Log burner underfloor air feed?

[Omi]

On 22/05/2021 at 17:41, Adsibob said:

I'm still on the fence. I've fallen in love with the Bora corner stove and had managed to get the manufacturer to sign off on the length and layout of our rather long air supply pipe

 

 @Adsibob - wondering where you eventually got to in this saga? We do not have the MVHR related issues but are rather confused around how best to route a direct supply line from the stove to the outside.

 

Out stove will sit against an internal wall so a duct directly though the wall wouldn't be an option as seen in the floor plan below:

 

 

My first thought was to take the ducting up the void of the internal wall and then running it along the joists to an external vent. The flue is at a height of 8.2 m while the vent would be at a height of 2.6 m. However, reading some of the previous comments, this may be problematic? Although this seems to suggest that it should be okay: https://www.stovesonline.co.uk/stove-ventilation.html (See "Positioning ventilation for a stove"). But maybe that only applies when the whole room is vented rather than ducted?

 

The other alternative I understand is to run a duct in the floor but I'm not entirely sure how this would work? The floor construction we have is:

 

 

If we run the duct along/under the floor slab, where does it come up again? Would it be inside the house itself, before being ducted out to the external wall?

 

Obviously Option 1 (ext. vent via joists) would be preferable due to simplicity/flexibility but if we must go with option 2, I'd like to understand how it works as it would have to be built into the construction plans pretty early on.

 

Thanks!

 

Edited June 18, 2021 by Omi

[Adsibob]

Hi @Omi. This was probably one of the things in our project that I ended up researching the most amount of time into after we had already started breaking ground, having really been naive about the complexities of combining MVHR with a stove that is not anywhere close to an external wall.

We haven't installed our stove yet, but we have pretty much confirmed our installation plan, although I haven't actually got an installer to confirm formally it's all okay, just various oral opinions that sounded very encouraging from installers who had plenty of experience.

 

In essence, what I've learnt is this:

1. If you have to have a long air supply pipe, it's all the more important that your exhaust flue is set up as efficiently as possible. One of the changes we made to improve our set-up was to bring the location of the stove closer to the centre of the house so that it was almost directly under the chimney that we want to use to vent out through two storeys above. This means our exhaust flue is about 10.5m long and almost perfectly straight, with just two minor dog legs. The installer who has looked at that positioning thinks that will "pull like a train".
2. Unless you go with a specialist product like Schiedel (mentioned by @dpmiller above) or Poujoulait (mentioned by @Trw144) no manufacturer is going to confirm in writing that it's okay to have vertical air supply pipe drawing air from above the location of the stove. They are basically freaking out that the air supply pipe might end up becoming an exhaust rather than an intake, even though simple laws of thermodynamics suggest that a 2m supply pipe will be overpowered by the much more efficiently positions 10.5m flue I was proposing. But hey-ho, I'm not going to bang my head against a wall repeatedly for the sake of it, so I had to look for another solution. 
3. Many stoves will actually have the recommended maximum length and number of bends for an air supply pipe somewhere in the  installation manual, but... there will also be some contradictory info like "HETAS approval only valid when installed without direct air feed". Ultimately, you don't need HETAS approval. The one we almost certainly going to go with, the DG Bora corner stove, says it can be up to 11m long and have 4 bends in it. I'm not going to push it that far. We are currently thinking of going straight out the bottom of the stove, then bending once towards a wooden step we are building between our lower ground floor and upper ground floor, going into that step and bending again so as to take the pipe to the side of the house. We have to bend a third time to get out above ground level, but whereas the first two bends are 90 degrees each, the last bend will be somewhere between 30 and 45 degrees. In total, this works out at about 6.75m I think, possibly as much as 7.5m. Can't remember.
4. I was very tempted by the additional peace of mind given by a DIBT certified stove, but... I'm probably not going to go with one. I have found some nice looking ones which are similar in price to the DG Bora, but none that have the corner glass that the Bora has. Ultimately, I put the question to our MVHR company, CVC Direct, and they confirmed that it doesn't have to be a DiBT certified stove, that they install MVHR with wood burning stoves a lot and that it's not a problem. They also said that worst case scenario they can set it up so there is a very slight positive pressure in the room where the stove is. Whether you have a DIBT stove or not, there is always going to a break in the seal when you open the stove door to light or refuel. I don't need to buy my stove for another couple of weeks, so if anybody can recommend a DIBT corner stove that works as well aesthetically for a corner as the DG Bora corner stove, please let me know as I would prefer DIBT, I just realise that it is not a "must".
5. Try and speak to several installers until you find one who will work with you in trying to recommend something that they can happily sign off on and will work for you. This is harder than it sounds. I've spoken to four different companies, and only two of them sound like they know enough about this. Of those two, one is extortionately expensive and the other one doesn't return my calls because he is so busy and booked up!

