The Pros and Cons of Direct Air for a Wood Stove (when there’s no mandatory requirement)

[carbs for arbs]

Hi all

 

Direct air / External air…

 

Additional air requirements are mandatory for stoves above 5kW in older houses, and for any stoves in newer houses (post-2008) or houses with an air permeability of 5 m3/hm2 or less.  I can understand the logic in having DA when the alternative would be an air vent (hole in the wall); although even then there are arguments on both sides.  But putting that aside a moment, when no additional air requirements are needed for a stove, is DA still a worthwhile option?  I’ve spent the day reading up on this and thought I’d post what I’ve found, as much to share my research as to get feedback and thoughts from people if anyone is interested in discussing it.  

 

***In Favour of DA***
- Draughts.  There are quite a few people on the internet who can’t say enough good things about DA and highly recommend using it whenever possible to reduce draughts and cold air in the room.  It’s an enticing idea!  However,  I did notice that this is usually as opposed to having to have an air vent.  When the alternative to DA is no air vent (for a 5kW or less stove), I’m not so sure how many of these people would still recommend it.  
- Draughts – Anecdotes.  That being said, I found instances where people were saying they used to have a stove which took air from the room, they weren’t happy with the draughts being created, and so retrofitted a DA feed and it made a world of difference.  
- Wasting Heated Air.  Some say that without DA, you are wasting air in the room that has already been heated and replacing it with cold air.  This quote sums it up:  “…I would definitely fit a direct air supply not only [for] the prevention of draughts but for the conservation of heat. Using room air you would be using air already heated and a wood burner will use a considerable amount of combustion air out of the room you have heated.”  REBUTTAL:  However, stoves actually use quite a small amount of air when compared to the average standard air changes required in a room.  The air changes will have to happen one way or another, so unless MVHR is used, won’t hot air still have to be replaced with a fresh cold supply regardless?  
- Colder Air Burns Better.  Quote:  “The colder the air – the denser it is and more oxygen per unit volume it contains, so it must be good to assist combustion.”  REBUTTAL:  However, I’m sure I’ve also heard the opposite, that colder denser air is detrimental to combustion.  Not sure which is right.  
- Cold Air Expands When Heated.  The claim therefore is that this can assist chimney draw as it is taken away up the flue.
- Control.  Although DA means less air changes compared to air from the room, it means you have more control over when you ventilate the house.  IE – opening windows in the morning, or before going out.  You can control the ventilation, and separate the times to ventilate from the times to heat.  Also, you can introduce more ventilation when required due to the weather (damp days) and less when not (dry crisp days).  As one person puts it, “Using the stove air to ventilate, means you wouldn't ventilate the right amount at the right time.” 

 

I thought this article was particularly useful in highlighting benefits, although note the “conditions” on page 2.  
https://mtbest.net/wp-content/uploads/sites/12/2023/02/cold-air-intake-1.pdf

 

 

***Against DA***
- Simply Not Needed.  Many people claim DA is not necessary, for various reasons.  Also, all of the installers and stove shops I have spoken to (which is quite a few!) have unanimously said not to bother and that we would not gain anything.  The “Outdoor Air Myth Exposed” article – link below – claims that it has been studied and DA (or even just air vents) are most often of no benefit.  
- Stoves Don’t Create Draughts.  Some say it is not true that you feel draughts.  They say to try putting your hand by the stove’s air inlet, where the air is being sucked through a small aperture and should therefore be the aggregate of all air being pulled.  If there were draughts you would feel it there, but you don’t.
- Cold Air Cools the Stove.  Pulling cold air directly into the stove will be worse for combustion and will cool the internal temperature of the firebox.
- Cold Air When Not in Use.  The duct will be cold and will emit cold air, especially when the stove is not in use (or even when the stove is in use, to be fair, as it’s still sucking in cold air).  Thermal bridge to outdoors.  
- DA is Worse for Ventilation and Humidity.  If the stove takes air from the room, it encourages more air changes in the house.  This in turn can increase ventilation and help with humidity too.  Outside air is almost always drier/lower RH, so frequent air changes help.  
- MVHR.  Of those people who think DA is a good idea, they sometimes add that MVHR is needed too for air changes.  Without MVHR, some say it’s best to vent the stove from the room air.  I don’t and won’t have MVHR, so that’s an “against” for me.
- Wind and External Air Pressure (Backdraught) Problems.  If the wind is at a particular strength and direction it can cause smoke to blow back into the room, as well as CO.  It can also make it difficult to start the stove.  Unpredictable and variable.  There can also be a potential fire risk if the reversal of the flue system flow results in the hot gases being expelled from the DA intake.  Also, if mechanical extraction in the house de-pressurises more than the chimney draw, smoke and gases can be drawn out of the stove and into the room.  REBUTTAL:  See “cold-air-intake” pdf (link above).  He says to have the external vent positioned in a place where there will be no wind (consistent known pressure), which would deal with this problem.  If that’s not possible, I wonder is something like the DR21 could help with this? 
- Air from Below Stove (and Entry Into Ash Pan Area).  See “cold-air-intake” pdf (link above).  He states that the DA needs to be supplied from below the fire as a requirement for it to be effective.  This is not possible for many wood only stoves as they have no ash pan.            
- More Money!  Kits are often £100+.  Fitting will add to the bill as well (unless DIYing).
- Historic Precedent.  It’s not how they used to do it in the old days!  Older houses need older methods?   

The strongest article I found arguing DA – and air vents in general – and claiming that studies have proved it:  

 

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

 

 

I appreciate the post is long, and probably quite boring! So feel free to ignore it; although if you've got this far then I guess it's a bit late to say that! But if it interests anyone, I’d love to hear your thoughts, arguments and agreements. Personal experiences always great to hear as well.

 

Cheers

 

Edited 8 hours ago by carbs for arbs

[Stubby]

Phew ! I think it was me who posted colder air is denser so contains more O2 per volume than hotter air . I have no proof that this aids combustion though . I was thinking in terms of the super charger on the front of an old Berkin 4 liter Bently forcing more air in to the enging there fore more O2 . Regards the outside air connection bringing cold air into the room when the stove is not alight . Firstly if the stove is not alight its probably not cold out side ? secondly it should be sealed , from the stove entry to the outside should be a pipe so it would only allow cold air into the stove not the room ( assuming you have the stove door shut ) . Having said all that I have a nominally 5kw stove ( I think it punts out about 7 or 8 on full chat ) and I don't have a direct out side air source . Enough air comes in through the dog flap in the back room wall . @carbs for arbs

Edited 53 minutes ago by Stubby