• Post Reply Bookmark Topic Watch Topic
  • New Topic

Wood stove: external air supply good or not?  RSS feed

 
Wytze Schouten
Posts: 26
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
I'd like to test an idea with you all.

So we have a vacation home in the woods in the Netherlands, and we are about to install a wood stove in the fireplace. Location: we have a temperate, moist climate (750mm per year) not unlike that of inland British Columbia or Washington state, except we're farther north on the globe, so the sun sits lower and isn't as powerful. Our land is fairly high and dry on sandy soil, but we do have moisture problems in the basement.

Whole house in the woods: https://www.dropbox.com/s/o2v72w2h2nihp8q/2013-05-18%2015.18.17.jpg

The fireplace installer technician mentioned the possibility of adding an external air supply to the wood stove. He advised against it, though, because in his experience this is only necessary for very new, highly insulated homes. In those homes, the fireplace would not be able to draw in enough new air through cracks and vents, causing incomplete combustion, underpressure, and other undesirable outcomes. For every other type of home, the recommendation is not to use external air supplies. Recent research online suggests there is not only very little benefit, but also a real risk because of the possibility of backdraught.

I accept this guy's expertise for installing in homes and modern buildings, but not when it comes to wooden vacation homes from the 1920s in the middle of a forest. I have reason to think we might still have a very good use for external air supply in this case.

First of all, the wood stove is calculated to use about 20 cubic meters of air per hour. That's one sixth of the volume of the room where we're installing the stove. So it's not much, but it isn't negligible either. Since the walls are all wood (planks, not logs), the house has very little thermal storage. So the approx 100 kg of air inside the living room is a significant part of its thermal mass. Also, letting the stove take its air from the living room will likely lead to a draught along the surface of the floor, which means cold feet and an unhappy dog. (Plus discomfort for any guests who might be sleeping over on mattresses on the living room floor)

Second, if we install the external air supply and let it take its air from the basement instead of the outside, we are likely to solve two problems at once. The tube would only be around 50cm (1,5 ft) long, or we could lead it all the way to the floor of the basement to make sure it sucks the coldest, moistest air out first. The basement has several big cracks and small vents to the outside. At 20 cubic meters, the basement's air would be fully replaced every hour and a half. This might help a lot with the moisture problem. Currently we can only avoid fungus formation in the basement by leaving the trap door open throughout winter. But ventilating moist air into the vacation home can hardly be called a solution.

I am wondering about three possible risks, some of which have been discussed in the forum on rocket mass heaters by Paul and Erica and Al:

1. Moist air and wood stoves. Taking moist air from the basement to go into the wood stove: how bad is that? I know moist wood is bad, so I'm not confident about this bit. How bad is a supply of moist air for the wood stove's metal parts? The air is preheated by the stove before it reaches the fire: will moisture help it to absorb more heat faster, or exactly the opposite?

2. Backdraught. In certain situations, wind may cause high pressure at the top of the chimney and low pressure in the places where the stove gets its external air. In that case the fire from the wood stove might blow back into the air supply and into the basement. This seems unlikely because the basement is shielded from the pressure changes that might occur around the outside of the house during strong winds.

Thanks to Al for pointing me in this direction.

I'd be happy to hear any thoughts!

Wytze



Basement: https://www.dropbox.com/s/4flhkpqjjc0egsy/2014-05-18%2017.53.59.jpg
Floor above basement: https://www.dropbox.com/s/l2nd1asaxm29tui/2014-05-18%2016.50.13.jpg
 
keith s elliott
Posts: 57
Location: Ruxton Island
4
  • Likes 2
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Wytze:

I think you are trying to convince yourself that you need an outside air source, even if it is from your basement.

I'm not so sure about that, and I'm inclined to go with your installers' suggestion. I have a small cabin here, 432 square feet on the ground floor, about another 350 in the loft, and I have no outside air source...other than leaks.

I looked at the pic of your cabin (very nice, by the way!) and I bet that there will be more than enough leakage there to feed your wood stove. To either confirm or deny this, how was the air supply to the existing fireplace? Did you need to leave a window cracked open to feed it, or did it manage OK on it's own? While I do not know the answer to this next question, I am going to hazard a guess that the wood stove will very likely consume less air than the open fireplace.

I think that the damp basement is another problem all together, and that you should perhaps try to devise some sort of ventilation not directly involving your wood stove. Either that, or perhaps try using some of the heat from the wood stove and route it into the basement area by pressure. Perhaps a 12v fan depending on your power source. A marine fan from a boat comes to mind, where you need to vent the engine compartment prior to starting the engine. If you were to push warmed air to the basement via one hole, you could add another on the opposite side of your floor in order to get some cross ventilation going down stairs.

Your stove is claimed to use about one cubic foot of air every 5 seconds. Considering that will be drawn from the entire room area, not just the floor, I don't think you will be getting cold drafts. Especially if you address the damp basement issue with some warmth.

I am at the coast in B.C., but in an area with a pretty good rain shadow. We get somewhere around 37" annual rainfall, or just under a meter.

