Air vent for wood burning stove?

Paul Tregouet's picture

I had scrolled through a few older threads on wood-burning stoves, but this topic does not seem to have been addressed.


So the deal is: We are putting in a wood burning stove, and the person doing the installation says that we have to install an air vent from the outside. It's not like this place is airtight (built 1845). We also have an air duct/exhaust above the stove.


I dug around, came up with a few sites, in English:


http://woodheat.org/the-outdoor-air-myth-exposed.html


http://www.stovefittersmanual.co.uk/articles/do-i-need-an-air-vent-for-a-wood-burning-stove/


And in french: http://www.acqualys.fr/page/regles-de-pose-de-grille-d-air-et-conduit-poele-a-bois-et-granules and http://www.bricozone.be/fr/poele-insert-pellets/t-prise-air-exterieur-poele-bois-69809.html and https://www.poelesabois.com/installer-un-poele-et-un-conduit/choix-emplacement-pose-poele-a-bois/arrivee-dair.php and http://www.debusquer.info/chauffage.bois/poele.a.bois.php.


What is the experience of others who have installed a wood burning stove? If you put in a vent, what size was it? Our stove is going to be 8kW. So we might need 50 cm2 (~7.8 sq. in), if this site is correct (https://www.poelesabois.com/installer-un-poele-et-un-conduit/choix-emplacement-pose-poele-a-bois/arrivee-dair.php)


And no, I have no real confidence in the "artisans", less so in that their record of keeping to schedule and showing up on the scheduled days is pretty doggone pathetic.


Roger Thomas's picture

Back to first principals.

You need a supply of air to burn logs.

Air comes from outside.

Air from the outside is cold

Air indoors is warm

You want to burn cold air and logs to produce heat

Therefore you want the source of the cold air to be as close to the wood burner as possible so that there are no cold drafts.

A vent under the Wood-burner and connected to it would be perfect, so that all the combustion air is supplied from the outside and the heat from the stove warms the room.

The most modern stoves, designed for BBC (Eco) homes, do just that.

Me? I've got a classic Godin (output c 13Kw) with a 180mm chimney and a 160mm air vent from the vide sanitaire immediately beside it. Connected to 230mm twin wall conduit for the hot exhaust where it passes through ceiling and the loft. not all artisans are qualified for the installation and commissioning of log burners. There have been issues with some Anglais experts who quite frankly should be charged with attempted murder for their total disregard of any Standards.

Roger Thomas's picture

 Here is the link to an earlier discussion on here about cowboy installers with a few useful hints about chimneys etc

Your duct should extend downward from the ceiling to close to the bottom of your woodburner so that you are keeping the hot air inside the room. Your total vent size should also take into account any VMC or other extractor fan(s) you use in the kitchen and W.C/bathroom.

It might be a leaky old house ATM but almost everyone wants to draft proof in the winter!

Roger Thomas's picture

Sorry thought you meant a duct beside your 'exhaust' duct 

Quote - "Now that you mention it, an intake duct alongside the exhaust duct would be the best solution"

vic evans's picture

Be careful  the fresh air entry is not in a low or negative pressure zone on the roof as you could end up with a double whammy if it starts sucking!

Roger Thomas's picture

 Provided that its in the same pressure zone as the chimney outlet should be OK  (à la balanced flue?)

vic evans's picture

it will be if it is physically joined to the combustion air inlet on the wood burner like a balanced flue heater where pressure changes are the same on flue & combustion air & have no effect on the flame stability.  If not it could try to suck air out the room & hence down the chimney if there is no other relief available in the room. The other problem could be the air wash system on most fires which effectively means that the combustion chamber cannot be sealed & therefor cannot be considered to be a "balanced flue" type of appliance.

Kent Shelley's picture

David Rosemont mentions the Baxi fire which took its air for combustion from under the “dummy” floor.  I used to work for the makers, Baxendales, some 50 odd years ago, my mother still has one of their fires.  The point is, as I remember it: a gas fire needs to change the air in a room 3 times per hour, a coal fire as much as 10 times and a wood fire is somewhere in between - so, either lots of draughts or, as per Roger’s info: a vent as close to the fire as poss.  I know it’s not true of many French houses but if you have a wooden floor with a space underneath it then you’re on a winner as the air drawn in from outside and under that floor, to a vent set near the fire, will be good for it.  Our place is like that and has air bricks around the base outside - so that’s in the snagging list - along with the other twenty million things...

BTW, when I was young and working for the Gas Board, I once visited a house where they were complaining about a “Gas Miser” burning badly.  The man of the house was permanently bed ridden with chest and breathing problems.  The wife had, kindly and considerately, blocked up every possible source of draughts into his room and sealed the windows very efficiently.  The fire was burning badly because it wasn’t getting enough oxygen, as a result, it was kicking out CO² into the room.  She was literally killing him with kindness.

David Rosemont's picture

We have fitted a C02 detector plus a few normal smoke detectors round our quite old house. I agree about the need for ventilation. Baxi fires in the UK used to have an underfloor duct which emerged under the grate. Certain models also had an ashtray which was accessed from outside the house. Going back 50 years! The cowboys are not all Brits by the way!

Dewi L Morgan's picture

Sorry I don't get this at all. In this house we had an open coal fire then a closed coal fire both of which burnt both wood and of course coal.  The only time we had a vent installed in a wall was when the house was converted to gas central heating. So my question is why do we need a vent for a wood fire ? 

James Higginson's picture

As I understand it, you don't need one, but if you have one the burner could operate more efficiently and it will not create a draught by drawing cold air across the floor to feed itself as it will take it's oxygen supply from the outlet.

