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Need advice on air intake changes for backdrafting wood stove…

emturnerNWO | Posted in Mechanicals on

We recently built a “pretty good” 1200 square foot bungalow (insulated slab on grade, no basement) in zone 7/8 (Canada) with in-floor heat (hydronic, electric boiler). It’s very tight. Last winter we were unable to use our small wood stove much because at the end of each burn, as the fire was dying down, we got backpuffing from the chimney near the ceiling (this is our wood stove… https://www.century-heating.com/en/products/stoves/wood-stove-on-legs-model-s245e/).

We don’t use the dryer while the fire is burning, of course (we are saving up for a heat-pump dryer, eventually) and we avoid using the range hood (although it is very low CFM and doesn’t seem to make a difference). It was suggested that our HRV is not properly balanced, but this occurs even with the HRV shut off completely.

There is an outside air kit attached directly to the wood stove (please see attached pdf). The 5″ tube runs up to the ceiling (10-11′ high), where it is joined to a 4″ tube that runs through the attic to the soffit. I could feel the cold air pouring in through this tube when I hooked it up to the wood stove. However, our HVAC/plumber suggested that we might be able to solve the backpuffing by running a 5″ tube directly out the back wall (see drawing), which would give us a greater volume of air. It’s a shorter, straighter run and there would be no 4″ section.

Does anyone have any guesses as to whether this would work or not? How much of a difference does the shape/size/length of the outside air intake tube make? Any suggestions would be appreciated…we would really like to use our wood stove this year to reduce our heating costs (we have 10 acres of trees and pay about 19 cents/kwh for electricity).

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Beth,
    I'm confused by one of your sentences: "We got backpuffing from the chimney near the ceiling."

    What does this mean? Do you mean smoke is emerging from the seam between two sections of metal chimney, near the ceiling? That would be very unusual. The chimney shouldn't be leaky enough at the seams for this to happen.

    Most backpuffing happens at the stove itself.

    If smoke is leaking out of seams in your chimney, it's not really backpuffing. It's something else -- something we might call "frontpuffing."

  2. emturnerNWO | | #2

    Yes, the smoke is coming out the seams. The chimney sections are just screwed together, so it's not really airtight (I'd never owned a freestanding wood stove before and I'd assumed it would be more tightly constructed...but I guess this is standard?). This mostly occurs when the fire dies down. With higher negative pressure (for example, turning on the dryer), it will backpuff from the stove itself.

  3. GBA Editor
    Martin Holladay | | #3

    You probably need to have a chimney expert examine your chimney. The seams shouldn't leak smoke.

    Perhaps an error was made when the chimney was assembled.

    Perhaps the chimney is too short -- it should be higher than the ridge of your roof -- and it is being affected by winds passing over your roof.

    Perhaps a bird built a nest in your chimney over the summer.

    Lots of possibilities.

  4. emturnerNWO | | #4

    Ok thanks Martin. I should have mentioned that we've been working on this for over a year now and have examined pretty much every possibility besides the air intake (chimney height, blockages, etc.). So increasing the airflow is sort of our last resort to get it working. I was thinking that perhaps it has enough of an updraft to get the smoke to the ceiling, but the cold air in the attic is pushing it back down, and it's coming out where the two converge?

    We will take another look at the seams/chimney construction though, if you think it is unusual.

  5. Expert Member
    Dana Dorsett | | #5

    One problem with the air intake as-installed is that it exits the house WAY above the firebox, which makes it behave as a secondary flue, which promotes backdrafting. It could even force backdrafting under some wind conditions. If using an exterior air supply tied directly to the firebox it has to be no higher in elevation than the firebox. That way you don't have competing chimneys (one masquerading as a combustion air intake.) The proposed installation is a big improvement (it at least would be more likely to comply with building codes), but it may or may not fix the problem completely. If the combustion air kit is really a proximity air supply communicating with room air, having it exit out the roof depressurizes the house (just not as much as the clothes dryer or kitchen fan.)

    The air kit piping also needs to be made out of non-combustible materials and have the necessary clearances to combustibles. Scroll down to Section R1006:

    http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%2010.html

    See the comments on this bit o' bloggery:

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/providing-outdoor-combustion-air-wood-stove

    It also doesn't need to be 5" in diameter to supply sufficient combustion air, but apparently that's the size of the kit they sell for it(?). Even a 3" diameter would be more than sufficient for a 40,000 BTU/hr wood stove.

  6. GBA Editor
    Martin Holladay | | #6

    Dana,
    Good point about the height of the air supply duct -- the "competing flues" possibility deserves investigation, for sure.

    Beth,
    Dana is referring to this code provision: "R1006.2 Exterior air intake. ... The exterior air intake shall not be located within the garage or basement of the dwelling nor shall the air intake be located at an elevation higher than the firebox."

