effects of primary air inlet

[ddug]

I noticed something curious (to me) last night while burning a fire in my stove. After an hour or so I got it good and hot with pipe temperatures reading about 325- 350 degrees, which seems pretty average so far when I get a nice fire going, but I've only been running this stove for a couple of weeks so I'm still trying to get a handle on what is "average".

I loaded it up again and watched as the temp. dropped, as it usually does after a fresh load. Slowly it climbed up to 350 again and burning real nice. I close the primary a bit (about 1/4 open) and went upstairs to read. My wife comes in about 1/2 hour later and out of curiosity I asked her what the temp. was reading. "575" she says, which is almost 200 degrees higher than I have thus far seen.

So I go down and close the primary a little more and watch for awhile as the temp. rises to about 625, then 675. Puzzled, I opened the primary to about half open and within a minute or two it was down to 550.

So I can see that closing the air inlet can raise the temperature but I am interested to know the reason behind it. To my simple mind more air = faster burn/ more heat, and less air should produce the opposite results.

Help me understand this.

 

[shawneyboy]

More air sends more heat up the flue and out of the house. Think of the ineffeciances of an open fireplace. You need enough air to get a good clean burn. If you get to much, the heat goes outside.

 

[firefighterjake]

I noticed something curious (to me) last night while burning a fire in my stove. After an hour or so I got it good and hot with pipe temperatures reading about 325- 350 degrees, which seems pretty average so far when I get a nice fire going, but I've only been running this stove for a couple of weeks so I'm still trying to get a handle on what is "average".

I loaded it up again and watched as the temp. dropped, as it usually does after a fresh load. Slowly it climbed up to 350 again and burning real nice. I close the primary a bit (about 1/4 open) and went upstairs to read. My wife comes in about 1/2 hour later and out of curiosity I asked her what the temp. was reading. "575" she says, which is almost 200 degrees higher than I have thus far seen.

So I go down and close the primary a little more and watch for awhile as the temp. rises to about 625, then 675. Puzzled, I opened the primary to about half open and within a minute or two it was down to 550.

So I can see that closing the air inlet can raise the temperature but I am interested to know the reason behind it. To my simple mind more air = faster burn/ more heat, and less air should produce the opposite results.

Help me understand this.

Common mistake of many folks new to burning . . . and to veteran burners with pre-EPA stoves.

It kind of seems wrong . . . I mean when you want more speed in a car you step on the gas and "open it up" right . . . and old-time burners were taught that the best way to have coals in your stove in the morning was to cut back the air on the stove right before you went to bed . . . when you wanted more heat during the day you would open up the air.

As mentioned with an EPA stove it's the other way around . . . and for the reason mentioned . . . when you have the air open all the way you may get lots of flames and you may think "OK, lots of flames is good since fire equals heat and I can feel this heat and the fire is obviously burning quite well." The problem is the fire is burning quite well since you are giving it lots of oxygen . . . and as such since the air control is not cut back a good portion of this heat goes up the chimney instead of being "trapped" in your stove for a bit.

By closing the air down after you've reached temp you are effectively slowing the draft so that the heated air takes a bit longer to leave the stove and go up the chimney . . . and this is a good thing . . . providing you have enough air, well seasoned wood and a high enough temp to maintain a secondary burn . . . since now not only is the hot smoke and heat going up your chimney quite so fast . . . now the combustible products in the smoke can ignite causing a second fire . . . or a secondary burn . . . and so these hot, combustible gases that would have just gone up the chimney are now burned resulting in more heat . . . and this is a very, very good thing.

Of course, shutting down the air too quickly, attempting to do so with unseasoned wood or closing it too much or before the stove is up to temp will result in a smoldering fire . . . and this is a very, very bad thing. The trick in the first year of burning with an EPA stove is learning how your stove works . . . how long it takes or at what temp you can start dialing down the air and how far you can dial down the air . . . since the lower you can go and still achieve a secondary burn is a good thing as it means more heat in your home . . . a clean burn . . . and a longer burn.

Seeing a free burning fire with the air open all the way is nice . . . it looks like a normal woodstove or fireplace . . . but you're losing out on a lot of potential heat for your home. Seeing a fire with a secondary burn is a whole other critter -- now there will be flames -- sometimes very intense (i.e. the Bowels of Hell, Northern Lights, propane BBQ jets, etc.) but this fire is typically mostly seen in the upper third of the firebox . . . there may be just a few or no visible flames "on" the wood and these flames may actually appear to be a bit "lazy."

