Extending burn times with secondary air control

[precaud]

A few months back I posted about adding a secondary air control to my Morso stove. I'm now discovering yet another benefit of this.

Actually, I should say "rediscovering", because it's basically what we did with our "airtight" stoves of the 80's and what the oldtimers did when they talk about banking their fire overnight.

By completely shutting off the secondary air supply, I have easily doubled the burn time of this stove. (By "burn time" I mean, the time between the end of the last fire's flaming phase and when I can easily revive a new fire simply by adding wood on top of the bed of coals.)

I always try to arrange my burning so the flames are dying out right around my bedtime. Last year, I would close the primary air supply, and wake up to find a cold stove with nothing but ashes on the grate. This year, I leave the primary supply just slightly open, completely close off the secondary, and I wake up to find the stove at 150-175 degrees with a bed of coals, mostly black but some still alive. A couple minutes of exposure to full primary air livens the coal bed enough to ignite a new load without building a fire from scratch.

The last two evenings, this period was 9.5 hours. Last night's flames died around 10pm and at 7:30 this morning it came back to life.

Just another benefit of adding a secondary air control to your stove, if it doesn't have one already (most don't.)

I'll also add, in normal operation, I have the secondary air set at about 60-70% open. Under most conditions, the amount of secondary air supplied is MUCH greater than needed.

 

[elkimmeg]

Important note so all other stove owners don'yt try this this works only with secondary air controls, that are not thermomatically controled
do not alter thye thermoatically controled stoves it automatically regulates secondary air supply

 

[Todd]

Sounds like a good idea as long as you shut it down during the coaling stage of the burn. Otherwise you may lose efficiency and have a dirtier burn. I always thought those secondary air inlets caused shorter burns due to lack of control. How did your glass look the next morning?

 

[precaud]

Sounds like a good idea as long as you shut it down during the coaling stage of the burn. Otherwise you may lose efficiency and have a dirtier burn.

That's easy: when the flames die, close it. Simple.

I always thought those secondary air inlets caused shorter burns due to lack of control. How did your glass look the next morning?

As good or better than it was.

 

[precaud]

Important note so all other stove owners don'yt try this this works only with secondary air controls, that are not thermomatically controled
do not alter thye thermoatically controled stoves it automatically regulates secondary air supply

Elk, I think you must get great pleasure posting unnecessary and irrelevant warnings on other people's ideas. This thread is about the benefits of ADDING a secondary air control to a stove that doesn't have one.

 

[Gooserider]

Important note so all other stove owners don'yt try this this works only with secondary air controls, that are not thermomatically controled
do not alter thye thermoatically controled stoves it automatically regulates secondary air supply

Elk, I think you must get great pleasure posting unnecessary and irrelevant warnings on other people's ideas. This thread is about the benefits of ADDING a secondary air control to a stove that doesn't have one.

I don't want to get into the middle of a flame war, but I think I'd back Elk on the need for the warning - it is a relevant warning if you have a stove with a thermostatic control, which is something I don't recall you mentioning whether your stove has or not (and I doubt that folks not owning your model stove memorize it's specs...)

It actually seems to me like Elk was showing a great deal of restraint judging by some of his other posts - I would have expected lectures about the "evils of modifying equipment" in ways the mfgr did not intend, and that could potentially cause you to violate EPA pollution standards. Perhaps he isn't as worried about stuff that doesn't bend Code... ;-P

Gooserider

 

[precaud]

I don't want to get into the middle of a flame war, but I think I'd back Elk on the need for the warning - it is a relevant warning if you have a stove with a thermostatic control, which is something I don't recall you mentioning whether your stove has or not (and I doubt that folks not owning your model stove memorize it's specs...)

Goose, I think what you're advocating is hyper-cautious. Experimenting and tinkering is an important class in the school of learn-by-doing. I don't want to warp this thread into talking about thermostatic control; it's a whole nuther subject, as they say. If my stove had one, I would clearly have stated so. Of the one stove I've examined that had a thermo-controlled secondary, I thought it's design was bass-ackwards and the stove's performance was sub-par as a result. And if I did own one, I'd be examining how it worked and decoding the assumptions made in it's design and implementation. Having been a designer and engineer much of my life, that's just what I do. Hardly anything I own is stock, and it all works better now than the factory made it.

