Log burner underfloor air feed?

[SuperJohnG]

Just opening this one up again.  Has anyone done anything recently with a direct feed through an insulated slab foundation? 

 

Looking for any pics/ advice. 

[joe90]

Yes I have (sort of). I have UFH in a screed on top of PIR on a slab . Stove on an inside wall so used two 90’ bends down and back under the stove/hearth. 4” soil pipe in the bottom layer of PIR , out through the outside wall with grill on to stop rodents. I am sure you can do the same with an insulated slab as long as the pipes are put in before the slab is laid like the drains will be.

[Peter M]

We are at the point of preparing our lounge floor (block & beam) with sand blinding, DPM and 150mm insulation but I'm unsure what can or can't be under the glass hearth.

Will it be sufficient for the glass hearth to be placed directly onto the finish floor screed and have the insulation under the floor just like the rest of the floor area minus any ufh pipes? or should I create a hard shaped raised area to match the shape of the glass hearth in the same way as I did for the shower base in the master en-suite and build it up to the same height as the insulation and have the liquid screed float over the top?

My thinking is that if there is no insulation under the hearth the heat generated by the log burner will be partially soaked up by the cold floor area and give a cold spot in the corner of the room when/if the fire is not lit, is there a regulation that dictates what is or isn't beneath a log burner?

Also I have a question about ventilation for the log burner given the fact that we have block & beam, can I run the fresh an air feed via a grill ducted from below the floor and the into the void and can it be non forced air or is this dependant on the log burner that we ultimately go for which will probably be >5kwh.

Appreciate any answers that these two questions may generate.

[ProDave]

For a stove that only needs a 12mm hearth, I would insulate under it and put exactly the same floor finish as the rest of the room and just put the glass hearth on top.

 

It is okay to draw combustion air from a ventilated under floor space it does not need any form of fan.  Assuming you are not getting a glass hearth with a hole in it for the air duct, it is best to build a length of duct into the wall behind the stove to draw it's air from below.

 

 

[Roundtuit]

3 hours ago, ProDave said:

It is okay to draw combustion air from a ventilated under floor space it does not need any form of fan.  Assuming you are not getting a glass hearth with a hole in it for the air duct, it is best to build a length of duct into the wall behind the stove to draw it's air from below.

Worth checking with your BCO perhaps.  Our BCO wouldn't let me draw air from the ventilated under-floor space; I had to duct it from outside (with 110mm soil pipe) and up in to the bottom of the stove.  We insulated all the way around the pipe and below the stove with pir, then 50mm of liquid screed, and tiles on top. The stove just sits on the tiles with a glass hearth in front.

[readiescards]

3 years on, drawing combustion air from my ventilated under floor space works fine.  If I was doing it again I'd make the vent a bit bigger, as although I made it 150% (I think) over sized. I wonder if the tight right angle I'm make the air flow go through is slightly limited draw 

[joe90]

24 minutes ago, readiescards said:

3 years on, drawing combustion air from my ventilated under floor space works fine.  If I was doing it again I'd make the vent a bit bigger, as although I made it 150% (I think) over sized. I wonder if the tight right angle I'm make the air flow go through is slightly limited draw 

Funnily enough (but not funny) my stove does not draw well, I never have to throttle it back. My air feed is 100mm pipe under the concrete floor but I had to make a manifold to bring it up to the stove with two tight 90’ bends. The back of hand test feels a good draught but I am not convinced it’s enough as the stove has a good rep. I am tempted to take the stove out and create a long 180’ bend so there is no restriction. It’s on my “to do” list.

[ProDave]

@joe90 I believe your stove is ducted air in?  What diameter is that pipe?

 

Mine has an 80mm diameter inlet pipe and the duct down through the wall to the under floor void is the standard plastic extract fan ducting the flat rectangular one.

[joe90]

From the stove it’s about 90mm but I put a 110mm (drainage pipe) under the floor for 4M. The manifold goes from pipe to rectangular box then back to pipe.

[Adsibob]

I'm about to start on this journey and I'm realising now that I've left it a bit late. I need to finalise exactly where my vent pipe for the direct air is going to go.

 

My first question is: am i right in thinking that the longer the air feed pipe and the more bends in that pipe the worse the draw of the stove? I ask because i have three choice of vent route. My options are:

1. straight out the back of my stove and then an immediate 90 bend taking the flue above the FFL where it will need to be boxed in against the wall, and then out the wall some 5.5m later = one bend plus 5.5m of duct;
2. down into the floor and out again in same position as above = will require at least three bends plus 5.5m of duct;
3. same as 2 but wall is 7.5m away.

If number of bends and the addition of an extra 2m doesn't matter, than option 3 is both easier for me to install and aesthetically much more pleasing as the pipe would come out the side of our house to an alley where aesthetics are a low priority because we never really see that elevation of the house. If however option 3 is going to give suboptimal performance, then I may have to consider options 1 and 2, but this really is sub-optimal aesthetically. I hadn't appreciated the pipe needed a diameter of 100mm, that seems really rather big, and will create a real eye sore on my rear elevation, particularly if - as @joe90 says earlier on in this thread - the terminal of the pipe needs to be 600mm above the external floor level.

