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 against 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

I have stoves both with and without DA in different houses.

I would now always go with DA.

The house with a stove without DA was OK until the windows were changed to modern sealed type. The room with the stove now gets v. stuffy and if the door is shut then headaches occur. Unfortunately retro fit is v. difficult.

The house with DA on the stove (and modern type windows) has none of the above issues and the stove works as designed. However a closer should be fitted to the DA pipe to stop the chimney effect when the stove is not in use otherwise the stove gets cold and condensation can form around the base of the stove. (Butterfly closers can be obtained operated by a bowden cable to allow the valve to be located optimally (e.g.outside) with ease of operation)

IMO the alternative regulation of a hole in the wall is an absolute nonsense.

Direct air every time for me. No wall vents, they are total madness - a hole in your house!
2 stoves in the farmhouse here, one with DA, one without.
I’m thinking about fitting DA to the other one, but I think I will need a new stove to do it (it’s pretty old and tired anyway)

Ideally would be insulation, airtightness, then MVHR, so no stove required.

The stone barn conversion has no stove - it has GSHP, MVHR, insulation, airtightness

The next conversion is not having a stove either. It’s having even more insulation, plus the other stuff.

Posted By: Cliff PopeSomeone mentioned the negligible effect of 20 degrees on air input to the stove. I would just point out the well known observation that cars run better when drawing cold air, because of the increased density of the air, causing a supercharger effect

If my school chemistry serves me right think a thermodynamic efficiency equation known as the Carnot cycle applies.

Posted By: Peter_in_HungaryIMO the alternative regulation of a hole in the wall is an absolute nonsense.

Totally agree. I have 2 DA stoves why would I want to draw air in from outside via a hole in the wall or as a neighbour has, a hole in her door. (because it was easier for the installer to do at the annoyance to my neighbour!)

I have the door slightly ajar when lighting as it gives that extra bit of draught. BCO advised having the MVHR slightly pressurised for the house so that when loading the stove there is less chance of fumes entering the room.

I do not think it would be a good idea to be able to open and close the air intake one could forget to open it with safety issues.

Someone pointed out that as my inlets are low down outside (the duct goes in the concrete slab) I should be aware that drifting snow could block them. Fortunately we get v little snow and the inlets are under a veranda. In Scandinavian countries for instance the air intake is at roof level to avoid the issue that was pointed out to me.

Is it really sensible to compare a modern internal combustion engine , with its complex array of sensors , injectors , turbos , exhaust gas recirculation , emission controlls etc etc etc , to a metal box loaded with very variable fuel that burns with the only control being the amount and size of wood you put in it and the amount of air you choose to allow into the stove usually via a manual control slide , or a thermostat controlled damper.
Additionally selecting the correct size stove for the room and the levels of insualtion etc is also going to be important, certainly more so than the temp of the air getting into the stove.

In a well insulated , air tight home with mvhr, is it really likely that anything more than a 5kw stove would be needed and even that is probably going to be a lifestyle choice and a bit of back up if you’re expecting to get regular prolonged power cuts.

<blockquote><cite>Posted By: Artiglio</cite>Is it really sensible to compare a modern internal combustion engine , </blockquote>

No, it wasn't a rigorous comparison, just an observation that a traditional petrol engine draws in more combustible mixture if the air is cold. The same is probably true of a bonfire on a cold day.

Interesting and I have considered putting DA on the list of jobs.

I struggle to see how/why DA would make a room feel less stuffy. If you sealed it as best as possible, to the point it was negatively pressurised and combustion began to be affected, within the firebox, what would be the difference to simply closing up the stove vents a bit? You'd be starving the fire of combustion air in just the same way that is achieved when the vents are throttled back, no? If it got to the point where the room was so negatively pressurised, the chimney draw could not draw sufficiently, the fire would be stifled, just like shutting the fire vents down.

I think that a simple test to see if a stove is under ventilated would be to run it in the room as closed as possible, then crack a window. If there is a noticeable increase in fire performance, akin to opening the fire vents more, it's likely that the room/house is actually restricting air supply to the stove.

Hopefully everyone with combustion stoves uses a CO monitor anyway. If any drowsyness is being caused by CO at least that should warn that there is a serious issue.