David Rosemont's picture

The problem comes about because houses generally are better sealed (especially round doors and windows) than they used to be. Old open chimneys used to be very large and these were common in French houses. Late better sealing and wood burners with restricted flues came along. There can be a danger of poisonous gasses building up and as these are not detectable by the occupants there can be the danger of poisoning. Many French houses have gas burners in kitchens too.

vic evans's picture

The truth is that you probably don't need an "extra" air vent as most house are sufficiently "leaky" I have a 6kw  wood burner in the kitchen & a 10kw in the lounge plus a VMC & have not needed extra ventilation despite my best endeavours to draught proof the house. In my defence I do have air  vents to outside but theses are always shut off except in exceptional circumstances when the aircraft wing shaped roof of the barn attached to my house via. the scullery seems to cause a negative pressure in the house with the wind in a certain direction meaning I need some air relief. Like David, I monitor the atmosphere closely with smoke & carbon monoxide detectors .   I am a retired building services design engineer & capable of making decisions such as these & DO NOT recommend anybody not well versed in these matters to follow suit & here is the nub of the matter. On a commercial basis It would be a fool or a crook who did not advise a client to install any fuel burning appliance with the "recommended" combustion air requirements. Who Or where these recommendations are obtained is another story so I can only suggest that a client ask for & verify that the installation is carried out in accordance with the current regulations of that country. If an installer could not demonstrate a knowledge & familiarity with these regs. & prove to the client he is complying with these,  avoid him like the plague & find one who is willing to demonstrate their competence. It's also worth pointing out  that in some cases it may not be possible to comply with the regs. so a bona fide company should decline the installation.

David Rosemont's picture

Agree Vic! I'm a retired architect. When advising people professionally it's always belt and braces especially in these days when everybody wants to sue everybody. My house is old and air leaky and I don't need extra ventilation. In the UK many years ago there was an organisation called the Solid Fuel Advisory Association which issued technical guidance. It was very good on open chimneys and "smoking" which was often a problem where the ratio of the surface area of the hearth aperture was I think more than 7 times of the area of the flue. However cold masonry flues, wind direction and other climatic variables can make a significant difference. Smoke doesn't always go UP I'm afraid!!!! There's also a frictional difference between smooth wall insulated flues and crinkly tin ones. Makes me sound like a fireplace nerd but there are frequent problems and of course it's always "the effing architect wot gets the blame"! Unfortunately practical experience and skills are often lacking these days.

vic evans's picture

As an aside to this thread :- many years ago I was building a fireplace & chimney in my home & researched the design requirements of this ancient art. It was interesting to find that no matter which way I calculated it  the cross sectional area of the chimney was always about 1/10 of the entrainment area of the fireplace ie. the size of the fireplace opening with a normal height chimney. This will explain why  Dickensian  boys could climb up chimneys serving huge fireplaces, they usually smoked & people sat in front of the fire on settles to protect themselves from the extreme draughts produced by the air requirements of these monsters.

Tip.--- If you have an open fire which smokes & the chimney is in apparent good nick, check the size of the fireplace opening & try temporarily closing it off a bit. You might be surprised! 

EDIT.David, you've beaten me to it. I was typing whilst you were posting. I always used 10x but that was a rule of thumb affected by many things.

David Rosemont's picture

We sold an old family house in Kent about four years ago after the death of an aunt. There were several monster fireplaces including inglenooks. One chimney had a door in the side of it to allow boys to climb up the chimney! The house was really quite old- going back to about 1450 and had been home to John Frith who was an English Martyr burned at the stake for his trouble! The whole house creaked and groaned at night, no floors were level, there was a murderer's grave in the garden (churchyard next door) and a home guard gun redoubt also in the garden, not to mention huge brick beer cellars and a coaching arch! One of the fireplaces had a brick oven. I've still got the fire dogs.

vic evans's picture

The estate agents call it character these days!

David Rosemont's picture

Remember Roy Brooks who really did not lie with the houses he was selling as agent!

Hilary Decaumont's picture

You would also get VAT at the reduced rate (currently 7% but to rise to 10% in January 2014) on stove, parts and labour.

Steve Taylor's picture

In UK there is HETAS approval, installers that are supposed to know everything about wood-burners etc. What is the equivalent in France, is there one???

I am looking for an installer, may be my plumber can do it but what am I looking for in terms of standard approval etc, any one know?

Sorry if this has already answered elsewhere here but can't see anything.

David Rosemont's picture

Just to mention my English neighbour had a French installer put in an expensive stove but it was done defectively and took months to sort out. The registered installer had to be threatened with legal action! The manufacturer sent a rep down to write a report.

vic evans's picture

Yeah but was it certified on completion as being fitted in accordance with regs? It must make a difference to take one's last fume filled breath knowing that it couldn't possibly be the woody 'cos  M le Plombier said it was fitted right!! I've seen some nasties & always wonder what the insurance take on it is. I fitted both of my woodburners & am content to bet my life on my work rather than some of the idiots around here who call themselves engineers.      Gotta go, I hear an alarm going off;-)

Roger Thomas's picture

   have a look here:-

http://renovation-info-service.gouv.fr/trouvez-un-professionnel

The site also gives an artisan selector

Ley Thompson's picture

Some good advice on here but the bottom line is that wood needs oxygen to burn. So if you want to continue breathing in the same room as the fire without worry, then you need an air vent - no argument. It should be the same size or bigger than the size of the chimney/flue. If the chimney/flue is 150mm in diameter, then the vent should be too. Put it as close to the fire as possible to avoid drafts/draughts. Simples.