External air ventilation for log burner

[SBMS]

Hi all

 

Currently debating whether to fit a log burner in our self build. We’ve got an external chimney so seems a shame not to use it and would be useful if we get blackouts! We are going for high levels of air tightness, good insulation, ASHP and are having MVHR. 
 

So we would need an external air ventilation source which, as I understand it, is a 4 or 6 inch pipe that goes out the back of the stove through the wall and cavity to the outside, to give the airflow required to feed the burner. 
 

My question is, does anyone know to what extent this compromises the thermal

efficiency of the building? Is it a cold bridge? And could it be a draught source and compromise air tightness?

[ProDave]

My stove (Mendip Stoves Churchill 5) the air intake is about 80mm 

 

A vent through the wall might be a bit of a thermal bridge but that will lose a LOT less heat than the only other alternative which is having a similar vent into the room to allow air in which will massively destroy your air tightness.

 

The pipe out, I used plastic, converting to flexible aluminium from the wall to the stove inlet.

[Gus Potter]

45 minutes ago, SBMS said:

My question is, does anyone know to what extent this compromises the thermal

efficiency of the building?

Ok on paper if you have a floor vent directy under the stove that is not connected to the stove air inlet and you do an air test.. it will contribute to a bad result.

 

Yes.. I acknowledge that folk want to have an air tight house and agonize about having a stove that is "room sealed" like a gas balcanced flue boiler.

 

But if this is your forever home you need to be aware that unless you want to change the door seals on your stove yearly you'll end up with a "stuffy room". Your strive for air tighness could come back and bite you!

 

For those that are concerned about PM 2.5 levels this is the last thing you want and for those that want an easily maintained house it is also the last thing you need.

 

If you don't create a negative pressure near your stove then these PM 2.5s are well into the house! Buy the time they get filtered out by any other system you'll have breathed a lot of them in.

 

You may have some semi automation or internet connected central heating thermostats that need some wireless control.  The idea of a stove is often to give you a "Flintstone" backup when all your electronics/ phone apps break down. This is not the time for "posing" it's a time for praticalities and thinking.. what if the gas / electricity goes off and god forbid.. my phone stops working!

 

Keep it simple and stupid.

 

Think about it.. are you really going to do this (change the stove door seals every year) with all the other things in life you need to do.

 

What I have is a duct from under the floor that rises directly under the stove.. there is a ~75mm gap. When the stove is working cold air gets sucked directly into the stove bottom air intake. When not it provides trickle venitlation.. in the summer. If the door leaks a bit during use there is some positive suction that stops the PM 2.5 from getting into the room as they are sucked mostly back into the intake flow by the Venturie effect.

 

I would encourage all to think about this in the round.. how you are going to live and use your home rather than blindly following the air tight current doctrine.

 

In summary it's your house so make it suit your and your way of living.

 

 

 

 

 

 

 

 

[SBMS]

 

4 minutes ago, Gus Potter said:

unless you want to change the door seals on your stove yearly you'll end up with a "stuffy room".

Thanks Gus but can you clarify what you mean by a stuffy room? We are planning on having MVHR which should be filtering and exchanging air throughout the house; and a direct air system for the wood burner would allow the burner to effectively be a sealed unit? But appreciate what you might be saying about how good the seals are. 
 

A wider question: would anyone advocate NOT getting a wood burner in a new build (I would like it as a ‘backup’ heat source; Mrs would like it for aesthetics 🤷)

[ProDave]

15 minutes ago, SBMS said:

A wider question: would anyone advocate NOT getting a wood burner in a new build (I would like it as a ‘backup’ heat source; Mrs would like it for aesthetics 🤷)

Plenty here will advise against a WBS. It is a very marmite subject.

 

In use, our stove draws very well, I don't personally believe there is any risk or particulates getting into the room.  The flue  draws so well I am sure the pressure inside the stove is less than room pressure.

[Gus Potter]

33 minutes ago, SBMS said:

Thanks Gus but can you clarify what you mean by a stuffy room?

Stuffy room.. You light the stove.. big flames.. warm the room up.. then you turn it down to ticking over. After a while you'll notice that the air in the room does not "smell fresh".. very difficult to desrcibe but you just know that the air in the room has changed since you lit the stove. Your nose is very sensitive so trust it.

 

Another way is to think of this. If you are a parent with a young child and you go into their bedroom.. instinctively you know if the air is bad.. "just not right". You open a window for example. That is the test.. it's in our genes as parents.