It actually seems to me like Elk was showing a great deal of restraint judging by some of his other posts - I would have expected lectures about the "evils of modifying equipment" in ways the mfgr did not intend, and that could potentially cause you to violate EPA pollution standards. Perhaps he isn't as worried about stuff that doesn't bend Code... ;-P

Perhaps you're right, and if so, I'll take it as a positive response on his part to our previous "encounters"... I think Elk is a good guy and it's great when he speaks about/from his experience. I know for a fact that he modifies his own stuff.

The bottom line for me is: I'm having ALOT of fun with stoves right now, getting good results, and I enjoy sharing it. And there's more to come. I have a unique opportunity to use three different stoves within a 2-week window, and it's very interesting to experience their differences. But that's for another topic.

 

[begreen]

Precaud, have you shut down the secondary during the daytime and monitored stove and stack temps over the day? If yes, what are typical readings?

 

[precaud]

BG, I don't tend to watch stack temps at all, only stovetop. But I haven't monitored temps in the way you describe. Though it would be interesting to see the difference in the temperature tailoff with time.

 

[laynes69]

I have a wood furnace thats about 20 years old. Not even close to epa. Last year I installed the forced draft kit, which the blower has a damper on it to control how much air goes into the firebox. Well that hole is about 1 1/2' to 2" in diameter. So Before I went to bed I would open that a 1/4 of the way and have the ash pan door damper open at about 1 to 1 1/2 turns. At night the fire would smoke sometimes bellow and when I woke up in the morning the fire had coals, but not letting the house get quite up to temps, therefore a cold house and the LP furnace running. Now this year I kept the forced draft damper open all the way, which lets that 1 1/2" hole pour in as much air as can come in. I set the ashpan damper open at 2 to 2 1/2 turns. My burntimes dropped by maybe an hour (Still I get 8 hours), and my home is much warmer in the morning because the house gets hotter at night, and I still have a good live bed of coals. What I have read in the past was at a certain point when the secondary burn is operating the stove of furnace will create a vaccum and let as much air in as needed but not over what you want. When I peek into the damper holes on the door, I will see dancing blue flames and no smoke from the chimney. At these hotter temps believe it or not the wood will not be engulfed in flames, but it will burn mostly the wood gas and smoke. I have always though the more air the faster the burn, but if the air comes in the right places, I have more secondary air than primary, I get a good hot clean burn all night. It amazed me. Now I do burn locust at night which I know helps.

 

[Roospike]

precaud , So your thinking is more secondary air during the first of the fire with new logs and flames from gases.

Than at coal stage to have less secondary air ?

Yeah , that seems opposite of how the EBT on the auto inlet control works on the PE Summit , as of now i agree with how it is now designed and also as laynes69 has described to control the more even heat.

I do agree that with less secondary air you will get longer burn times but like laynes69 stated it take out the needed heat to get the longer burn times and the over all burn temp within 8 hours is uneven.

My .02

 

[precaud]

precaud , So your thinking is more secondary air during the first of the fire with new logs and flames from gases.

Than at coal stage to have less secondary air ?

Absolutely. For clean combustion, the ONLY time you need to have secondary (over-fire) air is when it's in the flaming phase. After that, secondary air is doing nothing more than diluting the exhaust and cooling the stove.

 

[Roospike]

precaud , So your thinking is more secondary air during the first of the fire with new logs and flames from gases.

Than at coal stage to have less secondary air ?

Absolutely. For clean combustion, the ONLY time you need to have secondary (over-fire) air is when it's in the flaming phase. After that, secondary air is doing nothing more than diluting the exhaust and cooling the stove.

So you cut the secindary air on coal burn stage and now your air supply is less so your stove is going to cool. You would have to add main air after cutting secondary air to keep the heat rolling in the stove other wise the stove will cool anyhow.

 

[Roospike]

On the Pacific Energy stove the EBT controls the main air inlet air. So when the stove cools as it does in part of the coal burn stage it opens up main inlet air to the stove so ...........

Stove cools EBT opens air supply.
You have secoindary air + main air
stove cools in coal stage
EBT opens air supply


From the chimneysweeponline.com

"The EBT device is a pivot plate which is mounted over a hole in the stove's air intake plenum so that it covers and uncovers the hole when it pivots. A bimetallic coil is attached to an actuator arm that pivots the plate so that when the firebox begins to cool below a certain point, the coil contracts, and the entire assembly hinges open to uncover the intake hole and provide extra air to the fire. As the burn rate increases from the inflow of oxygen, the firebox heats back up and the coil expands, lowering the base plate back down to seal the hole in the air intake plenum, without changing the original slide draft setting.
"

http://chimneysweeponline.com/hoebt.htm

So precaud , once you cut off the secondary air how much does your stove temp drop?
Its the stove putting off enough heat once the air is lowered to heat your home? I "think" this is the same information BeGreen is looking for whaen he asked about monitoring stove and stack temps .

