Log burner underfloor air feed?

[dpmiller]

Is that inlet sitting above a flat roof? I'd be even more worried than normal about wind effects...

[Trw144]

44 minutes ago, Adsibob said:

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.

 

Difficult as you will presumably need someone to sign it off, as opposed to just finding a solutiuon that works. I would certainly advise oversizing the inlet ducting to keep resistance minimal. The issue as Drugasar say is avoiding the inlet acting in reverse (although scheidel seemingly don't see this as an issue)

[Adsibob]

2 hours ago, dpmiller said:

Is that inlet sitting above a flat roof? I'd be even more worried than normal about wind effects...

It's a pitched roof, but the drawing just shows a section cutting through the lowest point of the roof (ie the air inlet would come out the rear first floor wall, 10cm or 20cm above the lowest point of the sloped roof covering the ground floor extension). I thought wind would help stoke the fire, but if I'm wrong about that am I better routing the air supply tube under the floor to the side of the house, which is basically the alleyway between two houses (where each house has it's side passage access from the front of the house to the rear garden)? I had originally discounted that as a possibility because it's some 8m away and although there would be nothing blocking the air supply vent, it's fairly sheltered so I assumed it would get enough air. But you know what they say about assumptions ...

Edited April 15, 2021 by Adsibob

[Adsibob]

Further update: after a few days of vague info verging on pessimism from a leading stove supplier, I finally spoke to someone there who was more optimistic he could find a solution and hook me up with an experienced installer who would certify the installation. I will keep my fingers crossed he comes up with something soon, as builder wants to pour the concrete slab and start the floor build up next week, so if we do end up having to route the direct air supply under the FFL, i need to know asap!

[joe90]

Maybe a bit late but I bought my stove from “stovesonline “ and they have an in-house flue designer who was very helpfull for my installation.

[Adsibob]

On 15/04/2021 at 19:35, joe90 said:

Maybe a bit late but I bought my stove from “stovesonline “ and they have an in-house flue designer who was very helpfull for my installation.

Hi @joe90do you remember who you spoke to at Stovesonline? I've not had the same luck you have had.

We've settled on the Dik Guerts Bora as it meets our requirements and seems be quite efficient. Though I've not yet managed to find an independent review of it yet.  I asked the stove supplier to tell me what limitations there were in respect of the direct air supply and unfortunately they were pretty vague. They sent me the manual, but that was vague too. Impossible to adhere to the following guidance from HETAS with such a vague manual:

 

It is imperative at this stage for the installer to follow manufacturer specifications for the design of the air supply duct to the outside atmosphere, paying particular attention to the following areas:

• Minimum diameter or cross sectional area of the external air duct
• Maximum total length of the duct
• Maximum number of bends permitted
• Specification of the air inlet terminal

As the direct air supply for the Dik Guerts Bora comes in from underneath it,  the only option we really have is to have a 45 degree bend down and outwards, almost immediately after the pipe exists the stove, then about 75cm later have a 90 degree bend, followed by 7m of horizontal pipe under the floor and then another 45 degree bend and a rise of about 75cm upwards to bring the pipe above ground level just before it exits the house. So that's a total run of about 8.5m but with one 90 degree bend and two 45 degree bends. The vertical exhaust flue will have about 9.5m and no significant bends other than a dog leg to get around an RSJ. Even if I make the air supply 150cm in diameter, which I think I can just about do, the length and bendiness of it seems far from optimal in that surely the air supply should be significantly shorter than the exhaust, after adjusting for bends etc.

 

If @Trw144 or anyone else has any experience of the Dik Guerts Bora made by Drugasar, particularly on a direct air connection, please share.

[Adsibob]

I found the installation manual for the Dik Guerts Bora. Slightly worryingly things it states: "Prevent placing the outside air supply in an underpressure area on the outside of the house. If possible, place a supply pipe with T piece to 2 sides of the house" - does this mean i actually need my direct air supply to go in two opposite directions from the stove, so there is a current of air constantly travelling through this pipe with my Stove connected off that with a T junction? That changes things quite considerably. Has anybody done this, or is overkill?