 

Yes MHVR is great but I have Clients that send their kids to "outdoor schools" see my point? And no amount of technology is going to over ride a mother's instinct  in terms of air quality.

 

 

33 minutes ago, SBMS said:

A wider question: would anyone advocate NOT getting a wood burner in a new build

That is a diffucult question. For me having grown up in the country and abroad (Africa) I'm not that keen on relying on the gas / electric company / modern interconnected electronics to save the day in the short term when the shops are shut for a few days.

 

At least with a stove you could keep one room warm for your kids, boil a kettle and cook a meal on the top in the short term.

 

I'm not a "survivalist" say like in the US but have lived in the borders of Scotland where the power used to go off for days.

 

I admire what you are trying to achieve and hope this helps you to reach a conclusion that works for you.

 

 

 

 

 

 

 

 

 

Edited October 31, 2022 by Gus Potter

[Gus Potter]

1 hour ago, SBMS said:

Currently debating

Just edited my post to lay it on a bit thicker.

[Roundtuit]

We've got a 'room sealed' wood burner for supplementary heating.  The air supply is ducted in under the floor insulation. Cold bridge?  In theory, yes, when it's not lit, there will be cold air passing through it, but nothing that is noticeable.  Stuffy air?  Nope.  Mvhr does its thing, stove sucks what it needs from outdoors.

[saveasteading]

A year ago i was of the  view to have an air duct close to, but not connected  to, our proposed wbs. That would supply the air needed, without any room draughts, and with a standard wbs. But further thought peruaded me that direct connection provides much more control.    Also since then, many more manufacturers have offered direct connection. So that is what we shall have.

 

Experience of wbs in current home is that spmetimes it needs more air, and we have to open a door.  In reality, the room is stuffy and it is us who are short of oxygen or have too much smoke getting back.

 

Nasty gas can get back into the room, despite the seals and air flow. Proof comes when burning something that we shouldn't. Old bits of chipboard for example, when a nasty smell shows that the smoke is leaking into the room.

 

How much heat lost up the chimney when not burning?  With the direct air connnection there won't be room air going straight out. There willl however be heat transfer through the steel and up the chimney. Perhaps we need a stove cosey.

 

In my self centred opinion, it is ok for us to burn wood in the countryside, but not for anyone in the town. 

 

 

 

[dpmiller]

it's not difficult to set the MVHR up to hold the house at slightly positive pressure which will deal with any possible leaks around the stove.

[JohnMo]

We have balanced MVHR, WBS with an external air vent connected to to the stove.  Would have thought a disconnected air supply, is just an air leak in to house, not really understanding why you want it in an airtight house, with MVHR.  But everyone to there own thoughts on that.

 

Never experienced anything stuffy in the house (with or without stove on), have baked myself when we first tried the stove with room temperatures at over 27.  Buts its just a matter of getting the stove burning, very little air and not much wood, to controls heat output.

 

Don't select a stove with a high output.  A house that requires circa 3kW to heat at -5, doesn't need a stove that puts out 16kW for example.

 

When stove is off, just select air supply off, so no un-necessary air movements.  Simple

 

 

[saveasteading]

Looking at Charnwood 5kW. That looks to be the 'designed' size with the rest bring variation to some extent.  Have used smaller in rental properties and they are a pain to keep going, and fit standard logs into.

This is available quickly in black, and with some discount (thankfully as it is still expensive).

 

Re seals. They are easy and cheap to replace. A bit of glassfibre string in a clip of groove.

[ProDave]

9 hours ago, Gus Potter said:

Stuffy room.. You light the stove.. big flames.. warm the room up.. then you turn it down to ticking over. After a while you'll notice that the air in the room does not "smell fresh".. very difficult to desrcibe but you just know that the air in the room has changed since you lit the stove. Your nose is very sensitive so trust it.

If that happens in your house, there is something wrong with your stove.  It does not happen here.  Don't assume all stoves do this.

[JohnMo]

@Gus PotterDo you CO detector in the room - if you don't you should be looking to install

[SBMS]

30 minutes ago, JohnMo said:

We have balanced MVHR, WBS with an external air vent connected to to the stove.  Would have thought a disconnected air supply, is just an air leak in to house, not really understanding why you want it in an airtight house, with MVHR. 

Is this a direct air system (pipe through the wall connected to the burner, providing fresh air)?  Did you consider or calculate any losses with thermal bridging, or with any movement of cold air from the outside to the inside (of the burner)? I don't know if I am over thinking, but have been fastidious about making sure there's hardly any external penetrations and adding an 80mm one with fresh air from outside seems counter-intuitive?