Good luck with this, and I will be interested to see what you manage to come up with.
 
Wytze Schouten
Posts: 26
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Hi Keith,

Great to hear from someone in North America who has a cabin... at almost the same latitute... and rainfall amount... and temperatures! Thanks for the compliments on the cabin, we're very lucky to own it. It's a Swedish or Norwegian "package" from the 1920s and it's been in my family since it was built.

Thanks for your answers and thoughts!

You're right, at some level I might be trying to convince myself I need an outside air supply.

However I am not concerned about what you seem to think I'm concerned about.

I'm not worried at all about a sufficient amount of air reaching the wood stove. There will be plenty of air: you're right, there are enough leaks for that. The current open fireplace has no problem attracting enough air from outside (except sometimes it helps to open a door while starting the fire). Since a wood stove uses much less air, I'm not concerned on this count.

As for draught, I do think that if the wood stove were to take its air from the room, the draught would be along the floor. In any room with a heat source, air near the outside walls tends to cool and drop to the floor. Combine that with a wood stove near the floor which is taking in air as well as heating the air nearby it, and you've got all the ingredients for cold air moving over the floor towards the stove.

Fixing the basement moisture problem with targeted ventilation is probably the most effective solution. Using generated heat would probably be too expensive (the heating would have to be on for a number of months continuously), and I haven't really thought about electricity for this, with the simple reason that we have only installed electricity a few years ago, and there's still only one socket.

(Solar cells are not an option since this cabin is the target of molestation by bored youths every other year)

Again, thanks for your comments.

And any reader-along's comments are welcome too! I'm still curious about these issues:
- risk of backfire through the air supply tube
- effect of moist air going into the wood stove

Wytze


 
Tim Malacarne
Posts: 226
Location: South central Illinois, USA
2
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator

We've had wood heat for 40 years. I like it, warms you to the bone!

Outside air supply was a "fad" some years back, but from what I can read, it's sorta out of fashion now.

Our stove dries the air out something awful. We keep a large container of water on the stove to put moisture in the air. I wonder if you could somehow circulate the basement air and solve 2 problems at once? You could dry out the basement air and humidify the living space air at the same time. Heating the basement, however slightly, would also give you something of a thermal flywheel effect, I'd guess.

One cannot over-stress the importance of properly seasoning your firewood, and storing it where it will stay dry. This is important with a fireplace, and vital with a stove.

Best, T
 
Mike Cantrell
Posts: 555
Location: Mid-Michigan
28
bee books duck food preservation forest garden hunting solar trees
  • Likes 1
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
For what it's worth, The wood heat Organization (woodheat.org) calls it a myth. Here's their article:

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

As far as I can tell, their only agenda is to get more people to heat with wood, so I can't figure that would it bias them one way or the other. Seems reasonable to me to accept their conclusion as sensible and unbiased.
 
Wytze Schouten
Posts: 26
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Hi Mike,

Thanks for your response. I read the article you linked: it's very informative. It looks like it really is almost never a good idea to feed your wood stove direct air from outside the home's envelope.

However, my intention is not to grab the air from outside the building. All I want to do is to take the air from another place inside the house: the basement. The basement is sealed off from the rest of the home by a wooden floor: not airtight, but pretty good.

The part of your linked article that does leave me a little worried is the fact that the basement has some ventilation holes to the outside. This means that the air pressure in the basement is more strongly influenced by wind than in the living room where the wood stove is located.

So I'm left without a definite question, just a feeling that it should be possible to do something constructive with the air supply to the wood stove. Make it kickstart a circulation that was missing in the house.

 
allen lumley
pollinator
Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
58
books fungi hugelkultur solar wofati woodworking
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Wytze Shouten : I am not a big fan of most You-Tube videos, However a superior site that will improve your grasp of how to deal with efficient and affordable heating
and air exchanges in northern climates can be found at the Cold Climate Housing channel at U-tube, goto u-tube, and enter Cold climate housing channel

At the next page you should see a set of '' Playlists " ! The first 'playlist' is " Your Northern home ", I do recommend that the whole series gets watched over time, as
this is primarily about air exchange the single most important video in this 'playlist' is " Jorge and Pauls Adventure with HRV " Happy Viewing ! Big AL !

Late Note; Living in the Northern Hemisphere, we think we Know Cold ! the Cold Climate Housing Research Center, located at the University of Alaska, Fairbanks -
They KNOW COLD ! A.L.
 
keith s elliott
Posts: 57
Location: Ruxton Island
4
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Wytze Schouten wrote:Hi Mike,


The part of your linked article that does leave me a little worried is the fact that the basement has some ventilation holes to the outside. This means that the air pressure in the basement is more strongly influenced by wind than in the living room where the wood stove is located.



Wytze: Just a thought about air pressure from outside. If there are holes on opposing sides of the basement in question, one side will likely be slightly pressurized and the other in a slight vacuum as the wind passes around and over the house. I think they would come close to balancing each other out, not perfectly maybe, but close. I don't think it would worry me too much anyway. It looks as though your basement is all below ground anyway, is that right?
 