 

[Stump_Branch]

I noticed something curious (to me) last night while burning a fire in my stove. After an hour or so I got it good and hot with pipe temperatures reading about 325- 350 degrees, which seems pretty average so far when I get a nice fire going, but I've only been running this stove for a couple of weeks so I'm still trying to get a handle on what is "average".

I loaded it up again and watched as the temp. dropped, as it usually does after a fresh load. Slowly it climbed up to 350 again and burning real nice. I close the primary a bit (about 1/4 open) and went upstairs to read. My wife comes in about 1/2 hour later and out of curiosity I asked her what the temp. was reading. "575" she says, which is almost 200 degrees higher than I have thus far seen.

So I go down and close the primary a little more and watch for awhile as the temp. rises to about 625, then 675. Puzzled, I opened the primary to about half open and within a minute or two it was down to 550.

So I can see that closing the air inlet can raise the temperature but I am interested to know the reason behind it. To my simple mind more air = faster burn/ more heat, and less air should produce the opposite results.

Help me understand this.

Common mistake of many folks new to burning . . . and to veteran burners with pre-EPA stoves.

It kind of seems wrong . . . I mean when you want more speed in a car you step on the gas and "open it up" right . . . and old-time burners were taught that the best way to have coals in your stove in the morning was to cut back the air on the stove right before you went to bed . . . when you wanted more heat during the day you would open up the air.

As mentioned with an EPA stove it's the other way around . . . and for the reason mentioned . . . when you have the air open all the way you may get lots of flames and you may think "OK, lots of flames is good since fire equals heat and I can feel this heat and the fire is obviously burning quite well." The problem is the fire is burning quite well since you are giving it lots of oxygen . . . and as such since the air control is not cut back a good portion of this heat goes up the chimney instead of being "trapped" in your stove for a bit.

By closing the air down after you've reached temp you are effectively slowing the draft so that the heated air takes a bit longer to leave the stove and go up the chimney . . . and this is a good thing . . . providing you have enough air, well seasoned wood and a high enough temp to maintain a secondary burn . . . since now not only is the hot smoke and heat going up your chimney quite so fast . . . now the combustible products in the smoke can ignite causing a second fire . . . or a secondary burn . . . and so these hot, combustible gases that would have just gone up the chimney are now burned resulting in more heat . . . and this is a very, very good thing.

Of course, shutting down the air too quickly, attempting to do so with unseasoned wood or closing it too much or before the stove is up to temp will result in a smoldering fire . . . and this is a very, very bad thing. The trick in the first year of burning with an EPA stove is learning how your stove works . . . how long it takes or at what temp you can start dialing down the air and how far you can dial down the air . . . since the lower you can go and still achieve a secondary burn is a good thing as it means more heat in your home . . . a clean burn . . . and a longer burn.

Seeing a free burning fire with the air open all the way is nice . . . it looks like a normal woodstove or fireplace . . . but you're losing out on a lot of potential heat for your home. Seeing a fire with a secondary burn is a whole other critter -- now there will be flames -- sometimes very intense (i.e. the Bowels of Hell, Northern Lights, propane BBQ jets, etc.) but this fire is typically mostly seen in the upper third of the firebox . . . there may be just a few or no visible flames "on" the wood and these flames may actually appear to be a bit "lazy."

lengthy but very well put. hard to grasp, which is why reloading is time critical, or burn cycle critical not to over fire. and why thime needs to be spent after reload to let it settle in. not really a set it and forget it.

 

[ddug]

Thanks firefighterjake for the great explanation, even I could understand it!

I appreciate you taking the time.

 

[BrotherBart]

Also in a EPA stove cutting back the primary air input shifts the balance of air intake to the secondary air and the unregulated primary air inlets of the stove. Draft is going to pull air from the source of least resistance and when the primary is open it is that point. When it is closed the balance shifts to the other two sources. And the secondary input is pre-heated air.

 

[LLigetfa]

Not all brands and models of stove have comparable air controls and the makers are not standardized on nomenclature either. What one maker calls a primary air control could behave very different from another air control. We as a group, cannot always agree on the terminology either.

Some stoves, (like mine) have a primary air intake that feeds what is often referred to as the doghouse or zipper air. On my stove RSF chose to make it a fixed intake so the amount of "Primary air" is influenced by the pressure differential between the room and the interior of the stove. My stove controls only the "Secondary air" which combines the secondary tubes with the air wash of the door via a bi-metal thermostatic regulator. I long ago learned that "more is less" or "less is more" to some degree WRT how much air I give the stove. There definately is a sweet spot which happens to be the 25% mark on my stove's scale.