 

"When stove is off, just select air supply off, so no un-necessary air movements." - is this a feature of your WBS or something on the exterior wall where your air vent meets the world?

[SteamyTea]

Room sealed or not, airtight house or not, putting a WBS in your house is going to cause a change in air quality.  I don't think the seals, even when new, are that brilliant.

Ask yourself if you are happy to invite me around to smoke a packet of Marlboro in your living room one evening.  That is about 16gm of combustible material, WBS will take several kilos.

 

Why not fit a WBS with a back boiler outside the house, then pipe in some hot water to an emitter.  Then when the power cuts come, fire it up, let is thermosyphon and keep you warm.

[SBMS]

8 minutes ago, SteamyTea said:

Room sealed or not, airtight house or not, putting a WBS in your house is going to cause a change in air quality.  I don't think the seals, even when new, are that brilliant.

Ask yourself if you are happy to invite me around to smoke a packet of Marlboro in your living room one evening.  That is about 16gm of combustible material, WBS will take several kilos.

 

Why not fit a WBS with a back boiler outside the house, then pipe in some hot water to an emitter.  Then when the power cuts come, fire it up, let is thermosyphon and keep you warm.

I did wonder about the air quality @SteamyTea, but also wondered how good the MVHR might be at filtering the air - and wonder what real-term reduction in air quality there is with a 2022 EcoDesign Ready burner that is designed to reburn particulates etc etc.  Also, with as close a room sealed system as possible, bar opening the door to stick another log on, is there much leakage into the house from the burner?

 

It would be for a secondary, intermittent heating source (on those really cold nights) - or when they've got a 3 hour rolling black out 🙄

Edited November 1, 2022 by SBMS

[Conor]

If you are building a well insulated airtight house, you can easily go a day or two with no heating before the house starts being uncomfortably cool. So I don't buy the "backup" argument as I cannot recall ever being without power for more than a few hours at a time. Do not miss a stove at all.

[joe90]

I have a woodburner, live in the countryside , external air vent, occasional use and never noticed a stuffy feeling when lit, MVHR set to slightly positive pressure . Yes a marmite question but IMO it works for us.

[JohnMo]

1 hour ago, SBMS said:

Is this a direct air system (pipe through the wall connected to the burner, providing fresh air)?  Did you consider or calculate any losses with thermal bridging, or with any movement of cold air from the outside to the inside (of the burner)? I don't know if I am over thinking, but have been fastidious about making sure there's hardly any external penetrations and adding an 80mm one with fresh air from outside seems counter-intuitive?

 

"When stove is off, just select air supply off, so no un-necessary air movements." - is this a feature of your WBS or something on the exterior wall where your air vent meets the world?

My air pipe comes up through the floor connected to a socket on the stove.  All air for the stove goes through this pipe.  Not true for all stoves, some take primary air via the outside air connection and secondary air via the room.  So check before you buy.

 

Our stove has a primary and secondary air controlled via a rotary dial, this can be turned off fully.  You can also get a butterfly valve that goes in to the air supply pipe, but was not really required for our stove - some do need it.

 

I considered heat loss, but concluded if I was stopping continuous airflow though the stove in the grand scheme of things the heat loss was insignificant.  The air pipe is required if you want a stove.  So stove need air pipe, don't want air pipe - no stove.

 

But to be frank - well insulated house - do you really need a stove? - NO.  As mentioned a day or so without heat make a small difference, so put your jumper on.

 

My wife wanted one - different story, different argument.

[SBMS]

45 minutes ago, JohnMo said:

My wife wanted one - different story, different argument.

Yep. 

[ProDave]

2 hours ago, JohnMo said:

My air pipe comes up through the floor connected to a socket on the stove.  All air for the stove goes through this pipe.  Not true for all stoves, some take primary air via the outside air connection and secondary air via the room.  So check before you buy.

 

Good point.  That was one the "must have" list for my stove that it takes ALL it's air from the one ducted intake.

[SBMS]

Out of interest - did anyone end up foregoing a WBS and opting for an electric one instead? That's my fallback as other half would like one for aesthetics.

[saveasteading]

The air flow seems to be much simpler on a wood burner than with dual fuel, as coal needs air from below. So, from what i can see on drawings, wbs air is a simple flow up the back, down and sidewats into the furnace and up to the flue.