Secondary burn operation

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justincorhad

New Member
Jul 3, 2021
22
Canada
Hi there, just have a question about secondary burn operation in newer stoves. I have a droplet pyropak, works flawlessly, beautiful secondary burn. In the operation manual it says once stove is up to temp to close primary air supply fully and secondary burn will then commence, which it does, perfectly. Looking over the stove I see that the secondary air is open all the time with no control. I remember a friend of mine had an older model secondary burn stove and it had adjustment handles for both primary and secondary air.

My question is, is the primary air on my stove (or any newer stove) really shut all the way down? Or is there a stopper to keep it always open slightly? I’m just curious without any primary air whatsoever how the fire box gets enough cool air to flow to the bottom of the fuel to create the smoke for the secondary air to mix with an burn? Or is it simply that once operating in secondary burn mode it pulls enough air through the secondary air holes to both keep the wood shouldering and have enough left to burn the smoke?

I counted that there are 42 secondary holes that look roughly to be 1/4” maybe 3/16”, which gives a total surface area of about 2 inches squared, which seems like a lot, that’s probably not far off of the surface area of the primary air a lot wide open I would imagine.

Happy to learn and hear what you guys have to say!
 

ABMax24

Minister of Fire
Sep 18, 2019
1,226
Grande Prairie, Alberta, Canada
The primary air never closes all the way, there is either a stop or a hole in the damper plate that always lets air in.

With only a few exceptions, all modern non-catalytic stoves are built with the secondary air wide open all the time. Emission regulations have phased out an operator adjustable secondary air control. The only way secondary air is controlled, on the select few that are adjusted, is either by a direct linkage to the primary control, or through a thermostatic or draft based control. Generally on a standard install with an acceptable draft this "wide open" secondary control isn't an issue, the manufacturer has sized this system to work correctly at those draft levels. Excessive chimney draft can be an issue for these systems, but can be dealt with if that is encountered.

In general newer stoves burn hotter than the models of 30 years ago, the reason being the high fire box temperature in combination with the additional oxygen introduced from the secondary system achieves a more complete burn, increasing efficiency and reducing pollution.

Also once a fire gets going the wood doesn't need a lot of oxygen to keep the wood off gassing, the intense radiant heat from the secondary flames and the heat within the firebox can keep the cycle going.

Now this pertains to non-catalytic stoves, the catalytic models work a little different.
 
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justincorhad

New Member
Jul 3, 2021
22
Canada
I kind of figured it must not have closed all the way, I will have to take he cover off the front of the primary air control and have a peek.

You mentioned dealing with excessive chimney draft, would you deal with this by closing the flue damper? Or is there some other way to deal with excessive draft?
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,660
South Puget Sound, WA
Closing the flue damper in the stove pipe is the way to trim excess draft.