Woodstove with outside air source backdrafting
I’ve read the other posts about woodstoves with outside air sources backdrafting, but of course there are always so many specific details that don’t match up, so I’m posting about it as well.
I live in central Vermont. We moved from northern Vermont in October, so this is a new house for us.
We just installed a new VT Castings Dauntless, in front of our fireplace. This house is 1600 sq ft and terribly insulated, very leaky. We are working on it now. Before we installed the woodstove, I had a 3 1/2″ hole drilled through the side of the chimney, at a point where it should have lined up with the port for the outside air. They had trouble keeping a straight line when they hit the steel liner in the fireplace, so it didn’t. The duct leading from the woodstove (it is connected directly to the stove) exits the fireplace about 2″ higher than the woodstove firebox.
The chimney is brick and is on the south facing wall of the house, inside a screened-in porch (so outside – cold), with a 6″ flue. The top of the chimney is above the peak of the house.
We burned the stove for 4 days and 3 nights with no problems. Then last night, the fourth night, we realized our bedroom (upstairs) was smoky. It never set off our smoke alarms, but we could smell and feel the smoke in our eyes – not bad, but it certainly wasn’t right. We checked the stove, etc. etc. It did finally abate, as the fire burned down.
Today, during the day, as soon as I shut the damper on the stove (engaging the catalytic converter), we started getting that low-level smoke again. We had the installer out today to check it, and he thought we might have been shutting the damper before the stove was hot enough (I was following the instructions in the manual, which did not make sense to me). But this afternoon and evening I have deliberately gotten the fire hot, up at 500, and then shut it down – and we’ve gotten the smoke again. When we open the damper, the smoking stops.
This woodstove replaced an old, cracked Jotul with a damper that didn’t work. We couldn’t shut that down, so we never built the fire up that hot. We never had any backdrafting from that stove.
Any ideas on what is happening? It has been cold this entire time (20 degrees and below), and not particularly windy. It’s not windy now and we just got the backdrafting when we shut the damper.
Would it make a difference if we raised the stove on 2″ blocks of stone (hearth is soapstone – we could use blocks of soapstone)? Or would it make a difference if we extended the air duct outside so that the end of it dropped below the level of the firebox in the house?
I’m having a particularly hard time with this since the woodstove in our previous house was in the basement with an outside air source that exited the basement at least 5 feet above the firebox and we never had an issue. That chimney, however, went straight up through the center of house, through 4 floors -so lots of draft and it was warm. I suppose that may have made the difference?
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Replies
knh1VT,
I'm sure you will get some good advice here, but the best place for your question is: https://www.woodheat.org/questions-and-answers-about-wood-heat.html
I would bet that when the damper is closed, you do not have adequate draft. How is the stove connected to the fireplace flue. Is it closely sealed? Was this install checked by a building official?
knh1VT,
Check out this link from Hearth.com: https://www.hearth.com/talk/threads/regency-2450-backdrafting.183295/
This is a different brand but it seems there was a change in the EPA design requirements that added a more restrictive deflector and people are claiming it is choking the firebox causing smoke/backdrafting.
I have been using a Kozy Heat Z42 this season in a very tight house with no issues. This a fireplace as opposed to a stove and built in 2019 so any 2020 design change requirements would not apply. It has airtight box and OAK that provides air directly to the firebox. For 10-years prior to this season, I was using a QuadraFire 7100 with an OAK in a very drafty house. I did have to crack a window on start-up to get the draft flowing in the right direction, but once the flames started to "dance" I was fine.
I'd look to ensure that your stack flow isn't impeded. I've been running a small Lopi for 9 years heating about 1,400 ft-sq: manufactured home, so the stove has a fresh air intake. I run into problems with deposits building up above the bricks and restricting flow up the chimney, thus causing backdraft situations: but only after months of non-stop burning (it's our primary heating device). I clean the stove (and chimney- very short because the stove sits near an outer wall and the roof pitch is shallow) and it's good for many more months. I'll confess, the stove will get run damped down, which for sure causes more buildup (which then tends to fall back down).
Refer to section 4 chimneys and venting, of your installation manual. The first paragraph states "When there is a good match between all the parts, the system works well." If you read further to section C for masonry chimneys it states that a minimum size chimney is to be 8 in diameter. If your chimney is only 6 in then you may have a velocity problem associated with the drafting especially if you are burning a hot fire as you said you had. Once you close the damper, you are restricting this flow even more. Unfortunately, the damper is either full open or full closed with no intermediate positions for better balancing. Burning a smaller fire may be your solution to reduce the need for faster flow. This is even mentioned in the manual.
The faster flow moves move of a larger volume of exhaust that has to be matched by the intake which would have to be increased in size in order to match the flow by reducing friction. Hopefully you can see how closing the damper will affect this flow. So best to experiment and find what operating temperature works best which may only require one log every few hours or so.
Thank you so much for all the very helpful replies. I've been looking into all the different avenues suggested - then the installers came out again today (Jonathan Blaney asked if the install was inspected by a building inspector - that put a smile on my face. I live in the wilds of Vermont, where there are codes but no enforcement. The installers, however, are certified, work with the state on efficiency and have a very good reputation.)
The upshot appears to be (we need to test this over a few days) that the problem was caused by following the directions in the owner's manual. I'm not kidding. I am about to be 62 and have heated with wood for all but about 20 years of my life. I know how to use a woodstove, but this is a brand new model, blah, blah, blah - I decided to read the manual. We had had fires for three nights (and 2 1/2 days) with no problems. But I started reading the manual the day after the install - and in the manual, in at least 2 places, and maybe more, they specifically note that you should "rake the coals toward the back of the stove" before adding wood. That seemed odd to me - the airports are there. But the manual clearly directs you to do that. So I did it. When the installers came back (a second time) today, they found the airports choked with ash. Gee, I wonder why? Of course I felt like an idiot - but I was doing what the manual said to do! The manual also tells you that after raking the coals and adding the wood, to then shut the damper! Not to wait until the temp reaches 500F - just shut it down. The installer agreed that was also nuts.
At any rate, I think the installer fixed it when he cleaned the ash out of the ports.
I am going to keep a record of all the great responses I got here - I so appreciate everyone's time and energy on this. If the installers didn't solve it, I'll let you know. And by the way, if you're in Vermont, the installers were Montpelier Stove and Flag Works. They know their stoves and they know about draft and chimneys, etc. They've been very responsive - they have come out twice on this issue and made it clear they'll come back until it's solved.
Makes sense since the faster flow out the chimney is acting like a vacuum cleaner sucking the ash up and into the air ports.
We've been burning the stove, shutting it down, and no smoke! Every time I add wood now, I pull the ash and coals away from the back wall instead of following the owner's manual instructions to rake them back against the wall. Works perfectly. It's a great little stove. Too bad the manufacturers don't know how to run it! :)