Flue damper for stove with marginal draught.

[TomMcDonald]

Hi Hearth experts!

I have been busy learning my new secondary tube stove - 2.7cf firebox - and have noticed that this thing can get really hot quickly, especially on a larger load of wood.
The secondaries get pumping and sometimes even the airwash air ignites and a curtain of flame envelopes the door glass.

I have measured 850f on the stovetop using an IR thermometer even on a smaller fire with the air all the way down. I resorted to the usual methods of opening the door in steps, putting a pot of water on it etc. but am still concerned that this thing will easily overfire if I'm not diligent.

I have been experimenting with turning the air down very quickly which has worked fairly well.

I have a 14ft flue and with the milder weather here in Australia the draught at start-up is not always perfect. I sometimes have to have the door open for 5 or 10 minutes before it'll get going.

Wood is hardwood and well below 20%.

I'm wondering whether installing a key damper will assist in controlling the fire when it's cruising, or is it likely to make the stove top even hotter?
Additionally, is it likely that the presence of a key damper will negatively affect the poor draught on start-up and make it even more difficult to light?

I haven't examined the air intake mechanism as yet but I think it's pretty easy to get to.

Thanks for the advice and happy to provide more info if required.

 

[begreen]

14 ft is not a tall flue. It's not unusual for the secondary combustion to be highly active if the air is not turned down quickly. This is due to the rapid heating and outgassing of the wood. It may also be a combination of factors including very dry wood. If this is gum or eucalyptus, it will have a very high oil content too. If the splits are smallish in size, under 5" in diameter, then that can contribute also. Top down starts are slower, but often burn longer. On a reload, try letting the coals burn down more.

I would start with turning down the air sooner and if that doesn't work, then blocking off the boost air intake and see if that calms things down.

Can you add the stove make and model to your signature line? That makes it easier to answer stove-specific questions.

 

[TomMcDonald]

14 ft is not a tall flue. It's not unusual for the secondary combustion to be highly active if the air is not turned down quickly. This is due to the rapid heating and outgassing of the wood. It may also be a combination of factors including very dry wood. If this is gum or eucalyptus, it will have a very high oil content too. If the splits are smallish in size, under 5" in diameter, then that can contribute also. Top down starts are slower, but often burn longer. On a reload, try letting the coals burn down more.

I would start with turning down the air sooner and if that doesn't work, then blocking off the boost air intake and see if that calms things down.

Can you add the stove make and model to your signature line? That makes it easier to answer stove-specific questions.

Thanks Begreen,

I will try to be diligent in turning the air down faster - there is a delicate balance, due to the poor draught in the first 15 minutes.

The wood is eucalyptus yes - I will try to use bigger splits to see if that helps. Good suggestion.

As far as I can see, the boost air is blocked by the air control once it is reduce to half way or lower and due to the poor draught am reluctant to do so. However worth a try if other techniques don't improve things.

Will also try the top down method.

Thanks,

Tom.

 

[begreen]

The boost air hole should be separate but we don't have a stove make/model to be verify.

 

[EatenByLimestone]

How dry is your wood? Given the arid nature of much of your continent, could your wood be too dry?

 

[begreen]

How dry is your wood? Given the arid nature of much of your continent, could your wood be too dry?

I am watchful when I burn euc. It likes to burn hot.

 

[TomMcDonald]

How dry is your wood? Given the arid nature of much of your continent, could your wood be too dry?

The wood is eucalyptus and is below 20% generally, sometimes down to 14%.
I haven't tested this batch in a couple of months (since it all dropped below 20%) but will test again to see.
The western sun shines on my wood shed year round and moisture content drops very quickly.
How dry is too dry?

 

[TomMcDonald]

The boost air hole should be separate but we don't have a stove make/model to be verify.

Have updated my signature.

 

[EatenByLimestone]

I’d think much below 15 would be too dry. Doesn’t eucalyptus have an oil in it?

 

[TomMcDonald]

I’d think much below 15 would be too dry. Doesn’t eucalyptus have an oil in it?

There are over 800 species of eucalyptus and some can have volatile oils. I'll have to check the various species I have wood from and find out.
Thanks.

 

[begreen]

Have updated my signature.