Precaud, have you shut down the secondary during the daytime and monitored stove and stack temps over the day? If yes, what are typical readings?

 

[begreen]

precaud , So your thinking is more secondary air during the first of the fire with new logs and flames from gases.

Than at coal stage to have less secondary air ?

Absolutely. For clean combustion, the ONLY time you need to have secondary (over-fire) air is when it's in the flaming phase. After that, secondary air is doing nothing more than diluting the exhaust and cooling the stove.

So you cut the secindary air on coal burn stage and now your air supply is less so your stove is going to cool. You would have to add main air after cutting secondary air to keep the heat rolling in the stove other wise the stove will cool anyhow.

I could see where this is a fine balance, but I think he covered that - "This year, I leave the primary supply just slightly open, completely close off the secondary."

 

[Roospike]

precaud , So your thinking is more secondary air during the first of the fire with new logs and flames from gases.

Than at coal stage to have less secondary air ?

Absolutely. For clean combustion, the ONLY time you need to have secondary (over-fire) air is when it's in the flaming phase. After that, secondary air is doing nothing more than diluting the exhaust and cooling the stove.

So you cut the secindary air on coal burn stage and now your air supply is less so your stove is going to cool. You would have to add main air after cutting secondary air to keep the heat rolling in the stove other wise the stove will cool anyhow.

I think he covered that Roo - "This year, I leave the primary supply just slightly open, completely close off the secondary."

So is "slightly open" on the main air supply equal more or less of what is normally run ?
Does "slightly open" mean what air the secondary air was shut off the difference was added to the main air ?

 

[precaud]

On the Pacific Energy stove the EBT controls the main air inlet air. So when the stove cools as it does in part of the coal burn stage it opens up main inlet air to the stove so ...........

Stove cools EBT opens air supply.
You have secoindary air + main air
stove cools in coal stage
EBT opens air supply

Yes, I see. That's a different concept, for a different purpose. They're trying to maintain a more constant temperature output during the flaming phase by tweaking the primary air supply. And it makes sense, especially if you burn 24/7, like you do. But... once the flaming phase is over and you have a firebox full of red logs, how much primary air you want depends on when you'll reload next. If you'll be reloading soon, then you want more, to burn off those coals. If you want a bed of coals to start up from 8 hours later, then you only want enough primary air to keep the coals alive. BUT, and this is my main point, in either case you don't want or need ANY secondary air until you reload the stove again.

So I'd say the PE is a great stove for 24/7 use, and you've adapted your burning habits perfectly to it.

What I find with the Morso is that the primary air control lives in pretty much the same spot after the first 10 minutes or so of intense flaming. And I adjust the secondary air to match the flame height/intensity.

The more I use that system, the more it makes sense to me. I'm not trying to control the process, but to accomodate it.

So precaud , once you cut off the secondary air how much does your stove temp drop?

Hardly at all. And certainly much slower than if the secondary was open!

Its the stove putting off enough heat once the air is lowered to heat your home? I "think" this is the same information BeGreen is looking for whaen he asked about monitoring stove and stack temps .

Absolutely. Again, secondary (over-fire) air has no purpose except when flames are present up there. Think about it.

 

[Roospike]

pre, What size fire box does the stove have and home much wood can be loaded ?

I'm right along with you on your information and how your testing the stove and does make sense.
I think stove size and mass is going to change things too.

pre: "secondary (over-fire) air has no purpose except when flames are present up there. Think about it."

I agree to the point of air coming in from the top of the stove but i would think a stove that is set up with main air and secondary air coming into the stove is a set design on the lines to if one was to take away the secondary air it should be added to the main air supply. All the older pre-EPA stoves i have seen and used the main air was a lot larger inlet over that newer EPA stoves of today as the main air inlet was not the "only air" . So if building a EPA stove design over the Pre-EPA design the main air inlet would be / should be different as the secondary air inlet should be factored in to the design .

Just out-pouring thoughts of information here. I'm also side tracking off from the "main point" your trying to make but still agree to your test running of the stove and information.

 

[precaud]

precaud, What size fire box does the stove have and home much wood can be loaded ?

Both my stoves are smaller than yours, for sure. The Morso is about 1.25 cu ft, and the Jotul is about the size of your grandma's bread box. The size and amount of wood the Jotul takes would be considered large kindling by you, I'm sure!

I think stove size and mass is going to change things too.