 

I was also initially worried by this: "Please note that HETAS product approval remains valid for the appliance only when installed with its air supply taken from the room of installation and the room is ventilated permanently to outside air as necessary in accordance with the guidelines given in Approved Document J. HETAS product approval is not valid for appliances fitted with an external air supply ducted directly to the appliance." But having googled it, I now understand that HETAS is not a mandatory standard and that less and less modern stoves are on HETAS's approved list these days. Odd. 

[joe90]

I do find that second paragraph a bit worrying and would make me have second thoughts. The chap I spoke to was the owner and he was on this forums previous membership, but no longer. His wife was very helpful as well, perhaps it’s new management???. I would worry about taking your feed down and back up, any condensation or moisture will lie there and build up, luckily the ground outside my air feed is about a foot lower than the inside floor level. Never heard that about negative pressure area and 2 feeds from opposite sides of the house!!!and never read it anywhere.......

Edited May 2, 2021 by joe90

[ETC]

I would have thought a 4” duct through the external cavity wall into the room with the stove would be fine depending on the size of the stove. Just need to work out the ventilation requirement for the stove and ensure the duct gives sufficient air. If a 4” duct isn’t big enough go for a 6” duct. Need to fit a non-adjustable vent to both sides though.

[Adsibob]

1 hour ago, joe90 said:

I do find that second paragraph a bit worrying and would make me have second thoughts.

Well I’m definitely having second thoughts, but I do wonder if Hetas is just extra red tape. Ultimately what matters is whether the installation complies with building regs and whether the stove is 2022 Eco Design ready, which it is.

[joe90]

35 minutes ago, Adsibob said:

installation complies with building regs

Stove installation must be done/signed off by Hetas or the building inspector (who usually don’t know the regs regarding stoves. IMO). I installed mine and BC duly signed it off (saying “you’ve obviously done this before so it must be right” ?).

[CC45]

we put our own liner in & got HETAS chap to sign off for a few hundred. BC utterly not interested - complete waste of time.

[Adsibob]

So my saga to find an appropriate stove for a house with MVHR continues. I had settled on the Dik Guerts Bora Corner Stove. It is marketed as suitable for using with a direct air kit and after much persistence the importer was able to confirm that the air supply can be up to 11m long with up to 4 bends in it. That's plenty as we only 2 bends and 7.75m. So Stovesonline are pushing me to purchase it. I'm still prevaricating because it's so expensive and such a big decision; they reassure me that for extra peace of mind I can have a free survey with their recommended installers. So an engineer from a company called Bespoke Fire & Flue Services comes along to my house to survey it; it's all fine except he tells me that the DG Bora is not certified for use with MVHR!

I was initially rather annoyed but then started to doubt him as he said that actually "no stove is fully air sealed as you'll always get leakage when you open the door to light or refill it". I went back to Stovesonline to query this. They have now come back to me to say there are three levels of air tightness when it comes to stoves:

1. "DIP Test" ceritifed, which is a german standard adopted by passivehaus;
2. "100% direct air sealed" which is what the DG Bora is.
3. "direct air feed" which takes the majority of its air from the direct air supply, but not enough to be suitable for MVHR.

I have googled DIP Test and passivehaus and couldn't find anything. It's possible I misheard what Stovesonline were telling me. Maybe the dip it in water and test for air bubbles???

 

Anyway, stovesonline are telling me that:

• the DG Bora meets the second category 
• the reason Bespoke Fire & Flue Services didn't recognise the DG Bora as MVHR compliant is because they are HETAS certified and the DG Bora hasn't been tested to HETAS standards
• that doesn't mean the DG Bora wouldn't pass if tested, and they are still recommending it for me just that I should specify my MVHR system to run with some "positive pressure" in the room where I'm installing it.

So part of me wants to trust what Stovesonline is telling me, but part of me remains confused. Curious to hear other's experiences of air sealed stoves and MVHR. Did you go for the "DIP Test" certified one, whatever that is, or did you just go for a stove with an air supply. Do you have any issues with MVHR sucking smoke out of your stove when you open to light it or to refill it?

