Efficiency Tradeoffs of burning w/ max primary air

[Blowndiffuser]

I don't actually have a Jotul stove, I just thought it was some good advice I found on their website. I've got an Aga and Aarrow (both UK manufactures) and both have primary and secondary air controls. Most people in the UK close down the primary air control once the fire is going and then start to close down the secondary air control to achieve a nice gentle flame. Keeping the secondary air control fully open maintains an intense fire (as Jotul advises). Yes I probably go through more wood but I know there will be no risk of a creosote build up.

 

[Blowndiffuser]

Are you calling a flue damper as your "secondary" maybe?? And your stove air control as your "primary" ??
You mentioned "draught control". ????

I don't have a flue damper, both my stoves have a primary and secondary air control

 

[Clydeburner]

So you've got one of those stoves that when you turn it down it gets hotter? Could your experience be anomalous? The instructions for your stove likely indicate higher intake air settings for higher output.

My Clydesdale responds the same way once the fire is established, the further I close the primary air the higher the temps get. I start closing mine around 300°, fully closed by 400° and the stove will climb up to/over 600° all with my fan set to med-high(with good dry wood, stumbled on some so-so red oak in the pile and had to leave the air open a least a 1/3 for it to burn with minimal secondaries and heat output

 

[Highbeam]

My Clydesdale responds the same way once the fire is established, the further I close the primary air the higher the temps get. I start closing mine around 300°, fully closed by 400° and the stove will climb up to/over 600° all with my fan set to med-high(with good dry wood, stumbled on some so-so red oak in the pile and had to leave the air open a least a 1/3 for it to burn with minimal secondaries and heat output

Sure but what do you think would happen if you left the air control fully open? With an epa noncat you don't have much control but more air equals hotter stove.

 

[Clydeburner]

Sure but what do you think would happen if you left the air control fully open? With an epa noncat you don't have much control but more air equals hotter stove.

I have thermocouples on stove surface and flue probe and can tell you 100% running air full open will blast direct heat straight up your chimney and WILL cause your chimney to be over fired. I have gone to 1000° on reloads pretty quick, If you close the air down in increments the stove will retain the heat and lower the flue temps making a safer burn and a hotter stove.
These have been mine findings

 

[begreen]

Likewise. I have missed turning down the stove air once this winter. In 20 minutes I had a 1200F (probe temp) flue and the stove top was still at 250F. Too much air means less secondary burn which is where the efficiency and higher heat really kicks in. Decreasing primary air with a strong fire and draft pulls more air through the secondary manifold and increases the air feed at the top of the fire, burning off the bloom of gases coming off the wood.

 

[Highbeam]

There must be something lost in translation here. It is not true that reducing air to a fire causes the fire to get hotter.

 

[begreen]

When reducing the primary air, draft pulls in more secondary air. That will can cause the fire to burn hotter. In a modern non-cat there is still some flow of primary air going to the fire with the primary reduced but the bias changes to unregulated secondary air. Of course this depends on the stove and chimney. If draft is not strong then at some point reducing the air will also reduce the overall heat generated. But in some setups, ours included, one can reduce the air most of the way and achieve recommended temp limit for the stove. Increasing the air, say 25% at that point would cause the stove to overfire and the stove pipe to reach temps above its tested limits.

 

[Clydeburner]

^^^^^^^^ yup nailed it!
It could all depends on your setup, but closing the primary will allow the gasses to provide the heat instead of the fiber that makes up the wood we burn. Once the fire is established start to close the air until the flame becomes lazy, let that settle in and I'd bet you will have plenty of heat pumping out with way less wood burned.

 

[Clydeburner]

Also,
A good sign of your flue getting too hot ,may be bluing of your chimney cap and if you have to remove your surround/stove for cleaning you will notice the same if not more near the stove connection

 

[7acres]

I'm glad I posted this thread. It reminded me that last season (our first) my wife and I were vigilant with our loading and reloading regimen. We religiously followed the routine of enjoying the flames catch then cutting down the primary air till it was down at ~30% as soon as the fire was ready. This year we just haven't been as careful. Frankly we just forgot there was an important process to follow. We also both realized we have used more wood than we remember last year even though last Winter was colder. Running with primary air wide open a lot of the time is the reason!

It's nice when head knowledge syncs with experience! Thanks, begreen and others for helping me connect the dots!

 

[7acres]

One other thing I realized is that running with full primary air after the fire is established is akin to using the afterburners on a fighter jet. Yes, you can go 20% faster but you're burning through your fuel 100% faster. So there is a Top Speed vs Fuel Efficiency tradeoff.

With a wood stove the tradeoff is Peak Stove Top Temp VS Total BTU Output for a given load of wood. Running full primary air will make the stove top temps peak 200F hotter. But the load is petered out in 3 hours. As opposed to using only 30% primary air and being in the nice 400-600F range for 6-8-10 hours (depending on MC and wood species).

 

[Blowndiffuser]

From reading these posts I presume most of you don't have a secondary air control (airwash) located just above the fire chamber?