  7. GBA Editor
    Martin Holladay | | #7

    Beth,
    To gather data, why not just disconnect the air supply duct? Try operating the stove as an ordinary wood stove, without the outdoor air kit, so that it pulls its combustion air from the room where the stove is located. It's possible that it will behave much better.

  8. emturnerNWO | | #8

    Dana, I too wondered if that huge vertical section would act as a secondary flue...I think our contractor installed it that way so we didn't have to drill a hole in the wall, and because he thought it would act as a sort of p-trap for air (although I thought the air p-trap theory had been debunked). It is indeed directly ducted (not proximity air), and it's all up to code (WETT certified). The piping itself is an insulated class 1 duct for wood stoves...we will probably stick with that because we have a lot of it already. We are now doing the finishing details on the house, and need to box in the piping. It would actually be easier to box in with option 2 (directly out the wall), so if you think it will be better for combustion (or at least won't make it worse), then we will go ahead and make the change.

    Martin, at first, we did try burning the stove a couple of times without the outside air kit (simply because it was not hooked up yet). It was WAY worse. We could barely keep a fire burning at all. Since it made such an improvement, I wondered if a larger, more direct duct would make it even better...

  9. Kenny78 | | #9

    Have you tried burning the stove without outside air hooked up but a window cracked?

    These guys are giving good spot on advice, but if your willing to take and post pics I would also cross post on http://www.hearth.com obviously you have to weed out advice(like all Internet) but there are a lot of chimney experts/professionals there.
    Personally I don't think its a volume issue but a routing of OA piping that is contributing. As well as a possibly compromised etc chimey

  10. Jon_R | | #10

    Say that wind effects can be ruled out. And that we are talking about a given flue temperature to surrounding air difference (not much once the fire burns down). I expect that the pressure difference determining whether smoke enters the room is then a function of the stack effect (chimney height) vs indoor pressure. I'd either increase the chimney height or increase the interior pressure. I don't expect that changing the exterior air intake resistance/pressure will effect the smoke issue.

    >I could feel the cold air pouring in through this tube when I hooked it up to the wood stove.

    This suggests a negative room pressure. I'd also seal the flue seams.

  11. Kenny78 | | #11

    I'm fairly new to epa stoves and just had a new experience that could explain/add info. The wife was complaining it was cold so I threw a couple splits on. The stove did a great job and got the house to about 80. I closed the air all the way down(quadrafire 4300) and forgot about it. An hour later the wife complained of smoke and we observed a flashback or back puffing.

    When the smoke was coming out of the seams was there an adjacent flame up/explosion in the firebox? Too little air with too much fuel is the cause. This might be an additional cause along with your wonky make up air.

  12. user-5574861 | | #12

    Beth,

    I think the new fresh air intake on the bottom of the stove would be an improvement and I believe your issue is negative pressure. Here is why I think that is.

    Like Ken, I have a Quadrafire and I will never go back to a non-EPA fireplace. I have used it virtually every cold day for the past 4 years. My fireplace is in the family room which has a cathedral ceiling and is on the first floor. If temps are above 45 and I open the firebox before starting the fire, I rarely sense any downdraft. However, on days when temps are in the teens or lower, I can feel a heavy downdraft of cold air rushing into the firebox when I open the doors. I would imagine it would be worse if I also had a roof penetrating outside air kit (OAK). My OAK enters the firebox from the right side near the bottom. This entailed drilling a hole in the chase and definitely not as pretty as if were terminated near the soffit. On really cold days, I need to make sure the furnace is off and all of the upstair bedroom doors are closed before trying to start a fire. My fireplace has two doors and when I start it I keep the left hand door shut and start the fire on the left side of the firebox using some Superceder fire starters with good kindling, This is because the air coming from the OAK comes in from the right and blows across the firebox keeping the smoke away from the open door. I also crack the window next to the fireplace when starting the fire and leave it open until the flames start to dance, which means the pressure has turned positive. This only take about 15 - 30 seconds.

    You did not mention backpuffing on start-up, but I think maybe the reason for it near the end of the burn is similar. With a low mounted OAK, the fire has fresh air supply for combustion and in my case the fire burns to coals with no smoke and once I get to coals, there is no smoke. In fact, one of the lessons I learned from the site that Ken mentioned, Hearth.com, is you burn "coals to coals" so you never get smoke with you refuel. Now with both your chimney and OAK terminated above, as the fire draws down, the negative pressure starts creeping up and that downdraft builds to a point where it inhibits combustion before you get to coals, thus causing smoke. One way to test this theory would be to crack a window when the backpuffing starts. If you had a lever on your OAK that allowed you to close it, it would interesting to see if that makes a difference with and without the window cracked.

