Secondary burn operation

[justincorhad]

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]

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.

 

[justincorhad]

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]

Closing the flue damper in the stove pipe is the way to trim excess draft.

 

[MR. GLO]

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.

Great info. Can you explain pros and cons of controlling secondary? I'm guessing one is starting fire.

 

[ABMax24]

Great info. Can you explain pros and cons of controlling secondary? I'm guessing one is starting fire.

Increased efficiency, decreased emissions, longer burn times could all result from a controlled secondary air supply.

The problem is this can't be a user controllable setting, it has to be done either through a linkage to the primary air control or through some other mechanical means such as a thermo-sensitive device such as a bimetallic strip.

It could very easily be done with electronics and a lambda sensor in the flue and a temperature probe, but there is pushback by many against computerizing a wood stove, and there is little justification for it as relatively simple mechanical controls or something as simple as a basic orfice still pass current emissions regulations.

 

[MR. GLO]

Have you seen any videos on controlling it?

I've seen someone with a ball valve on secondary air inlet before.

 

[Dave_in_ABQ]

Not sure why you'd want to control 2ndary. Controlling primary determines the rate at which your wood burns. Having lots of warmed 2ndary air allows for burning as much of the gasses coming off the wood as possible before they go up the chimney. No reason not to burn them and every reason to. If you want less output, turn down the primary more.

 

[ABMax24]

Not sure why you'd want to control 2ndary. Controlling primary determines the rate at which your wood burns. Having lots of warmed 2ndary air allows for burning as much of the gasses coming off the wood as possible before they go up the chimney. No reason not to burn them and every reason to. If you want less output, turn down the primary more.

In an ideal scenario you are correct. But in a case such as high draft from a tall flue this ratio of secondary air to primary air goes out of wack. Eventually it's possible to get so much secondary air that it really isn't all that hot, and the fuel air mixture becomes so lean it is difficult to burn and unstable when it finally does.

All this being said I'm still a believer that controlling draft is the most effective way to operate a stove through a device like a flue damper, but it some cases it's difficult to install one and controlling the secondary air becomes necessary.

 

[MR. GLO]

Thanks.

I must have misunderstood . I thought he wrote you could get these results or better results by controlling secondary air...

"Increased efficiency, decreased emissions, longer burn times could all result from a controlled secondary air supply." I agree better
...

what's the purpose of the sensor? If your not regulating it.

 

[ABMax24]

Have you seen any videos on controlling it?

I've seen someone with a ball valve on secondary air inlet before.

Yeah there's different ways to do it, any type of valve would do the trick, butterfly valve or a simple gate valve are probably best.

A ball valve may not be the best, they generally have plastic (often Teflon) seats that can be damaged by high heat.

 

[MR. GLO]

In an ideal scenario you are correct. But in a case such as high draft from a tall flue this ratio of secondary air to primary air goes out of wack. Eventually it's possible to get so much secondary air that it really isn't all that hot, and the fuel air mixture becomes so lean it is difficult to burn and unstable when it finally does.

All this being said I'm still a believer that controlling draft is the most effective way to operate a stove through a device like a flue damper, but it some cases it's difficult to install one and controlling the secondary air becomes necessary.

Can you explain what draft figures you would like to see...seems like with turing air almost all the way down and then setting draft to guessing. 08
I would get dirty chimney....the stove pipe temp would drop but most likely later in burn anyway.

But maybe since it's in secondary mode it would be cleaner anyway...

Have you seen good results with chimney..

 

[ABMax24]

Thanks.

I must have misunderstood . I thought he wrote you could get these results or better results by controlling secondary air...

"Increased efficiency, decreased emissions, longer burn times could all result from a controlled secondary air supply." I agree better
...

what's the purpose of the sensor? If your not regulating it.

You still can achieve better even on an "ideal install". A standard stove is a compromise in attempt to cleanly combust the variable fuel variable loading patterns inherent to a wood stove. At times (likely most of the time) you will have an excess of secondary air, but it is there to ensure complete combustion in the event of more wood gas being driven off the fuel. This extra air cools the stove to some extent and decreases the amount of heat entering the room. Now are we talking 5%, 10%, that really varies by stove.

You have to remember the combustion of wood is not consistent, it goes through stages. The offgassing stage is when secondary air is needed the most, but as we get to the coaling stage the secondaries could probably be shut off completely, minimizing heat loss up the flue. Without some kind of sensor, air control/actuator and logic processor it is difficult to know when this occurs, so for decades stove builders have been designing stoves to have enough secondary air for the offgassing stage and giving less concern to this extra air that enters and then leaves the firebox unused during other stages of the burn.

For the average stove user it's not really a topic worth considering, modifying a stove has certain liability issues associated with it. If excessive draft is present a flue damper is by far the simplest method of control.

 

[ABMax24]

Can you explain what draft figures you would like to see...seems like with turing air almost all the way down and then setting draft to guessing. 08
I would get dirty chimney....the stove pipe temp would drop but most likely later in burn anyway.

But maybe since it's in secondary mode it would be cleaner anyway...

Have you seen good results with chimney..

Depends on the stove, consult your manufacturer or stove manual for the draft specs for your model. Cutting secondary air too much is also a bad thing, and is almost guaranteed to produce soot, smoke, creosote, or a combination of all three. I'd rather have 50% too much secondary air than 5% too little.

If flue temps drop below enough to allow moisture to condense in the chimney then yes you will see creosote build up much faster. That's the kicker with any combustion appliance, you can only extract so much heat before condensation issues rear their ugly head. For traditionally vented natural gas, oil or propane the efficiency number this occurs at is around 80%, try to increase the efficiency beyond this and condensation begins to occur. Wood stoves have pretty much the same 80% limit, it a problem not uncommon to those running the BK stoves, many get icicles forming at their cap, some even get water inside the chimney, and this is due to the lower flue temps associated with the ~80% efficiency these stoves are achieving.