Air supply impact on efficiency and emissions

[paddyb]

Long time listener, first time caller...

I've got a large insert stove with damper control, primary air control and airwash that I can't control.

Currently burning ash ~20% moisture.

I've been thinking about the effect of air supply on efficiency and emissions, when the stove is up to temperature and I reload.

With damper open and primary supply open there is huge off gassing resulting in large jets of flame from the splits.

Closing the damper reduces the intensity of the flame but doesn't significantly reduce the size of the flame.

Closing the primary air supply reduces the intensity and size of the flame. This implies less gas is combusted.

Given that off gassing is caused by heat, I assume that it happens regardless of air supply position, and it continues while the stove box temperature is sufficient.

If that is correct, then closing the air supply during the off gassing phase results in a reduction in efficiency and an increase in emissions because there is not enough oxygen supplied to burn the gasses and they go up the chimney.

The conclusion is that if I want to maximise efficiency and minimise emissions I should only close the air supply during off gassing as long as it does not visibly impact combustion.

Obviously this impacts reload cycle time and makes the stove temperature more peaky.

What is incorrect about my thinking/what am I missing?

 

[bholler]

Long time listener, first time caller...

I've got a large insert stove with damper control, primary air control and airwash that I can't control.

Currently burning ash ~20% moisture.

I've been thinking about the effect of air supply on efficiency and emissions, when the stove is up to temperature and I reload.

With damper open and primary supply open there is huge off gassing resulting in large jets of flame from the splits.

Closing the damper reduces the intensity of the flame but doesn't significantly reduce the size of the flame.

Closing the primary air supply reduces the intensity and size of the flame. This implies less gas is combusted.

Given that off gassing is caused by heat, I assume that it happens regardless of air supply position, and it continues while the stove box temperature is sufficient.

If that is correct, then closing the air supply during the off gassing phase results in a reduction in efficiency and an increase in emissions because there is not enough oxygen supplied to burn the gasses and they go up the chimney.

The conclusion is that if I want to maximise efficiency and minimise emissions I should only close the air supply during off gassing as long as it does not visibly impact combustion.

Obviously this impacts reload cycle time and makes the stove temperature more peaky.

What is incorrect about my thinking/what am I missing?

We need to know what stove you are running. Different types of stoves react very differently

 

[EbS-P]

Your thinking assumes the off gassing doesn’t change. In the minute or 5 harder you close down the air you maybe correct. I look at it this way. More air= more flame =more heat =more off gassing. The hotter it gets the more off gassing. But if the think the primary air controls the temperature the. It also control the off gassing rate.

New secondary combustion stoves generally have unregulated secondary air. So the fuel air mixture never runs rich. There is always excess air (in theory).

Generally your efficiency goes down the higher the burn rate. Ie you send more heat up the flue.

I wouldn’t worry to much about it. Burn hot enough to burn clean, and hot enough to keep the house as warm as you want. As long as the chimney stays clean you are doing it well enough.

If you have an old pre EPA smoke dragon the efficiency increase if a new stove is something to consider.

 

[stoveliker]

Sidebar:

The efficiency may change less than you think.
Running high the flue is hotter, more BTUs per minutes te going up indeed. But you also get more BTUs per minute.in the room, so the ratio, i.e. the efficiency, can be the same. (In fact my mfg says it is, for my stove.)

Or, in another way, a load making the flue hot does so over a short time but load burned low makes the flue less hot but for a long time. Integrate those and it may actually be close.

On topic: the above may simply mean you should not worry about it, and do as ebs-p says: burn hot enough to be clean, and hot enough to be comfortable. Let the stove design do its thing (unless your draft is higher than what the stove was designed for).

 

[EbS-P]

The efficiency may change less than you think.

I’m going to guess less than 10% or 15%

 

[EatenByLimestone]

Could be tall flue or waiting too long to close off the air.

Since on a new stove the air cannot be completely closed off, on a hot stove that is running away, more than enough air cannot still get in to ignite the gasses.

 

[gthomas785]

It totally depends on the stove. In older stoves without secondary combustion, closing the air definitely reduces efficiency and increases emissions. This is why the secondary combustion systems were invented.

In a modern stove, closing the primary air will theoretically lower the firebox temperature while still combusting all the organics that are offgassing. So you get an extended burn time with lower heat output but still the benefit of high efficiency and low particulate emissions.

 

[stovelark]

Wow my head hurts. Burn good dry wood into a well drafting chimney at the correct STT's = warmth and a smile, we'll let the magic go on inside the stove on its own accord. Stay warm all, warming up this week in SE CT, snow is gone, looking forward to green returning...

 

[paddyb]

Thanks all for the various replies, sorry for the delay.

The stove is a Seguin (Axis) Poele 1000 https://www.cheminees-seguin.com/en/stove/PO1000SF-POELE-1000/

No secondaries or cat. Seems to use a LOT of brick and an overhead baffle to get a lot of heat in the box and prolong the time gas spends in the combustion area for more complete combustion.

It's not designed to burn overnight, the manual indicates reloading every 45 minutes...a lot of EU manufacturers seem to have gone this way as a method of meeting emissions regs.

I take the point about primary air supply controlling temperature, but there is so much brick in this stove I think the effect of the primary is quite delayed and smoothed out.

 

[EbS-P]

Thanks all for the various replies, sorry for the delay.

The stove is a Seguin (Axis) Poele 1000 https://www.cheminees-seguin.com/en/stove/PO1000SF-POELE-1000/

No secondaries or cat. Seems to use a LOT of brick and an overhead baffle to get a lot of heat in the box and prolong the time gas spends in the combustion area for more complete combustion.

It's not designed to burn overnight, the manual indicates reloading every 45 minutes...a lot of EU manufacturers seem to have gone this way as a method of meeting emissions regs.

I take the point about primary air supply controlling temperature, but there is so much brick in this stove I think the effect of the primary is quite delayed and smoothed out.

I think you are correct about the delay. The key is to light the load with air full open and keep turning it down in stages. You turn it down a bit more a bit more than you think you should then let the fire re-establish. Then turn it down again, as many times as you need to get down to your desired burn rate. But for smaller EU batch burners that’s probably not going to work that well.