That said, I had long ago noticed that the "fixed" zipper air was influenced so much by the draft and by a pressure deficit in the room that I started to experiment with it to regulate the fire. Reducing the size of the zipper air reduced the stove's propensity to run away when stiked full but it also caused the coals to not burn down as quick resulting in a build-up. My stove now has a zipper air mod that I leave in the reduced mode when the stove is unattended but that I open up for more heat output and faster coal burn-down when the stove is monitored.

 

[LLigetfa]

Also in a EPA stove cutting back the primary air input shifts the balance of air intake to the secondary air and the unregulated primary air inlets of the stove. Draft is going to pull air from the source of least resistance and when the primary is open it is that point. When it is closed the balance shifts to the other two sources. And the secondary input is pre-heated air.

While I understand what BB is saying, how can one "cut back" something that is "unregulated"?

 

[pen]

Also in a EPA stove cutting back the primary air input shifts the balance of air intake to the secondary air and the unregulated primary air inlets of the stove. Draft is going to pull air from the source of least resistance and when the primary is open it is that point. When it is closed the balance shifts to the other two sources. And the secondary input is pre-heated air.

While I understand what BB is saying, how can one "cut back" something that is "unregulated"?

you can't! That's the whole point. Since these are EPA stoves if you got your fire over roaring on the primary air, then close it down, the 2ndary air will provide whatever necessary to maintain a clean burn. Sometimes that means one very hot stove.

For future reference, the obvious: Don't get a load of wood going that hard before trying to close the air down on it.

Also, find out where your secondary air enters your stove and if necessary, have a means to plug this so that you could whoa it down.

For me, (since I have a wife who is occasionally forgetful) I simply installed a key damper on the pipe. This way, if she pulls the "oops, I left the air too far open for too long" maneuver, she can close key damper which will reduce the draft and reduce the amount of air that can be pulled in through the 2ndary air pipes or any other unregulated source. For me, this is a lot easier than telling her she needs to go behind the stove she has turned into the fires of hell and plug something up.

pen

 

[begreen]

On many modern stoves there is more than one air inlet. The regulated inlet is controlled by the air control. Unregulated intakes are smaller and provide air to the door glass air wash and to the secondaries. When the air control is wide open, air takes the path of least resistance via the regulated air control. But as the primary air control is cut back, draft starts pulling air through the unregulated ports. This is why the fire can actually grow hotter and the air is cut back.

This is not universally true, some stoves have simpler variations on this design, but it is very common on today's EPA stoves.

 

[LLigetfa]

Again, every stove may perform different with regard to regulating air. Chimney draw, room air pressure, OAK pushing, the wood and how it's loaded, etc, are all factors. I generally find however that increasing "zipper air" will make a fire go faster than the opposite, provided there is no change made to the secondary air. Once again, it may boil down to what one considers primary versus secondary.

On my stove, the stock primary air intake was just 3/8ths of one inch in size whereas the secondary air is 4 inches. The path of least resistance alone would not create the sort of shift you speak of but fiddling with the primary could move the sweet spot of the secondary.

As always, YMMV.

 

[LLigetfa]

On many modern stoves there is more than one air inlet. The regulated inlet is controlled by the air control. Unregulated intakes are smaller and provide air to the door glass air wash and to the secondaries...

This certainly is not the case with my stove. As mentioned, unregulated zipper air to the doghouse is just 3/8ths of an inch while the 4 inch regulated air feeds both the secondary tubes and the air wash for the door. The air wash for the door is often called "primary air" as after it has travelled down the door, some of it combines with the zipper air at the base of the coals. On my stove the ratio of how much air goes to the tubes and how much rolls down the door cannot be controlled and varies by the amount of draft and the amount of OAK "push".

 

[begreen]

Yes, I was referring to stoves and inserts per ddug's question. I could see how in a big fireplace there could be a variation on this theme.

 

[LLigetfa]

Yes BG, but do you know it to be a fact for the VC Aspen or are you again speaking in general terms?

 

[begreen]

General terms for sure, I tried to make that clear. I'm not sure how the airwash is done on the Aspen, though ddug's description of the burn (cutting back air and getting more heat) fits the general pattern. Corie rebuilt an Aspen and could provide greater detail. It looks like there might be a secondary air intake to the side of the main air inlet, but it's hard to tell from the pics.

https://www.hearth.com/econtent/index.php/forums/viewthread/7011/

 

[LLigetfa]

I too am speaking in general terms. Usually the air wash system uses preheated air like what the secondary tubes use. Whether or not they are regulated independently, and/or are tied to zipper air would/could factor in how much the "less is more" sweet spot phenom is at play.

Perhaps other VC Aspen users could chime in here about their sweet spot anecdotes.