Wytze Schouten
Posts: 26
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Hi Keith,

That's correct, the basement is mostly below ground. It's rectangular: 1.5 meters wide, 7 meters deep. And 2 meters (6ft 7in) high.

The top 40cm (1ft 4in) are above ground and that's where the holes are. They are indeed on opposing sides. There is also a big opening, just below the ground-level floor, to the crawlspace under the rest of the house. And that crawlspace has holes on all other sides. So you're right, if the wind blows in from any direction there is always plenty of opening on other sides, and it's unlikely that pressure will build up.

Best regards,

Wytze
 
Cj Sloane
pollinator
Posts: 3734
Location: Vermont, off grid for 24 years!
87
bee books chicken dog duck fungi solar trees
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Wytze Schouten wrote:
Fixing the basement moisture problem with targeted ventilation is probably the most effective solution.


If the basement is like an earth cellar than I don't think it'll work because new moist air will replace the old. If it's a finished basement, what's causing all the moisture? You could put a few vents in the floor/ceiling to creat a little airflow or vent it out of the cellar.

Our house is pretty tight, R38 on 6 sides but our builder told us if we had a dog we wouldn't have to worry about an outside air supply (how many times do you open the door to let the dog in & out).
 
Glyn Tutt
Posts: 14
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Dear Wytze,

The general impression regarding responses to your post have not been in favour to install an external air inlet directly into your woodstove. Personally, I would not install a woodstove without one. Not for building code or regulations purposes, but simply due to trying to increase convection within the space you are trying to heat. Any 'suction' of air form the space you are heating (and you have already stated that there is very little mass in your building - the air making up a significant proportion of it) will remove heat. Yes, your building may be leaky, but I too live in very old properties and want my draughts to go around the house, taking warm air with them - I don't want them to go up the chimney!

The document sited below about scientific evidence states that increased system energy is unfounded, but then did not go into any detail. Yes pressure differentials and fluid dynamics are very important in reducing airflow and in modern housing that creates a problem for humans as air gets stale and humid - not good for people, or indeed the fabric of the building. Clearly in such an airtight building, to combust wood would then require additional air, so that is why your installer mentioned the need for such air supply in modern builds.

However, you, like me, live in older houses (mine is over 400 years old) - and I still value the idea that heat dissipated from my stove is heating air that will remain in my house. I always draw air from an unheated part of the house (i.e. an outbuilding) or directly from outside. It certainly reduces that 'rush' of air that does indeed appear to hug the floor as it travels to end of the room where the fire/woodburner is, I also think that it helps ventilate the outbuilding/cellar that it draws air from.

I am not sure that your suggestion to draw the air from the 'worst' bit of your cellar - but providing more airflow generally will help with your damp problem and then using that air in your wood burner won't affect the burn quality as the air as it is heated within the burner (you mentioned that your model does this, Jotul or Clearview I presume), it's ability to absorb water increases and it is the wood and how you burn it (good secondary burn) that has the most effect on performance.

If you do wish to passively ventilate the air from the bottom of your cellar, why not add a secondary flue that enters your chimney flue near about 1 metre above your woodburner so that it draws a small amount of air as the expended gasses pass through from the woodburner?

At the end of the day, your little woodland cabin will never be as efficient as your main place of residence, but at least you should have a stove that quickly heats up the internal air without all of it going out of your chimney every 5-6 hours.

 
Wytze Schouten
Posts: 26
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Hi Glyn,

Great to hear from someone who seems to share my intuitions. Yes, the arguments against external air supply are mostly arguments in a debate about modern homes vs ultramodern homes. Where no thought is given to ultra old or ultra thin homes.

I do think the risk of blowback / backdraught / whatever you call it, should be seriously considered. Does this ever happen with your wood stove?

As an example: the interior of the room with (soon) the wood stove has wood panelling up to about 1m80 (6 feet) high, and above that it's burlap nailed onto a frame of thin wooden joists (unsure if that's the right word). Behind the burlap some air, then wooden planks about 1 or 2 inches thick that make up the outer wall. Then you're outside. Recently a candle slowly bent over one evening and burned a hole in the burlap (fortunately it stopped there), and now you can feel the wind sucking through that hole like nobody's business.

You're also the first one to directly answer my question about moist air going into the wood stove. Bonus points, thank you! So maybe not the very wettest fungi-ish air from right at the basement floor.

The wood stove is going to be a Dik Geurts (Dutch builder). It's designed to be a lot of visual fire and a minimum of steel frame, but its performance is still pretty good.

http://www.geurts.nl/assortiment/houtkachels/vidar_triple.aspx

Thanks again!
Wytze


 
We've gotta get close enough to that helmet to pull the choke on it's engine and flood his mind! Or, we could just read this tiny ad:
FT Position Available: Affiliate Manager Who Loves Permaculture & Homesteading
https://permies.com/t/69742/FT-Position-Affiliate-Manager-Loves
  • Post Reply Bookmark Topic Watch Topic
  • New Topic
Boost this thread!