Perhaps... what are you thinking?

precaud: "secondary (over-fire) air has no purpose except when flames are present up there. Think about it."

I agree to the point of air coming in from the top of the stove but i would think a stove that is set up with main air and secondary air coming into the stove is a set design on the lines to if one was to take away the secondary air it should be added to the main air supply.

But no EPA stove is set up that way. That's a recipe for runaway fires, ala Vogelzang or whatever they're called.

 

[Roospike]

By completely shutting off the secondary air supply, I have easily doubled the burn time of this stove. (By "burn time" I mean, the time between the end of the last fire's flaming phase and when I can easily revive a new fire simply by adding wood on top of the bed of coals.)

I always try to arrange my burning so the flames are dying out right around my bedtime. Last year, I would close the primary air supply, and wake up to find a cold stove with nothing but ashes on the grate. This year, I leave the primary supply just slightly open, completely close off the secondary, and I wake up to find the stove at 150-175 degrees with a bed of coals, mostly black but some still alive. A couple minutes of exposure to full primary air livens the coal bed enough to ignite a new load without building a fire from scratch.

The last two evenings, this period was 9.5 hours. Last night's flames died around 10pm and at 7:30 this morning it came back to life.

Just another benefit of adding a secondary air control to your stove, if it doesn't have one already (most don't.)

I'll also add, in normal operation, I have the secondary air set at about 60-70% open. Under most conditions, the amount of secondary air supplied is MUCH greater than needed.

I think "I" was a little confused so i had to go back and re-read your first post on how you were running the air controls. I'm better updated now. ( i think :roll: )

#1 So about the same amount of air but just from a different inlet.
#2 Longer burn time ("doubled the burn time") from before.
#3 Warm stove with coal vs a cold stove in the A.M. no coal
#4 Stove putting of same heat but for around double the time.

Unsure what the stove was rated for on efficiency as listed but sounds like if you double your burn time with the same wood , same load of wood (lbs) and there has been no drop in heat out put then you basically doubled your efficiency assuming it was running at max efficiency as listed in the first place.

What is the stoves efficiency rated at per the factory ? What was the factory claim of burn times?

 

[Roospike]

So with that said ( after i had to write it out in crayon for myself - lol) If you say had and EBT design set up on your stove that controlled the air supply per main air with secondary air to just main air after the stove cooled cutting off/down secondary air then you would be all set up so you didn't have to wait and watch for the flames to go down and into coal stage.

 

[pistonslap]

I have an older pre-EPA wood furnace. The only air supply when I got it was a spring controlled flap on the ash door. I had a hard time getting fires started and buiding a good set of coals. I drilled a 2 1/2inch circle of 1/4 inch holes in the loading door and covered them with an electrical junction box cover that swivels on a 1/4 in bolt. I open that all the way up and leave it open until I get a good bed of coals. I have found that once that happens and I close my homemade secondary, I get more heat and longer burn times. If I leave it open it just eats more wood. This is only my second year burning and the weather hasn't been cold enough during the day to stuff the furnace full of wood and close the ash door flap and try to control the long burn when I could watch the temps all day. I will give your method a try on a cold day and let you know if it works for my furnace. I'm always open to try any ideas that will safely extend my burn times and keep my flue temps in the safe range.

 

[jtcedinburgh]

This is fascinating stuff. I have a Morso Owl, which I have found to have fairly short 'burn times' (by your definition) and I shall make a point of trying your method tonight. I have generally shut down the primary air (i.e. from below the coal grate) and opened up the secondary (i.e. from above) to between 1/3 to 3/4 and that leaves me with no heat and just ash in the morning. Trying smokeless coal briquettes last week, whereby I closed down the secondary and had a little primary air, I found that lasted until morning - but I don't really want to get into a smokeless coal habit (non-eco).

So, I shall try your method and report back. It would be good to extend the burn time as you claim.

 

[precaud]

JC, I'm pretty sure that, like my 2110, your Owl has a front-of-fire air source, so you should close down the primary all the way after the last fire dies.

Last night I got 8 hours out of my Jotul F602 using this method.

 

[jtcedinburgh]

Just out of interest - when you speak of primary air, do you mean air from below (which is used for smokeless coal briquettes and provides air from below the riddling grate) or do you mean the airwash which comes in from the top of the glass (which I refer to as secondary). The Owl also has a (fixed) tertiary air supply which takes heated air and provides air through little holes at the rear of the stove.

So, in summary: two adjustable levers - left (pri) and right (sec) - and one at the back (tert., fixed).

john