Brief Overfire Flue pipe Explanation

[travelerstro]

Good morning,

Was running my Hearthstone Manchester last night with some well seasoned live oak on a cold start. (hybrid stove with secondary burn and Cat) Everything was going good but I normally have secondaries off gassing thru the first couple of hours once the wood gets charred but not much was going last night. I take the air intake down in stages to prevent the fire from going out like usual. Cat gauge showed in the lower part of the active zone on my last stage of shut down so I engaged it and shut the air almost all the way like I normally do which is around 3/16" from being fully shut.
Stove top temp inside grates at this time was around 350ish according to infrared (Stove has a cast iron jacket over stove top) and the condor pipe probe inside my double wall was reading around 650.
Pipe key damper was wide open at this point and I stepped away for 15 minutes.

When I cam back the condor probe read 1200 and I could smell paint. No visible flames whatsover in the box. The Cat probe was basically on the border of the "too Hot" zone. I panicked and closed the key damper all the way (Icc damper still allows 20% of flue gas even during the completely shut state) and the pipe temp came down to 850 ish within a 5 min time frame. Stove top on the grate/fin area read 450 to 500.

Ive got a 27" stack straight up with double to the ceiling, then class A through my attic and out the roof. Pulls hard which is why I installed the damper.

Was hoping someone could explain to me why and how did this happen? I've never seen the temps rise that fast, especially with no flames in the box. As mentioned I am use to seeing a hard secondary combustion but there was none during this time period last night.

Should I have used my key damper earlier in the burn?

 

[kennyp2339]

Fuel rich environment, no flames in the fire box because not enough air to support it, rich in fuel gases, hit the cat and the cat was eating as much as it could getting very hot.
Next time load the stove, keep air fully opened to allow the wood to catch, then turn down as you normally would both the air and pipe damper, this will keep the off gassing of the wood in check.

 

[travelerstro]

Fuel rich environment, no flames in the fire box because not enough air to support it, rich in fuel gases, hit the cat and the cat was eating as much as it could getting very hot.
Next time load the stove, keep air fully opened to allow the wood to catch, then turn down as you normally would both the air and pipe damper, this will keep the off gassing of the wood in check.

Thanks Kenny! I was hoping someone like yourself would respond. That makes sense. I also only have a couple of fires under my belt since the flue damper was installed and have exhausted my resources trying to figure out the best way to use this thing with my setup.

Basically I need to feed enough air after initially engaging the Cat to make sure the wood stabilizes. or Like you mentioned take the draft down while reducing the Air to ensure correct combustion.

Also, can I run all night with the flue damper engaged at say half closed or will this build creosote in my pipe?

 

[EatenByLimestone]

You shouldn’t have any unburnt fuel past the cat to attach to the wall of your flue and cause creosote to build. Just make sure the flue stays warm enough to keep the gasses going up!

 

[Crab]

Please read the sticky thread " Starting a fire and running an EPA stove" to see the lag in flue temps on a Condor vs. an Auber digital probe. If you read 650 on the Condor there is no doubt the real flue temp was much higher. As advised earlier, turning down the air sooner will help.

 

[begreen]

Please read the sticky thread " Starting a fire and running an EPA stove" to see the lag in flue temps on a Condor vs. an Auber digital probe. If you read 650 on the Condor there is no doubt the real flue temp was much higher. As advised earlier, turning down the air sooner will help.

^^ This is exactly what I was thinking when reading the posting. As the wood heats up, the release of volatiles increases. Sometimes, this can be fairly linear and predictable and other times it is not. The fire may start in one area or one side and not heat up the other side (say a denser, thick split) for 15 minutes or more. When this happens there can be a delayed bloom of wood gases.

A digital flue probe will provide real-time feedback. It's made a big difference in how I run the stove. The alarm function would have caught the rise immediately. It's worked so well for me that I ponied up for the wireless version so that I can also watch the stove temp remotely in my office while moderating Hearth.com.

 

[travelerstro]

Please read the sticky thread " Starting a fire and running an EPA stove" to see the lag in flue temps on a Condor vs. an Auber digital probe. If you read 650 on the Condor there is no doubt the real flue temp was much higher. As advised earlier, turning down the air sooner will help.

Thank you all for the help. It’s is very much appreciated I will read through this link.