Returning to your specific situation, depending on the kW rating of your stove, I think you will probably want to use a 110mm soil pipe, lots here favour that size and it's also safer to upsize if you are concerned about sufficient air. That means you have two options I think, either:

• you have a cold part of your floor where instead of UFH you have your air vent. This would mean having slightly less insulation under that as well, so you will need to check with your SE that structurally that will still be okay and wouldn't cause any issues - might be tricky;
• you go further down and through your concrete slab - again your SE will need to sign off on this, but should be okay i imagine if you add some extra steel rods under and over the pipe, perpendicular to it.

I don't think going down any further is feasible as the more you go down, the more you end up with a deeper U shape which the installer probably won't like as not as efficient at the other end when you have to come up; and then there is also the issue of having to go through the DPM, which is definitely do-able, but if not done properly risks damp issues. Of the two options above, I'd be leaning towards going through the slab, just get the SE to spec how it should be done.

 

I'm sure others will have views, as I'm really a complete novice having only just planned my install and not yet executed it.  

 

Suffice to say that if you do go down, you save yourself a bend if you install a stove that can already be vented from underneath.

 

Oh, and I'm sure that the link you posted isn't talking about a direct air feed situation. It is just talking about a regular installation, not a direct air supply installation.

 

Keep us posted with what you end up doing, as will be helpful to compare notes.

[Omi]

@Adsibob - thanks for the very detailed response. Beggars belief that balanced flue systems like the Schiedel or Poujoulait with similar lengths of intake/exhaust are seen to be okay but not something with a shorter vertical run that you had been talking about. Out stove is situated directly underneath the flue so at least that part will be a nice straight vertical run.

 

Think I'll need to call around and start speaking to some installers to find one willing to work with our constraints. Really hoping to avoid further SE changes as we're expecting our building warrant in the next couple of weeks with a view of breaking ground towards end of July.

 

[Omi]

Having spoken to two HETAS installers over the phone, the vertical duct doesn't seem to be much of an issue - at least in this part of the country!

 

I proposed the following which has a flat channel duct going up to the joists and then connecting to a fairly long 100 mm duct out to an external wall vent on the rear elevation:

 

 

I asked at least 3 times to confirm. The supplier we went to see in person didn't bat an eyelid. He's happy as long as there is a way to get fresh air to the stove which will be a Burley 5 kW room sealed unit. There's still quite some time to go before we're ready to go ahead with the install but at least have a quote in place.

 

[Adsibob]

@Omi you are lucky to have found an installer that will do this. Common sense suggests it should work, but I not that the installation manual for the Burley does not provide for your suggested solution as an option: https://burley.co.uk/wp-content/uploads/2019/02/9103-9112-Burley-Wood-Burner-Installation-Instructions-JANUARY-2019.pdf (pages 9-10).

[SteamyTea]

@Adsibob

You are in London right?

Are you not concerned about the legislation changing that will ban woodburners.  The currently updated legislation is pointing in that direction already.

[Adsibob]

25 minutes ago, SteamyTea said:

@Adsibob

You are in London right?

Are you not concerned about the legislation changing that will ban woodburners.  The currently updated legislation is pointing in that direction already.

I am in London yes. My understanding is that when the legislation changes it will be prospective, not retrospective. Any idea when the new law is due to come into force?

[Omi]

14 hours ago, Adsibob said:

I note that the installation manual for the Burley does not provide for your suggested solution as an option: https://burley.co.uk/wp-content/uploads/2019/02/9103-9112-Burley-Wood-Burner-Installation-Instructions-JANUARY-2019.pdf (pages 9-10).

