Cat stoves: closing down the air, how fast?

[edge-of-the-woods]

So a lot of advice I have seen says to close down the damper (primary air, not the cat) a little at a time once the fire gets going.

Why?

I have done both, meaning that I have started closing the air down by 1/4 once the fire gets up to about 1000-1200 on the cat probe temp, and just closing it all the way down at once. I don't seem to see any difference in output.

 

[fire_man]

I asked WS that exact question when I was at the Open House last year. I did not get a technical answer, but was told to close it slowly "just because". I agree with your assessment and have noticed no difference either way.

 

[Woody Stover]

I have done both, meaning that I have started closing the air down by 1/4 once the fire gets up to about 1000-1200 on the cat probe temp, and just closing it all the way down at once. I don't seem to see any difference in output.

Once the cat probe is reading that hot, it's probably glowing already and you can set the air wherever you want, you don't have to step it down. OTOH, right after I've closed the bypass and the cat probe is much lower in temp, I will leave a little flame in the box until the cat catches. When it does, the probe temp will start rising by 100-200 degrees a minute. I have a different stove, though. And I follow procedures with different cat stoves that I run. What does your manual say about when to close the bypass? Do they want a particular temp on the cat probe before you close the bypass?

 

[Highbeam]

I asked WS that exact question when I was at the Open House last year. I did not get a technical answer, but was told to close it slowly "just because". I agree with your assessment and have noticed no difference either way.

I haven't asked but also don't understand why, especially on my thermostatic stove which should soften the blow. I suppose the answer is that a rapid shut down of air could produce a rapid plume of smoke from the snuffed fire that could overwhelm the cat and send its temperature too high to fast.

 

[Woody Stover]

I asked WS that exact question when I was at the Open House last year. I did not get a technical answer, but was told to close it slowly "just because".

If I have pretty lively flame in the box, I'll cut the air some and let the flames settle down a bit before I close the bypass, but usually leave a little flame in the box for a while after I close the bypass. Once it starts catching, I may close the air down in a couple steps. My objective is to have the cat burning smoke as shortly as possible after closing the bypass. On the Dutchwest, the cat probe may be below the active range, but the cat temp starts rising quickly so it is burning, even though it may or may not be glowing yet. Every light-off is different, depending how hot the stove is, how much the load has burned in, and what kind of wood I've loaded (dense woods lick Hickory or BL gas slower than, say, soft Maple or Cherry.)

 

[Woody Stover]

a rapid shut down of air could produce a rapid plume of smoke from the snuffed fire that could overwhelm the cat and send its temperature too high to fast.

If you cut the air that much, the gassing would be reduced pretty quickly, before the cat would get much hotter at all. You can see the glowing in the coals, and the flames, react pretty quickly when you close the air. On your BK, air input may lag a little when you are trying to manually override the thermostat, and are trying to guess how far to move the knob...

 

[tarzan]

With the BK I can begin to shut the air down slowly as the cat becomes active and the cat probe thermometer will rise steadily and settle in to a predictable spot.

On the other hand, if I get a vigorous fire and then suddenly shut the thermostat the cat temp rises quicker and higher. I'm sure doing this is essentially creating a bunch of smoke to feed the cat.

 

[mellow]

If you don't have good true seasoned wood if you shut it down to quick you will get puff backs,you kill your active flame and then once the cat ignites all the smoke in the firebox... whoooof!