Control Secondary Burn by Blocking Secondary Intakes

[ColdNorCal]

Would like to know if its a good idea to block the secondary intake ports when the stove is approaching the point of uncontrollable and it looks as if the gates of hell have opened up. I read comments in the past about blocking secondary intake ports but have not read if it works well or if its a good idea.

It seems that the best burn temps for longer burn times and still heating the house is when the STT is ~500-600 degrees. It would be nice to have a bit more control of the burn and temps.

We have a Century FW3000 and the secondary inlets are in the back, on the bottom of stove.

 

[brenndatomu]

I wouldn't block them completely...restrict some would be OK...something that can be easily removed after the "firestorm" part of the burn is over. You might consider restricting the primary air about the same amount too...since it cannot be fully closed on modern stoves.
I had to do this on an easy breathing VZ stove I had...especially when it was real cold out...I would have rather had a damper in the stovepipe, but on this particular install that would have been difficult to do.
That would be the first step, if you haven't already...install a manual damper...control that draft!

 

[ColdNorCal]

I wouldn't block them completely...restrict some would be OK...something that can be easily removed after the "firestorm" part of the burn is over. You might consider restricting the primary air about the same amount too...since it cannot be fully closed on modern stoves.
I had to do this on an easy breathing VZ stove I had...especially when it was real cold out...I would have rather had a damper in the stovepipe, but on this particular install that would have been difficult to do.
That would be the first step, if you haven't already...install a manual damper...control that draft!

Yes, partially block/restrict intake ports.

The flu goes into an interior chimney so can not install flu damper.

Dont know how to restrict primary air? Their is the small hole in the dog house. I tried blocking that but did not do much to change the burn. Are you referring to the adjustable damper on the front of stove? How did you modify yours?

 

[brenndatomu]

The flu goes into an interior chimney so can not install flu damper.

Not following...? You can't access the stove pipe?

Dont know how to restrict primary air? Their is the small hole in the dog house. I tried blocking that but did not do much to change the burn. Are you referring to the adjustable damper on the front of stove? How did you modify yours?

On mine there were two square holes underneath, I wadded some foil up and wrapped it around a wooden dowel so my "plug" had a hole in it...then I would stick them in (1, or both) in as needed.
On the primary air I partially covered the main intake hole with foil duct tape...also made a little foil ball to stick in the dog house hole sometimes too (I did not restrict/plug everything all at once)

 

[ColdNorCal]

Not following...? You can't access the stove pipe?

On mine there were two square holes underneath, I wadded some foil up and wrapped it around a wooden dowel so my "plug" had a hole in it...then I would stick them in (1, or both) in as needed.
On the primary air I partially covered the main intake hole with foil duct tape...also made a little foil ball to stick in the dog house hole sometimes too (I did not restrict/plug everything all at once)

The stove sits partially inside a large fireplace.. Stainless steel flex stove pipe is used and goes into the masonry chimney. No black stove pipe used or exposed in the room.

Thanks for the info!


Update:

Decided to look closer at the primary vents, When fully closed, it only leaves a very small opening. One pic is primary open. The other pic is fully closed. Their are two primary intakes. I took a pic of nly one as the other intake is very very close to being closed completely.

I think restricting the secondary intake ports only on this stove is the way to go.

 

[wagne223]

If over firing is a normal occurance for you then I would probably make an attempt at throttling the air. Preferably with a damper but if that is not an option then modify the intake.
However, if it only happens occasionally, then what I do now is slowly open the door.
Secondary combustion stops, and a lot of heat goes up the flue.
Note: be sure to use your stove gloves!

 

[ColdNorCal]

Its just occasional. Did the "open door" before and would rather do a safer method.

 

[ColdNorCal]

Its just occasional. Did the "open door" before and would rather do a safer method.

Update:

The stove went into full secondary with a STT of only 575 and I decided this would be a good time to try restricting the seconday intakes and the small intake port in the dog house. Blocking secondary intakes definitely reduced flames in the mid to rear of the stove. Blocking the dog house intake slightly reduced front to mid flames. Restricted together made an obvious impact of flame reduction. In short, I like it

Its unfortunate these stove do not allow more control of the burn. Waiting until STT are 500+ degrees then as desired, having the ability to throttle the air would be a helpful feature for longer burn times and managing any possibility of over firing of stove.

Are their any reasons not to manage the stove this way? To be honest, I dont see doing this every day or every burn.

 

[begreen]

The reported SSTs are not exceptionally high. Adjust burning habits, not the stove. Full secondary burn is proper behavior when there is a major bloom of hot wood gases in the firebox.

 

[peakbagger]

My guess is the manufacturer designed the stove to meet some sort of emission standards at a certain range of outputs. You are may be operating it outside the range it was designed for. Cutting back on secondary air can possibly increase emissions.

 

[ColdNorCal]

The reported SSTs are not exceptionally high. Adjust burning habits, not the stove. Full secondary burn is proper behavior when there is a major bloom of hot wood gases in the firebox.

Todays 575 STT was for test purposes only. Reading the entire thread would help in understanding the point of it all.

 

[ColdNorCal]

My guess is the manufacturer designed the stove to meet some sort of emission standards at a certain range of outputs. You are may be operating it outside the range it was designed for. Cutting back on secondary air can possibly increase emissions.

Yeah, possibly, hard to know. I would think that cutting back air when STT is exceeding 650 would not change emissions. Its possible the manufacturers allow only so much air control so that the air is not throttled back too much and too early in the burn. That would certainly impact emissions.