Earlier in this thread @Temp posted a helpful link (http://www.stovesonline.co.uk/direct-air-supply-stoves.html) but I think what would be even more helpful is any reviews of stoves installed (and used) with a direct air supply.

 

[PeterW]

 

4 hours ago, Adsibob said:

hadn't appreciated the pipe needed a diameter of 100mm, that seems really rather big, and will create a real eye sore on my rear elevation, particularly if - as @joe90 says earlier on in this thread - the terminal of the pipe needs to be 600mm above the external floor level.

If you’re building a new wall then just insert an air brick on a telescopic sleeve, then use an airbrick to round duct adapter below ground, and the whole thing will be hidden inside the wall.  
 

 

[joe90]

5 hours ago, Adsibob said:

if - as @joe90 says earlier on in this thread - the terminal of the pipe needs to be 600mm above the external floor level.

this is what I was told but because of poor “pull” on the stove I removed the upstand, it now exits just above path level with a rodent guard over it. Might be worth giving “stovesonline” a call, they were very helpful and have a dedicated tech guy who designs chimneys etc so will give a view on what’s best.

[SteamyTea]

6 hours ago, Adsibob said:

am i right in thinking that the longer the air feed pipe and the more bends in that pipe the worse the draw of the stove

Yes

 

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

[epsilonGreedy]

46 minutes ago, joe90 said:

this is what I was told but because of poor “pull” on the stove I removed the upstand, it now exits just above path level with a rodent guard over it. Might be worth giving “stovesonline” a call, they were very helpful and have a dedicated tech guy who designs chimneys etc so will give a view on what’s best.

 

 

I am trying to picture this setup.

 

Unless your FFL is very high it sounds as though the duct is u-shaped with a low point between the outside intake and the internal upstand feeding into the stove. Might a puddle of water be developing at the low point?

[Iceverge]

6 hours ago, Adsibob said:

I'm about to start on this journey and I'm realising now that I've left it a bit late. I need to finalise exactly where my vent pipe for the direct air is going to go.

 

My first question is: am i right in thinking that the longer the air feed pipe and the more bends in that pipe the worse the draw of the stove? I ask because i have three choice of vent route. My options are:

1. straight out the back of my stove and then an immediate 90 bend taking the flue above the FFL where it will need to be boxed in against the wall, and then out the wall some 5.5m later = one bend plus 5.5m of duct;
2. down into the floor and out again in same position as above = will require at least three bends plus 5.5m of duct;
3. same as 2 but wall is 7.5m away.

If number of bends and the addition of an extra 2m doesn't matter, than option 3 is both easier for me to install and aesthetically much more pleasing as the pipe would come out the side of our house to an alley where aesthetics are a low priority because we never really see that elevation of the house. If however option 3 is going to give suboptimal performance, then I may have to consider options 1 and 2, but this really is sub-optimal aesthetically. I hadn't appreciated the pipe needed a diameter of 100mm, that seems really rather big, and will create a real eye sore on my rear elevation, particularly if - as @joe90 says earlier on in this thread - the terminal of the pipe needs to be 600mm above the external floor level.

Earlier in this thread @Temp posted a helpful link (http://www.stovesonline.co.uk/direct-air-supply-stoves.html) but I think what would be even more helpful is any reviews of stoves installed (and used) with a direct air supply.

 

You could increase the diameter of the duct to compensate for the increased flow resistance. If there's space the price will be negligible. 

 

[dpmiller]

what kind of flue/chimney are you having? Would SwiftAir be a possibility? Takes wind direction out of the equation...

 

It's a smart system and I found it to be a no-brainer.

 

https://www.schiedel.com/uk/products/ceramic-system-chimneys/swift-air/

[Adsibob]

28 minutes ago, dpmiller said:

what kind of flue/chimney are you having? Would SwiftAir be a possibility? Takes wind direction out of the equation...

 

It's a smart system and I found it to be a no-brainer.

 

https://www.schiedel.com/uk/products/ceramic-system-chimneys/swift-air/

@dpmiller this sounds like a genius idea! I have contacted Schiedel and will report back with their comments. 

 

My architect also pointed out that our stove is actually only about 1m across and 2.7m up from the roof of our ground floor extension, so we could vent in the direct air supply from there with just one or possibly two u bends and a much shorter run of pipes. But that does expose my install to a wind direction issue, which as you suggest, the Schiedel would eliminate. Have you installed the Schiedel Swift Air? Any downsides of their system?

[joe90]

2 hours ago, epsilonGreedy said:

 

I am trying to picture this setup.

 

Unless your FFL is very high it sounds as though the duct is u-shaped with a low point between the outside intake and the internal upstand feeding into the stove. Might a puddle of water be developing at the low point?