 

I had noticed that but it also doesn't explicitly prohibit it ?. Not going to keep pressing the point if the installer (who is Burley authorised) is happy.

[SteamyTea]

6 hours ago, Adsibob said:

I am in London yes. My understanding is that when the legislation changes it will be prospective, not retrospective. Any idea when the new law is due to come into force?

I would not count on that, cars that were legal on emissions a few years back are now effectively banned.

Can't comment on government future timetables, if I could I would write a cheque and get you all the labour and materials you wanted.

[Adsibob]

3 hours ago, SteamyTea said:

I would not count on that, cars that were legal on emissions a few years back are now effectively banned.

Can't comment on government future timetables, if I could I would write a cheque and get you all the labour and materials you wanted.

There is no chance in hell that they will simply outlaw all stoves regardless of when they were installed. The industry would die overnight and there would be many claims for judicial review on the basis of a legitimate expectation that law Is not enacted with retrospectives effect. We are going for an extremely efficient drive that complied with the 2022 EU requirements.

[Bob77]

This is an interesting thread. I'm planning to install an inset woodburner to replace the existing freestanding stove on what is currently an external wall. An extension will be added on the other side of that wall so there will be about 4 metres from the stove to the outside. The stove has a combustion air inlet on the underside which can either be supplied by vents inside or from external air.

 

Although my house is nowhere near "passive" it seems like it would be wise to supply air from outside rather than having it sucking in cold air through all the gaps. So the obvious solution seems to be to put a supply pipe underneath the floor of the extension with a vent on the new external wall. However some of the comments above suggest that having what would effectively be a "U-shape" (with the inlet above ground level, then dipping down under the new floor and then rising up to the stove) could cause problems with condensation. Does this need to be taken into account? 

[joe90]

I ran a 4 metre underfloor air feed pipe (110mm drain pipe) within the insulation layer but luckily the outside ground level was low enough to not have a “u” section, in fact I sloped it down towards the outside in case any moisture found it’s way in there.

[Bob77]

50 minutes ago, joe90 said:

I ran a 4 metre underfloor air feed pipe (110mm drain pipe) within the insulation layer but luckily the outside ground level was low enough to not have a “u” section, in fact I sloped it down towards the outside in case any moisture found it’s way in there.

I am on fairly level ground, there is a small slope but if the air brick/vent is meant to be 600mm above ground then no chance! With a vent or air brick low down to the ground I would be worried about it getting blocked with dirt or leaves etc being blown up against the wall. (Or snow, if we ever get that stuff again!)

 

I will ask the fireplace supplier what they suggest. Want to get it straight in my head and on the plans before the floor goes down!

Edited May 31, 2022 by Bob77

[joe90]

4 minutes ago, Bob77 said:

but if the air brick/vent is meant to be 600mm above ground then no chance!

 

5 minutes ago, Bob77 said:

but if the air brick/vent is meant to be 600mm above ground then no chance! With a vent or air brick low down to the ground I would be worried about it getting blocked

Funnily enough I was told about the 600mm but mine is only 100mm and i Put a grill on it (to stop mice and leaves and it’s fine), yes you have to be careful about snow I guess (if it ever happens again with global warming 🤷‍♂️

[SteamyTea]

1 hour ago, joe90 said:

yes you have to be careful about snow I guess (if it ever happens again with global warming

Going to be more likely in some places.

[joe90]

23 minutes ago, SteamyTea said:

Going to be more likely in some places.

Well you could always use a 90’ bend and turn it upwards 600mm as long as you drill a drain hole in the base to drain any  moisture

[JohnMo]

That's what I will do with mine is there are any complaints. My duct actually terminates 400mm below DPC level but about 200mm above outside ground level, I will have decking above it. It is an L shaped duct the horizontal flows slightly downwards to the outside.

[Adsibob]

Finally had my stove installed a couple of weeks ago. Seems to work very well. Our air supply pipe enters the house on the side, about 400mm from ground level, then bends downwards at a 40 degree for about 60cm, then it runs straight and level for about 8m then it does a right angle turn to the left and slopes up a bit at maybe 30 degrees for about 50cm to where it connects into the underside of my stove. Probably not the most efficient design, but probably also the only one we could achieve without trying something much more experimental. Ultimately as long as the length of your exhaust is materially longer than your supply pipe and you don’t have too many bends (I think more than 3 is too many) you shouldn’t have any problems.