    I am getting ready to start construction of a non-certified Passive House and I have been struggling to find a small enough wood burning fireplace. Of the units that I have found, they all have an OAK and they show the intake terminating below the level of the firebox. When I check the install manual for your unit I noticed that they show the OAK terminating either at the same level of or below the level of the firebox. Ironically I also found this paragraph:

    "In Canada, wood stoves are not required to have a supply of combustion air from outdoors (except in mobile homes) because research has shown that these supplies do not give protection against house depressurization and may fail to supply combustion air during windy weather."

    I had my unit fired up during Hurricane Sandy in anticipation of the inevitable power loss and it never backdrafted during the storm.

    Ken - with regards to your backdrafting issue, I had to turn dow the throttle to about 1/4 last night because we hit 77 degrees and it made for a really cool flame. I did not get any backdrafting though.

  13. emturnerNWO | | #13

    This is all great info, thanks everyone. The deed is done. At first I missed that comment from Martin/Dana about the code provisions (i.e., intake not being higher than the firebox). Not sure if those apply in Canada, but I figured they were there for a reason. We drilled the hole in the wall last weekend and boxed it in today. Looks nice and it was probably easier than boxing in the vertical tube (gives us a little bench in the front hall). However, we will have to wait until it gets colder to test it out. We've had freakishly warm weather for November (probably because of climate change) and we really can't run our wood stove until it's at least -15 C or we will overheat in our small, super-insulated house.

    Jon, what do you mean by seal the flue seams?

    To answer Ken, no there is no corresponding explosion in the firebox when the back puffing starts...well, not that I have noticed anyway.

    Jonathan--you're worrying me with that paragraph about OAK "not giving protection against house depressurization". Is there a link to that research anywhere? It's so hard to find scientific research on this issue, but that's exactly the type of thing I needed before we began this endeavour of trying to put a wood stove in a super tight house.

    Either way, it's done now...In a few weeks, I can report back on whether it's improved the backpuffing if anyone is interested.

  14. user-5574861 | | #14

    Beth,

    I am betting you have solved your problem and you just need cold weather to confirm it. Speaking of freakishly warm weather, I noticed this morning that my rose bushes have flowers on them - in NJ in November!

    My point regarding the depressurization was that if my house did not depressurize during Hurricane Sandy with my OAK open, then it will never happen. I think in a tight house an OAK is no longer optional but needed.

    Btw, I have seen some videos of Passive Homes in Europe with wood stoves/fireplaces. It can be done. I think the 3 issues that arise are 1) overheating, 2) air infiltration in the winter when fireplace is not being used and in the summer when it allows warm air and 3) the thermal bridging effect as you have a continuous metal object breaking the thermal barrier.

  15. Jon_R | | #15

    > what do you mean by seal the flue seams?

    Something like this.

  16. dinnerbellmel | | #16

    Jonathan, Thanks for your explanation above. We have the same Quadrafire! I like the secondary burn on your attached pic. I am jealous though that your glass is so clean. Mine can't seem to stay that way.

  17. user-5574861 | | #17

    Mel,

    I bet you are never cold when your Quadrafire is lit. Truth be told, we had family visiting this weekend so I cleaned the glass on Friday. It usually needs to be cleaned once a week. I found a cleaning trick that works great - simply rub the dirty glass with a wet paper towel and dry with a clean one. The carbon comes right off.

  18. yogumon | | #18

    Spray cleaner for kitchen ovens works very well for cleaning fireplace glass.

  19. emturnerNWO | | #19

    Just wanted to report back on the results of the air intake changes. We have had several fires now at various temperatures and the new makeup air configuration has made a big difference. The backpuffing problem is resolved and we can even close the damper down now for a slower burn (we couldn't do this before without the fire dying out). So it would seem that the size and routing of the makeup air duct does indeed make a difference (at least in our case...it's not exactly a scientific study but does provide some insight, perhaps). I'm thinking that the "acting as a secondary flue" theory was correct.

    We still need to crack a window while lighting, and be sure the dryer and range hood are not in use, but other than that, I think it is feasible to use the wood stove in our tight house. I will also seal the flue seams when we have a chance as suggested because they are definitely a source of leakage. We even got a little frost around one of the top seams when the temperature dipped down around -30 this week. I'm sure the whole wood stove set-up is a measurable contributor to heat loss (when the stove is not in use) with the roof and wall penetrations, but it's still worth it to have a low-tech backup heating method in case of power outages. Especially in a climate where it regularly gets dangerously cold.

  20. dinnerbellmel | | #20

    I love a happy ending!

  21. Expert Member
    Dana Dorsett | | #21

    Whether the arm-chair theory regarding competing flues is the full story or not, there's no point in seizing defeat from the jaws of victory with more experiments just to find out! I'm glad it worked out!

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