Caked Cap and Reverse draft

[claybe]

It's been a while since I have posted. Been following in the background. Tonight I went to start a fire as normal and could not get a draft. It was a little windy outside but nothing crazy. As the fire started all the smoke blew back into the house. I shut the door and the thermo was open (blaze king 15' pipe) and smoke was pouring into the house through that. Ultimately had to pour a little water on the fire to get it to go out. Needless to say there was smoke everywhere in the house. I got out the ladder and got up on the roof in the dark and my chimney cap has creosote all over it. It wasn't all clogged but one side was pretty caked up. Now I am wondering if creosote has blocked the pipe. Or?

I am quite surprised at how much creosote was up there. Last year when I cleaned the pipe and cap half way through the season the cap was caked and hardly anything came out of the pipe when I cleaned it. What causes this caking on the cap and why only one side? I won't be able to investigate further until this weekend.

My wood is dry and seasoned. Burning "city mix" with a lot of pine. Tests at about 8%. We run about 3 humidifiers in the house. Wondering if the humidity from these is going straight up the pipe and condensing on the cap?

I plan to clean everything this weekend. Anything else I should be looking for?


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[Highbeam]

Is there a mesh filter in your cap?

 

[moresnow]

Is there a mesh filter in your cap?

Ditto. Not so good in many cases. May work for some but known to plug also. Interesting to hear what you find.

 

[claybe]

There is an expanded metal section before the top of the cap. No wire mesh though.


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[sportbikerider78]

Pop the cap...have a looksee. All else is speculation.

 

[Highbeam]

There is an expanded metal section before the top of the cap. No wire mesh though.


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That expanded metal is a filter. If it's possible to remove it, the clogging will no longer be possible. Can we assume the filter is what you refer to as being caked?

 

[xman23]

The cap, is not insulated, so it's is. When the smoke hits it you get creosote build up on it. So remove or open up the grill. Leave the top.

 

[claybe]

If the expanded metal is a filter then yes, the filter is clogged, but not 100%. It is only about 40% clogged. Still don't know why that would reverse the draft and force smoke back into the house. Also, I figured when it is hot and when it is blowing snow that creates creosote? If that is the case then I need to get a new cap. Any recommendations?!?


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[xman23]

If the expanded metal is a filter then yes, the filter is clogged, but not 100%. It is only about 40% clogged. Still don't know why that would reverse the draft and force smoke back into the house. Also, I figured when it is hot and when it is blowing snow that creates creosote? If that is the case then I need to get a new cap. Any recommendations?!?


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If it's plugged up the smoke can't get out fast enough. It fills the chimney until it's coming out the loading door. This normally begins with a cold start up. It just get worse. I plugged up my screen in half a season. I removed the screen 16 years ago. No more issues. I don't know your cap, but a small modification and it will be fine. I would cut the sides wide open with tin snips. Leave just supports for the top.

 

[willoweep]

Was the creosote plug on the east side of the cap? Convective heat loss, cooling the flue gases to the condensation point. When the wicked winds of the west start to blow, it is a common occurrence. Get up there and scrape it, don't remove the screen, you need a spark arrestor in most counties in Colorado due to fire danger.

 

[claybe]

Yes most of the gunk is on the east side.


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[willoweep]

The west wind pushes the smoke east, cooling it as it hits the screen. BKs have low flue gas temps to begin with. How much cottonwood in your 'city mix?

 

[webby3650]

Do you use some paper when you are starting the fire? A negative draft isn't uncommon on a cold stove. It won't usually reverse until there's a good flash of heat. A box full of wood and a fire starter isn't enough sometimes, a piece of newspaper will usually do the trick.

8%MC? Are you sure about that? I've never ever had wood that dry, nor have I had a clogged cap, ever. I don't think it's even possible to get wood that dry around here. Did you split the piece right before you tested the room temperature piece?

 

[jetsam]

I clogged my cap once, burning low with wet wood. After seeing that completely plugged screen, I was sure that the flue was going to be filled with a block of creosote, but it was really not bad. Brushed it out anyway!

The lower the surface temperature, the more condensation.

 

[willoweep]

Living lodgepole pine was running 12-15 % MC in the forest last summer. Cut, split and stacked it easily reaches single digits.

 

[webby3650]

Living lodgepole pine was running 12-15 % MC in the forest last summer. Cut, split and stacked it easily reaches single digits.

So living trees can be cut and burned the same day? 12% seems too low to keep something alive.

 

[jetsam]

So living trees can be cut and burned the same day? 12% seems too low to keep something alive.

It sounds like they almost face selective pressure to burn well. That's kind of amazing.

https://en.m.wikipedia.org/wiki/Lodgepole_Pine

They're demon kamikaze trees. Born to burn, needs fire to reproduce. ...I would not have guessed that!

 

[Corey]

A partially plugged cap can certainly be a contributor to low draft, but I would expect it to be a slow process where you loose a bit of draft at a time as the cap slowly plugs. I would not expect it to be an 'all of the sudden one night' type thing. Plus, the plugging cap would reduce the draft, but it would not create a reverse draft on its own.

It is possible the wind might have been acting against the plugged section to create the back draft. But another big factor is temperature. Any sudden warm-up of the air outside - while the house is still relatively cool - can create a backdraft.

The key scenario for me is typically a weekday with a cold night / cool morning which chills the house, followed by a sudden warmup and some humidity in the afternoon. No one would be home during the day, so a 58F house would be no big thing. But when I get home in the evening, I'd like a fire to chase the chill away. Indoors at 58F and outdoors around the same would mean essentially no draft and any little thing could tip that into a backdraft situation.

If I know the temps are close, or don't feel a bit of 'normal' / correct draft, I will fire a propane torch up the flue for 30-40 seconds to get things rolling in the right direction, then light off my pre-staged bundle of kindling and wood and close the door. I also tend to set up my kindling to get a fairly big/hot fire going really quickly. You want the fire to be feeding self-sustaining heat up the flue as soon as possible - and certainly before the heat from the 30-40 second propane torch blast goes away.

 

[Zack R]

The draft on my recently installed Jotul F45 (Sept 2017 install, used nearly every day since) has become progressively weaker over the last few weeks as the cold temperatures have finally settled in (low this morning was 7F, highs in the 30's). Loading mid cycle resulted in smoke spillage where there was none before so I called and setup and appointment with the chimney sweep for Monday.

Curious about the chimney cap situation I looked through my binoculars and couldn't see through the mesh. With a low fire down to coals and a nice sunny day I figured why not take a look up close. Lets just say I was surprised how clogged it was, that I was even able to have a fire. I didn't pull the cap off but did clean it out with a flat head screwdriver, the heat pushed the creosote dust away as I went.

Now the draft is strong (two story, 6" liner the whole way) but I will keep the appointment for Monday so the chimney itself can get a complete cleaning. For those of you with a screen mesh I recommend taking a look sooner than later. I forgot to take an after photo but you get the idea.