[SteamyTea]

1 hour ago, Adsibob said:

Ultimately as long as the length of your exhaust is materially longer than your supply pipe and you don’t have too many bends (I think more than 3 is too many) you shouldn’t have any problems.

Not sure if it is as simple as just pipe length once adjusted for bends.

The inlet air density will be pretty constant until it reaches the combustion chamber, the density is then greatly reduced, but as it exits the chamber it rapidly cools and becomes denser again.  Cool it too much and it does not flow i.e. exhaust gases at same density as incoming air.

There will also be an optimal inlet flow rate for maximum thermodynamic efficiency.

 

 

Bends and elbows

https://www.engineeringtoolbox.com/minor-loss-air-ducts-fittings-d_208.html

[Adsibob]

11 hours ago, SteamyTea said:

Not sure if it is as simple as just pipe length once adjusted for bends.

The inlet air density will be pretty constant until it reaches the combustion chamber, the density is then greatly reduced, but as it exits the chamber it rapidly cools and becomes denser again.  Cool it too much and it does not flow i.e. exhaust gases at same density as incoming air.

There will also be an optimal inlet flow rate for maximum thermodynamic efficiency.

 

 

Bends and elbows

https://www.engineeringtoolbox.com/minor-loss-air-ducts-fittings-d_208.html

You are of course correct to point out these subtleties. But the reality is that this is quite tricky to model accurately, so a certain amount of finger in the air/rule of thumb can be helpful. It will also depend on stove selection. Mine has a comment in the manual that said not to exceed four bends, so to be safe I went for no more than three. And my exhaust is virtually straight up and is about 11m long, so will have a greater draw than a pipe that is around 90% of that and mostly horizontal.

Edited June 1, 2022 by Adsibob

[SteamyTea]

12 minutes ago, Adsibob said:

Mine has a comment in the manual that said not to exceed four bends

Any mention of minimum combustible material amounts.

A small burn will cool quicker in the flue, so draw less air into the chamber, causing an even more inefficient burn (running rich in automotive terms).

This may be part of the reason that it is recommended to have a short, hot burn. Quite the opposite from what people actually want, sitting around the camp fire discussing the merits of red wine over white wine, or a vegan diet over a lamb dhansak.

[Adsibob]

3 hours ago, SteamyTea said:

Any mention of minimum combustible material amounts.

A small burn will cool quicker in the flue, so draw less air into the chamber, causing an even more inefficient burn (running rich in automotive terms).

This may be part of the reason that it is recommended to have a short, hot burn. Quite the opposite from what people actually want, sitting around the camp fire discussing the merits of red wine over white wine, or a vegan diet over a lamb dhansak.

No, nothing in the manual but the guys at Stovesonline advised us to undersize our stove so that we can run it on max, which I think is good advice.

Edited June 1, 2022 by Adsibob

[markocosic]

FWIW triple wall exists to ease this issue.

 

 

[Hastings]

On 22/05/2021 at 10:11, jamieled said:

We've got one with just a direct air supply to the stove. Also have mvhr.

 

Mvhr commissioned to ensure a bit of positive pressurisation (essentially supply fan speed higher than extract). No obvious smoke leakage when the stove door opens.

 

Did you not go with a room-sealed stove because the house is not particularly airtight?

 

Only ask because I have almost completed an MVHR/airtight build only to discover  too late that the direct-air stove I bought (Morso 3116) is not suitable for an airtight building because the air supply kit does not fully seal around the stove, effectively introducing a permanently open vent to the outside.

[ProDave]

2 minutes ago, Hastings said:

Only ask because I have almost completed an MVHR/airtight build only to discover  too late that the direct-air stove I bought (Morso 3116) is not suitable for an airtight building because the air supply kit does not fully seal around the stove, effectively introducing a permanently open vent to the outside.

Some stoves are not fully room sealed and only take primary or secondary air from the duct, not both.  Our Mendip Stoves Churchill is properly room sealed.  Not a good time to find out after you already have the stove.