Exit Gas temps on alternative stoves?

[rashomon]

Hi,
I should first ask for clarification regarding the concerns I see here regarding Flue Temps / Cap temps / Exit temp, or any other temps after gases have left the stove (I hear flue, chimney, and exit temps being used interchangeably, although I'm sure distance from the stove is important -- cap temps I assume being the end).

I assume this is important for keeping creosote buildup down and for making sure you get the draw needed for your specific chimney situation (location, length..). Please add to my understanding if it's more than that.

But it appears these concerns may also depend on the type of stove and how well the stove is able to fully burn material? For instance I believe I've read some Masonry Stoves, Russian Stoves, & Rocket Mass Heater Stoves intentionally have lower exit gas temperatures. I know certain designs claim to burn the fuel much more efficiently -- because of higher internal temperatures in combustion chambers where all the gases are burnt. Those hot gasses are then used to heat the large masses surrounding the route the gases take to escape.

The FlueGuard thermometer I have above my more traditional though EPA certified Green Mountain 60 appears to indicate around 375 degrees is the minimum for "optimal operation". But I assume this EPA stove similarly is able to fully burn the gases as it passed through my catalyst. I am fully double walled once leaving the stove, but considering complete combustion, couldn't I have lower temperatures exiting? Or, could I have had single wall directly above my stove to allow more heat to transfer into my living space (and then transfer to double wall)?

Is the reason Masonry and Rocket Mass Heaters allow for lower exit temps simply because they ensure the complete combustion?

 

[30WCF]

It’s probably not as drastic as you think. I did a little test a month or two ago. Seems it scaled with increased temperatures. At low temps, the difference was about 30 degrees different from the stove to the cap. At high temps, it was a 100-150 degree difference from the stove to the cap. See my posts starting at post # 170 on page 7 then a second test on page 8 when the outdoor temps cooled a bit more.

Regardless. I think the standard answer is keep it above 250F to stop creosote, but if you want to keep the cap at 250, keep the flue at 280 for my set up.
Here is the link. Start at page 7, post 170-197

Vermont Castings Aspen C3 2022

Looked around the forum rules, didn’t see anything against an ongoing review. ( only prohibiting promotions and bashings, this may contain both in the same post😁 ) I have a few other open threads on this stove, but since reviews on this stove seem very hard to come by online, I thought I would...

www.hearth.com

 

[rashomon]

It’s probably not as drastic as you think. I did a little test a month or two ago. Seems it scaled with increased temperatures. At low temps, the difference was about 30 degrees different from the stove to the cap. At high temps, it was a 100-150 degree difference from the stove to the cap. See my posts starting at post # 170 on page 7 then a second test on page 8 when the outdoor temps cooled a bit more.

Regardless. I think the standard answer is keep it above 250F to stop creosote, but if you want to keep the cap at 250, keep the flue at 280 for my set up.
Here is the link. Start at page 7, post 170-197

Vermont Castings Aspen C3 2022

Looked around the forum rules, didn’t see anything against an ongoing review. ( only prohibiting promotions and bashings, this may contain both in the same post😁 ) I have a few other open threads on this stove, but since reviews on this stove seem very hard to come by online, I thought I would...

www.hearth.com

Wow, these are the kinds of experiments I can get behind. Very interesting! Thanks for sharing.

 

[Todd]

Watch you flue temps and look for smoke out your stack. With my stove burning a full load in the cruising stage ( about 1-2 hours in) I will see smoke out the stack if my internal flue temps are less than 400.

Masonry heaters and Rocket stoves are a whole different animal where they burn wide open and very hot for the whole burn so less of a chance of creosote compared to a longer drawn out wood stove burn.

Keep your double wall pipe, it helps maintain draft better than single wall. Heat with the stove not the pipe.

 

[rashomon]

Watch you flue temps and look for smoke out your stack. With my stove burning a full load in the cruising stage ( about 1-2 hours in) I will see smoke out the stack if my internal flue temps are less than 400.

Masonry heaters and Rocket stoves are a whole different animal where they burn wide open and very hot for the whole burn so less of a chance of creosote compared to a longer drawn out wood stove burn.

Keep your double wall pipe, it helps maintain draft better than single wall. Heat with the stove not the pipe.

Great information, thanks.
I do love my EPA stove, but I wish my home had more thermal mass which is why I'm interested in the Masonry and Rocket Mass Heaters.

My EPA stove does have a fan on it so I can extract much more heat before the exhaust exits. Controlling the fan I can easily drop temps from 700+ to 350 in minutes. So yeah, I may as well keep the double wall pipe. And I'll keep an eye on the smoke to see what temps it appears. If my EPA stove is rocking hard maybe I can assume it's hot enough to drop temps a little further considering the smoke could possibly be cleaner.

 

[stoveliker]

But I assume this EPA stove similarly is able to fully burn the gases as it passed through my catalyst

One never fully burns the gases in all parts of the burngin cycle. Upon reloading or starting a fire, and upon changing the air settings (thus changing the amount of fuel the cat has to process), changes in draft (or having too high draft in general) etc. one has some smoke. This is because the capability of the catalyst to process quantities of fuel depends on its temperature. I.e. to process more fuel the cat has to be hotter.
E.g. If one decreases the primary air, one will provide more smoke to the cat to eat, but to do that completely it needs to heat up - which it will slowly do so. But before that's the case there's a (10 minute?) time of smoke.
Having too high draft also results in incomplete combustion - because the time the gases spend in the cat is not long enough if they flow too fast.

All this is not bad IF the pipe is warm enough. If it is below 212 (at the coldest point, likely the top), water (a normal product of combustion of even 0% moisture content wood) will condense on the walls - and sending smoke through a pipe that has condensation on its walls is the way to create creosote deposits.

TLDR: no one has zero smoke at all times during a burning cycle. To avoid condensation, the pipe needs to be kept warm enough, also with a cat stove.

 

[rashomon]

.......TLDR: no one has zero smoke at all times during a burning cycle. To avoid condensation, the pipe needs to be kept warm enough, also with a cat stove.

Great information, thank you. I use a FlueGuard probe at the proper distance above my stove so I'll keep it at least on the bottom end of "optimal". My flue goes straight up and is double walled.