No, the ground outside at this point (luckily) is quite low, the duct in within the 200mm floor insulation below a 70mm screed with UFH pipes in it. When I did have an upstand outside I drilled a drain hole at the lowest point in case of any moisture/condensation.

[dpmiller]

3 hours ago, Adsibob said:

@dpmiller this sounds like a genius idea! I have contacted Schiedel and will report back with their comments. 

 

My architect also pointed out that our stove is actually only about 1m across and 2.7m up from the roof of our ground floor extension, so we could vent in the direct air supply from there with just one or possibly two u bends and a much shorter run of pipes. But that does expose my install to a wind direction issue, which as you suggest, the Schiedel would eliminate. Have you installed the Schiedel Swift Air? Any downsides of their system?

 

Yes, one system at each end of the house even tho we'll likely only ever have one stove needing direct air. Whilst you can  build it stand-alone, ours is inside a standard block chimney. There's some piccys on my build thread.

 

[Adsibob]

So I've now heard back from Schiedel. The only option they can offer us for a standalone stove is to go for either a Sirius or a Sirius 3G. Both options described in the attached brochure. 

This is not really the look we had intended, though we are trying to see if it will work. The main issue is the stove is really quite high. But if it reviews well, we could probably be persuaded as it does solve a lot of issues for us, given how far our stove location is from an external wall to link up the direct air supply. @dpmiller I'm curious to know which stoves you opted for.

 

It's a shame Schiedel don't sell their Permeter Smooth Air Chimney technology for use with any direct air supply stove. Does anyone know of a competitor who does? @Trw144 do you know?

940003846_Sirius_Stove_System_Brochure_16-12-20.pdf

[Trw144]

1 hour ago, Adsibob said:

 

It's a shame Schiedel don't sell their Permeter Smooth Air Chimney technology for use with any direct air supply stove. Does anyone know of a competitor who does? @Trw144 do you know?

940003846_Sirius_Stove_System_Brochure_16-12-20.pdf 2.76 MB · 0 downloads

 

Poujoulait produce one but I believe it is only marketed for pellet stoves. Having used it though I always found the stove struggled more with the air trying to draw down the chimney - my personal view is its a long way to be drawing fresh air downwards. Haven't seen this system operate on a log stove though.

 

PS. I'm a schiedel customer so have asked what the situation is in terms of getting the flue without the stove.

Edited April 15, 2021 by Trw144

[dpmiller]

Well I've not ordered any stoves yet but will most likely be going with a Woodfire Passiv as the primary unit; at the other end of the house it'll probably be something LPG.

 

We're lucky in that both chimneys are on outside walls, it was easy to core through into the starter chamber at the bottom of the stack at the requisite height for the supply pipe.

[Adsibob]

1 hour ago, Trw144 said:

 

Poujoulait produce one but I believe it is only marketed for pellet stoves. Having used it though I always found the stove struggled more with the air trying to draw down the chimney - my personal view is its a long way to be drawing fresh air downwards. Haven't seen this system operate on a log stove though.

 

PS. I'm a schiedel customer so have asked what the situation is in terms of getting the flue without the stove.

Thanks @Trw144. Funnily enough we've just had a look at the poujoulait system. My gut reaction is that with either the Schiedel or the Poujoulait it seems crazy to draw direct air downwards over such a long distance, and that my original idea of taking direct air from the external wall accessed by going up one level was always going to be better, but unfortunately I've yet to find a stove manufacturer that will happily confirm their stove would work that way. E.g. here is the response the vendor of a Dik Guerts I'm looking at received from Drugasar (the Dik Geurts UK importer)

 

[Trw144]

12 minutes ago, Adsibob said:

Thanks @Trw144. Funnily enough we've just had a look at the poujoulait system. My gut reaction is that with either the Schiedel or the Poujoulait it seems crazy to draw direct air downwards over such a long distance, and that my original idea of taking direct air from the external wall accessed by going up one level was always going to be better, but unfortunately I've yet to find a stove manufacturer that will happily confirm their stove would work that way. E.g. here is the response the vendor of a Dik Guerts I'm looking at received from Drugasar (the Dik Geurts UK importer)

 

 

 

Do you have a sketch of what you are trying to do?

[Adsibob]

3 minutes ago, Trw144 said:

 

 

Do you have a sketch of what you are trying to do?

Yes, please see below. Since drawing this I've realised I need to either move the stove to the right so that it is closer to the Chimney, or create a new flue exit point for the exhaust, as this drawing shows the horizontal height of the exhaust flue that is diagonal as being more than 20% of the overall height of the exhaust flue, which i understand is not good. But the image helps show the general idea, particularly in respect of the (blue) direct air supply. Simple physics would suggest there should be no problem with having a supply of direct air coming from above if it is much smaller in height that the exhaust flue, but clearly Drugasar don't want to take the risk.