Ah, thanks, that helps. The AU docs for this stove are just for installation. They don't include the operation and parts diagrams. My docs for this stove in the US are from 2010. Is the air control still set up like this? If so, I am unfamiliar with the boost air location.

 

[TomMcDonald]

Ah, thanks, that helps. The AU docs for this stove are just for installation. They don't include the operation and parts diagrams. My docs for this stove in the US are from 2010. Is the air control still set up like this?
View attachment 312149

Although I haven't opened up the side shield that hides the air control, this looks like the same setup.
I am led to believe that the rod E closes the boost air (D and C) once the lever is set to half way and below.
Part A is the lever and the control for the air wash.

 

[TomMcDonald]

I made a top down fire tonight using the same wood.
I turned it down much earlier than normal and this is it burning after 1 hour.
Air control is just under half way and I assume boost air is blocked.
Stove top cruising at 650.
Single wall flue at 285 surface temperature at 18 inches.

 

[TomMcDonald]

And this is 90 minutes after lighting.
Air control on minimum setting.
STT 565
Flue temp 250

 

[TomMcDonald]

2 hours in.
Intake at minimum.
580 STT
280 flue.
I'm impressed. Turning it down earlier has made it much more manageable and longer lasting.
I'll be surprised if there aren't usable coals in 10 hours.
Thanks for all the help.
Oh and wood is between 17 and 20%.

 

[EatenByLimestone]

Those burns look great!

 

[TomMcDonald]

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I will keep practicing turning down earlier. I think this will work.

 

[TomMcDonald]

Those burns look great!

Yes thanks. Way less vigorous than when I was waiting too long to turn down the air.

 

[begreen]

2 hours in.
Intake at minimum.
580 STT
280 flue.
I'm impressed. Turning it down earlier has made it much more manageable and longer lasting.
I'll be surprised if there aren't usable coals in 10 hours.
Thanks for all the help.
Oh and wood is between 17 and 20%.

That sounds much better. You're doing well. The fire looks beautiful. It takes time to get fully acquainted with a new stove and flue's burning characteristics. There is some art along with the science of burning.

 

[EatenByLimestone]

I almost think it’s more art than science. Science is 100% repeatable.

While wood burning principles are based on science, due to different wood species, split size, firebox packing, atmospheric conditions and even tree growing conditions within the same species , each load is different. A lot of wood burning comes down to controlling the variables you can, then looking at the fire and knowing to give a little bit more air or take a little bit away.

 

[TomMcDonald]

So the thing went thermo nuclear again this morning even after turning down to full closed very early. Was getting stove top temperatures of 850f.

I decided to expose the intake to have a look and see what's going on.

As per my understanding, the boost air is fully closed with a plug at half air setting and below so that's already blocked.

The air wash is left partially open even at the lowest setting as expected.
I found a magnet and placed it partially over the aperture. Will have a chance to test later.

Does anyone see an issue with partially blocking the air to control the burn early on? Apart from the possibility of increased emissions?

If successful I'd look at shaping a magnet to fit more closely and finely tune the opening.

Thanks again.

 

[EatenByLimestone]

If thats what you have to do to control the air, that's what you have to do!

 

[EbS-P]

I made a top down fire tonight using the same wood.
I turned it down much earlier than normal and this is it burning after 1 hour.
Air control is just under half way and I assume boost air is blocked.
Stove top cruising at 650.
Single wall flue at 285 surface temperature at 18 inches.

I was just going to suggest a top down fire. I have found that adjusting the amount of kindling and the size of the splits under it can have an effect on. Urn rate. I’d probably install a damper. I messed with trying to restrict primary air but it never balanced with the secondary. You might have better luck.

 

[TomMcDonald]

I was just going to suggest a top down fire. I have found that adjusting the amount of kindling and the size of the splits under it can have an effect on. Urn rate. I’d probably install a damper. I messed with trying to restrict primary air but it never balanced with the secondary. You might have better luck.

Thanks. I'll see how this magnet goes and look at a damper after that.

Am I right in thinking the damper will theoretically slow down air coming in due to slowing down gases going out?

Thanks.

 

[begreen]

Am I right in thinking the damper will theoretically slow down air coming in due to slowing down gases going out?

Yes, the damper will reduce the strength of the draught (vacuum) pulling air through the stove.