 

[peakbagger]

There are some companies with Lamda controls that control excess air to the stack. If you know excess air and the gas temp then secondary air can be throttled but at some point you also need to deal mess with primary air.

I will be messing with a 4 story wood story high wood boiler that I think puts out 18,000 pound of steam per hour in the few weeks. that has all sorts of combustion controls to play with and screw up.

 

[begreen]

As stated in other threads, cutting back the air based on SST is not the best guide, especially for cold starts. In some stoves it's ok to start turning down the air at 300ºSST. Flue temps are a better guide. For an insert this usually means a digital thermometer. In lieu of that, a trained eye can usually see when to start turning down the air.

 

[brenndatomu]

If you cut the air back sooner (lower STT) on the VZ stove I mentioned earlier, it would still go nuclear...it just delayed the event a bit, but it would still get there...if not, it was because I did it too soon and it "stalled".
I was restricting air just enough to get STT back into the 6-700* range...

 

[begreen]

If you cut the air back sooner (lower STT) on the VZ stove I mentioned earlier, it would still go nuclear...it just delayed the event a bit, but it would still get there...if not, it was because I did it too soon and it "stalled".
I was restricting air just enough to get STT back into the 6-700* range...

Yes, different stoves have different configurations for secondary air. The other methods of control are thicker splits, loading orientation and less air between the splits. Of course, if the draft is well above spec then alternative methods may need to be employed like blocking boost air and reduced primary air. A short flue system of 15' is not likely to need hardware alterations. Secondary air is doing what the stove needs to do, keeping emissions low.

 

[Das Jugghead]

Would like to know if its a good idea to block the secondary intake ports when the stove is approaching the point of uncontrollable and it looks as if the gates of hell have opened up. I read comments in the past about blocking secondary intake ports but have not read if it works well or if its a good idea.

It seems that the best burn temps for longer burn times and still heating the house is when the STT is ~500-600 degrees. It would be nice to have a bit more control of the burn and temps.

We have a Century FW3000 and the secondary inlets are in the back, on the bottom of stove.

View attachment 273007

I installed stainless nuts in the ends of my secondary tubes to slow down the air to them.

 

[Slimdusty]

I installed stainless nuts in the ends of my secondary tubes to slow down the air to them.

Picking up an old thread here, but curious if you are still using your stove with the nuts in the tube ends and how you like it? I have been thinking of ways to slow down the burn cycle in my insert.

 

[begreen]

What is the goal of slowing it down? Too much heat or wanting a longer burn time?

Does the insert have an unusually tall liner, say over 25' tall? If not, perhaps changing the way the insert is loaded or how quickly the air is turned down is better than modifying it?

 

[Slimdusty]

What is the goal of slowing it down? Too much heat or wanting a longer burn time?

Does the insert have an unusually tall liner, say over 25' tall? If not, perhaps changing the way the insert is loaded or how quickly the air is turned down is better than modifying it?

Longer burn times. My liner is short at 14’, draft is fine and definitely not over drafting. Overall I like my setup, however my flue temps do run a tad high in my opinion. Auber probe just above appliance adapter, so this may contribute to higher temp readings. For example, last night, full load of wood, temps spiked at 830 with primary air closed, secondaries vigorously firing. No more control. My thought is if I can slow the secondaries down a bit, it would allow for slightly cooler temps and in theory a bit longer burn. I feel like the size of my wood and loading is appropriate for the insert. What say you?

 

[begreen]

Flue temp will read high when taken right at the stove exit. To extend the burn, try changing the way the stove is run by packing the firebox tighter with thicker splits and smaller pieces filling in the gap. Turn down the air aggressively once the fire starts going. Turn it down enough to make the flames lazy, but not out. Then wait for the fire to regain strength and turn down the air some more. That should help with preventing too high flue temps on startup and help lengthen the burn time.

 

[Slimdusty]

I am already turning the air down as early as I can get away without stalling, and with decent sized splits. Last night’s example, I had the primary air closed off, temps dropped to 650 or so, then slowly proceeded to climb up and peak at 830 over the next hour. Nothing I can do but watch it climb. 830 degrees doesn’t make me panic but it’s plenty high and wasted heat up the flue, hence my pondering of limiting secondary air. I get the feeling you dont want me tinkering on my stove controls 😂

 

[bholler]

I am already turning the air down as early as I can get away without stalling, and with decent sized splits. Last night’s example, I had the primary air closed off, temps dropped to 650 or so, then slowly proceeded to climb up and peak at 830 over the next hour. Nothing I can do but watch it climb. 830 degrees doesn’t make me panic but it’s plenty high and wasted heat up the flue, hence my pondering of limiting secondary air. I get the feeling you dont want me tinkering on my stove controls 😂

Messing with the stove should be a last resort. And with your chimney height it shouldn't be needed at all. What type of wood are you using how dry is it and how long are your burns?

 

[Slimdusty]

Messing with the stove should be a last resort. And with your chimney height it shouldn't be needed at all. What type of wood are you using how dry is it and how long are your burns?

I burn a lot of willow and then locust later on when it gets cold. Wood is dry, have not checked moisture content lately but it’s had two summers in an arid climate to dry. My burn times average 6-8 hours on a full box, 2.2 cubic foot so they say. Definitely can’t put 2.2 of wood in the box. Not terrible burn times per say, but I go back to higher than ideal flue temps equals wasted btu’s and lack of controlling the air enough.

 

[Todd]

I’ve messed with my secondary air ports with my previous Jotul’s. It definitely made a difference partially blocking them with magnets. It was hard though finding that sweet spot since so many variables in burning. It would be better if one could figure a way to make an adjustable secondary air slide instead of just a fixed blockage.