimneys, ideally on a cold and windless day, and see what a smoke match does in the fireplace below the open flue. The warmth of your house should generate a gentle draft (hence cold and windless) which mimics least draft conditions. Betcha the smoke slips straight up that flue. 
 

However….

 

having two flues connected does give the opportunity for a strong draft in one sucking smoke down the other (as I’ve experienced). If you’ve bought fires that shut off the airflow well then it should be ok using one then shutting the other off.   I wouldn’t, however assume without investigation that you can use both at once. 
 

What stage are you at re flue liners?