 

[dpmiller]

Dibt tested stoves are the gold standard. They've self closing doors, the doors are tested to seal after thousands of openings etc.

 

Remember that you do NOT have to use a HETAS installer, your BCO can sign it off.

 

It's not difficult to balance the MVHR to bring the house to positive pressure. Elsewhere in Europe room pressure monitoring is used to guarantee this/ alarm in case of issues as far as I recall from my research a while back.

[Trw144]

As @dpmiller says - Dibt is what you should be looking for as the standard the stove meets. Used to be quite uncommon but I would think quite a few manufacturers now have appliances that neet it.

[Adsibob]

Now that I understand there is a gold standard, that sort of makes sense.

I emphasis the “sort of” because now I’m confused about whether I should just stick with the Bora and make sure I’m running a mild positive pressure, and for a pressure alarm if necessary, or ditch the Bora completely and try and find a Dibt compliant stove that meets my needs. 

 

Generally it’s very frustrating to have specifically requested Stovesonline to recommend an MVHR compatible stove, and for them not to have mentioned this gold standard until now. It would have been nice to get the steer on that. Really feel like they’ve lead me down the garden path.  It is also confusing because this means that the majority of stoves marketed as direct air supply stoves suitable for use with MVHR are not.

 

[joe90]

My take on this is no house is 100% sealed, doors open and shut, people come and go. My woodburner is air feed from outside, yes when the door is open to start it and  wood is added it’s “leaking” but mine has never been a problem. I too like the idea of positive pressure from the MVHR and in summer i switch it off as we open windows and patio doors.

[dpmiller]

I think tho that it's only HETAS in their ar*e-covering wisdom that "require" Dibt tested. It's no surprise that only the very dearest Euro stoves have it.

 

I got a really snotty response from the local big stove place and they flat out refused to even quote me for a non Dibt unit for purchase at my own risk... Don't need the turnover evidently.

[jamieled]

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.

 

Thinking about this a bit more, I was wondering to what extent this is a problem just for mvhr, or if it's just related to ventilation generally? As others have said, no house is 100% airtight,  the mvhr replaces ventilation from trickle vents and leaks. I've been in older houses with no mvhr and in certain weather conditions the leakage from the stove is really obvious when you open the door, presumably to do with internal/external pressure difference. It's not exclusive to mvhr so the idea the stove needs to be mvhr compatible is a bit questionable. A high rated stove such as dibt might be required for passivehaus certification, but only to meet airtightness criteria.

[TonyT]

http://www.hetas.co.uk/wp-content/mediauploads/Ventilators.pdf
 

hetas guide to be vent, a few years old but you will get the idea

[Adsibob]

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, but it's not DibT certified, but maybe it doesn't need to be and this is just a lot of hot air from the HETAS engineer. He did try to scare me though, by saying "it's not just smoke that will leak, but carbon monoxide too". 

Now considering:

• Spartherm Cubo L

• Spartherm Seo S

• Rais 600 Max

• Rais Q-Be

all of which are DibT rated (i think!), but still need to find out if they are all Defra approved for smoke control zones (as I live in one).

Maybe I should just go with the Bora corner stove and set the MVHR with a bit of positive pressure and just find a non-HETAS approved installer.

[jamieled]

2 hours ago, Adsibob said:

"it's not just smoke that will leak, but carbon monoxide too". 

I'm pretty sure building regs require a hardwired carbon monoxide alarm within a certain distance of a solid fuel appliance (In scotland anyway). Ours has never gone off and we only have a direct air stove.

[joe90]

7 minutes ago, jamieled said:

I'm pretty sure building regs require a hardwired carbon monoxide alarm within a certain distance of a solid fuel appliance (In scotland anyway). Ours has never gone off and we only have a direct air stove.

My carbon monoxide is battery only and was passed by BC (In England)

[CC45]

same here.

[Adsibob]

The CO alarm point is a good one! Hadn’t thought Of that