 

[begreen]

Correct. Typically the airwash air comes from the primary air manifold. There are some old pre-epa stoves that had the airwash set up that way, but having an actual secondary manifold has replaced these older systems for more complete burning.

 

[7acres]

Correct. Control over primary air only.

 

[Clydeburner]

The only other thing that could help with control and overall burn could be a flue damper, but again with newer epa stoves these shouldn't be a necessity like they were with the old smoke dragons. I would think in most cases when a flue damper is needed it is due to excessive draft and even when it is throttled back the secondary air intake is still under adjustable from the stove manufacturer.

 

[Blowndiffuser]

Almost all stoves sold in the UK have a secondary air control. The idea is that the air is already hot when it meets the hot gases from the burning wood. The gasses therefore ignite in the upper part of the stove making the stove much hotter and releasing more heat energy from the wood. Hence why I keep my secondary control fully open with the primary fully closed.

 

[begreen]

I believe that Jotul and Morso stoves sold in the UK do not have secondary air control, nor most continent clean burners like Rais, Wittus, Nestor Martin, etc.. The mfg. can achieve more predictable clean results by taking user error out of the operation.

 

[Blowndiffuser]

I believe that Jotul and Morso stoves sold in the UK do not have secondary air control, nor most continent clean burners like Rais, Wittus, Nestor Martin, etc.. The mfg. can achieve more predictable clean results by taking user error out of the operation.

Some of them do like the Jotul MF 3 and Morso Squirrel

 

[begreen]

This could be due to different emissions standards. Where does one find the emissions for UK stove models? I'm curious about how they are tested and what the testing results for emissions are. In Dovre and Stovax stove literature I am only finding %CO listed.

 

[Blowndiffuser]

I'll have a look to see what I can find

 

[begreen]

It also could be due to different fuel options. Many UK stoves are multi-fuel and are setup to burn a wide range of fuels, some which are not available here.

 

[Squisher]

My experience has been the same as cClydeburner and Begreen. Once established my air is turned down to closed or nearly closed for most burns. If I require more heat I'll bump the air open a bit more but temps would skyrocket if I ran wide open. I let my stove get a little to hot on a reload last night and it took hours and hours to come down from 750 stove top to 500 with the air right closed. Also before I 'remembered' actually smelled that I'd left the air wide open to long I was reading 660 exterior single wall stove pipe about 12"s above stove collar(digital auberins) and at about 18"s up my condor magnetic thermometer was nearly buried. So I had flue temps heading into the danger zone but my stove top was still in the 700-800 range. Wasting fuel and creating a potentially dangerous situation.

 

[Clydeburner]

My experience has been the same as cClydeburner and Begreen. Once established my air is turned down to closed or nearly closed for most burns. If I require more heat I'll bump the air open a bit more but temps would skyrocket if I ran wide open. I let my stove get a little to hot on a reload last night and it took hours and hours to come down from 750 stove top to 500 with the air right closed. Also before I 'remembered' actually smelled that I'd left the air wide open to long I was reading 660 exterior single wall stove pipe about 12"s above stove collar(digital auberins) and at about 18"s up my condor magnetic thermometer was nearly buried. So I had flue temps heading into the danger zone but my stove top was still in the 700-800 range. Wasting fuel and creating a potentially dangerous situation.

Glad to hear you caught it before it became too serious. I would imagine your flue gas temps were above 1200°....scary

 

[Highbeam]

One other thing I realized is that running with full primary air after the fire is established is akin to using the afterburners on a fighter jet. Yes, you can go 20% faster but you're burning through your fuel 100% faster. So there is a Top Speed vs Fuel Efficiency tradeoff.

With a wood stove the tradeoff is Peak Stove Top Temp VS Total BTU Output for a given load of wood. Running full primary air will make the stove top temps peak 200F hotter. But the load is petered out in 3 hours. As opposed to using only 30% primary air and being in the nice 400-600F range for 6-8-10 hours (depending on MC and wood species).

This. If this is the consensus then I am now on board with you guys. The confusion came from the original post which was about high output, not 600 but up to 800 degrees, and you won't get there with the air control closed. To get there you need to run your air higher and waste more heat up the flue. Of course you can't run 100% open intake air because at some point you overtemp the flue which is the biggest reason I have a temp gauge on my flue. The secret is running the flue as hot as possible and watching the surface temp climb much higher than some measly 600.

With a modern non-cat you can only run hot or hotter. There is no low temperature option as can be shown when you all get full 500-600 temps with the air closed.

Especially valid is the last paragraph of the above quote. I can verify a 3 hour burntime when running my stove as hot as possible up over 700 to get max heat. This puts me right in the using way for fuel for only 200 degrees more heat club. This is less efficient than when you run the stove at low intake air settings and only get that 500-600 temperature.

I have missed turning down the stove air once this winter. In 20 minutes I had a 1200F (probe temp) flue and the stove top was still at 250F. Too much air means less secondary burn which is where the efficiency and higher heat really kicks in.

You see, if you had just reduced your intake air to where the flue was at 1000, eventually the stove would have gone much higher than you got at zero intake air. When you want more heat (700-800), you need more air.