Air sucked through burn pot. Why?

[markg136]

Hello everyone, new to the forum but will be posting alot in the future.
Currently working on a small wood pellet project. Just a question about the process by which air is introduced into the burn pot.
Why is the air sucked through the burn pot? Does it make a difference if the air is blown into the burn pot?
I.e. would you get the same result if the burn pot was enclosed in an airbox and air was forced into the airbox and consequently through vents in the burn pot and into the burn pot itself?
Thanks in advance.
MG

 

[hossthehermit]

Hello everyone, new to the forum but will be posting alot in the future.
Currently working on a small wood pellet project. Just a question about the process by which air is introduced into the burn pot.
Why is the air sucked through the burn pot? Does it make a difference if the air is blown into the burn pot?
I.e. would you get the same result if the burn pot was enclosed in an airbox and air was forced into the airbox and consequently through vents in the burn pot and into the burn pot itself?
Thanks in advance.
MG

Q. If ya blow it into the pot, then where does it go?
A. Wherever it it can find a way out.

Q. If you suck air THROUGH the pot, where does it go?
A. Wherever it's directed.

BUT, that's just mu opinion.

 

[wilbilt]

Hello everyone, new to the forum but will be posting alot in the future.
Currently working on a small wood pellet project. Just a question about the process by which air is introduced into the burn pot.
Why is the air sucked through the burn pot? Does it make a difference if the air is blown into the burn pot?
I.e. would you get the same result if the burn pot was enclosed in an airbox and air was forced into the airbox and consequently through vents in the burn pot and into the burn pot itself?
Thanks in advance.
MG

I would guess that safety is a large part of the reason.

As the owner of an older positive pressure stove (air is blown into the burn pot), I will testify that things can get very interesting within a very short period of time should the firebox door be opened while the stove is in operation.

Even a marginally leaky firebox door gasket can enable sparks to be propelled from the stove.

I often think about what would happen should the gasket or latch fail while the stove is running on "high". It does keep me up some nights.

 

[crausch]

I agree with hoss...on auto ignite stoves the pellets are lit from the bottom. If you blow air at them the air will hit the top pellets and bounce off in all directions. Drawing the air through the pellets get the air through all the pellets and down to where the heat is causing a more efficient combustion.

 

[fyrfightr]

I feel it is for safety. I can open the door to clean the burn pot out with the stove running and not have any smoke come into the room.

 

[hearthtools]

Older bottom fed units are normally Positive air
Martin, Pellet master, Welenco, Scott stove, Fabco and some others
But the all the stove and HOPPER need to be sealed or smoke will come out and a burn back into the hopper.



It is much easier to build a Negative air stove due to the fact nothing has to be sealed.

 

[stoveguy2esw]

to expand on rod's statement also , a positive draft pellet stove would also usually have to be connected to a drawing chimney following the same rules as with a woodstove , we used to build one as well "back in the day"

 

[hearthtools]

to expand on rod's statement also , a positive draft pellet stove would also usually have to be connected to a drawing chimney following the same rules as with a woodstove , we used to build one as well "back in the day"

Yes
and we still get people here that buy a used pellet master and argue with us about having A min of 8 feet of vertical pipe.
then when they get smoke in the house they ask
"WHY DO I SMELL SMOKE"

Here is a cut away of a Positive draft stove with a Ventrury in the vent to help force the smoke out.

 

[slls]

Learn something new every day, and I thought all pellets stoves were negative pressure. I thought negative pressure was an ingenious idea, now I find out it came out later.

 

[markg136]

Thanks everyone.
So you have been talking about the possibility of smoke coming out if it is not sealed properly.
But what about the whole combustion process? I can control the amount of air entering an air box in which the burn pot is installed (and the amount of pellets). So will i still get the same combustion result i.e. temperature and efficiency if the air is forced in as opposed to sucked through?

 

[wilbilt]

Thanks everyone.
So you have been talking about the possibility of smoke coming out if it is not sealed properly.

...as well as sparks and flames...(!)

I don't think it really matters in terms of combustion efficiency whether air is blown in from the bottom or drawn out the top of the firebox. This is assuming the rest of the design corresponds to the method used.

 

[stoveguy2esw]

one of the tough things for a positive draft unit is intake v/s exit air balance. to perform at its peak the intake blower would have to move the right amount of air to first burn the fuel at the fed rate , but also it would have to be tuned to not over power the flue , think about it this way , you have a flue that can pull up to 30CFM out the stack , if you blow 50 cfm in , the flue may not be able to keep up , this would result in backpressure and forcing the exhaust to find other places to go.

a direct vent negative pressure unit pulls what it can through the unit and pushes it out through a pipe that is small enough that you would maintain velocity up to a point without relying on convective draft.

the old 25-pfs we used to builld actually was a pretty successful stove , it worked quite well but the design had one issue that eventually decreased its popularity, you had to have a full blown woodstove chimney you couldnt just dump it through a wall like a direct vent many newer houses were built without chimneys and floks opted for the much less expensive direct vent style due to ease of install , and a lower install cost in most cases. im sure most of the positive draft stoves kinda died out for similar reasons.

 

[markg136]

one of the tough things for a positive draft unit is intake v/s exit air balance. to perform at its peak the intake blower would have to move the right amount of air to first burn the fuel at the fed rate , but also it would have to be tuned to not over power the flue , think about it this way , you have a flue that can pull up to 30CFM out the stack , if you blow 50 cfm in , the flue may not be able to keep up , this would result in backpressure and forcing the exhaust to find other places to go.

a direct vent negative pressure unit pulls what it can through the unit and pushes it out through a pipe that is small enough that you would maintain velocity up to a point without relying on convective draft.

the old 25-pfs we used to builld actually was a pretty successful stove , it worked quite well but the design had one issue that eventually decreased its popularity, you had to have a full blown woodstove chimney you couldnt just dump it through a wall like a direct vent many newer houses were built without chimneys and floks opted for the much less expensive direct vent style due to ease of install , and a lower install cost in most cases. im sure most of the positive draft stoves kinda died out for similar reasons.

Thanks for the reply.
This makes sense. So could this problem be overcome by having a larger flue that is capable of pulling out more than what is forced in? How would you test that you are forcing the right amoung of air in? Is it as simple as analysing the emissions for excess oxygen?