Improving efficiency of an old stove

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Relivinmychildhood

New Member
Jul 31, 2021
4
Wisconsin
What does a baffle do? How does adding one improve efficiency of an old top vent stove? Does adding a layer of insulation on top of the baffle help?
Does adding secondary air directly below the baffle help? What ratio of primary vs secondary air? This stove doesn't have a window so I would have no idea what's happening inside
 
Nov 21, 2020
144
NeK Vermont
A baffle in an old stove is simply an old way to increase efficiency by reflecting heat back down again to help with a better burn, and save some of the heat that would otherwise rush right out. The baffle blanket on top just adds insulation and helps with the process of reflecting more heat down. (Some newer Jotuls have the whole firebox surrounded with insulation now, behind all the burn plates, for the same reason, hotter fire=more complete burn)A secondary air tube below the baffle was basically the next step in stove evolution. It adds fresh air and o2 to create a “secondary burn” thats more effective then just baffle alone in getting a better burn, alot of the the time those secondary air tubes run a pattern through the stove in which the air is pre heated as well.
 

begreen

Mooderator
Staff member
Nov 18, 2005
88,996
South Puget Sound, WA
The baffle extends the burn path and residence time for the smoke in the firebox for more complete combustion. If air is properly introduced to the smoke as it turns around the baffle then basic secondary combustion can be achieved.
 

Blazzinghot

Member
Dec 5, 2019
132
New Plymouth, Idaho
You have asked some good questions. As far as the ratio of primary air vs secondary air goes. The primary air is usually adjusted by the owner of the stove. If the stove has secondary burners you need to have the primary air open all the way when you first start the fire. Then add more wood until you get the outside temp of the stove up to about 350 to 450 degrees this is when the secondary burners start working by burning the hot gases in the stove. Then you start turning down the primary air as needed until you hit that sweet spot when you have the best combustion inside the stove from the secondary burners. Some newer wood stoves secondary air can operate when at the right temp without primary air.

As far as the secondary air or knowing how much you need can get very technical and over my head. I have just tried to copy what I have seen in other stoves and the secondary burners work great. There are many variables when it comes to primary and secondary air such as the height of the chimney how air tight your home is and your elevation. Some people who have put in secondary air tubes make them so they can adjust the air flow. Then you have control over both primary and secondary air which is a good idea when retrofitting.

You say your stove has no window then the only other option you have is to keep an eye on the smoke coming out of the chimney. If you can get your fire just at the right temp then you start turning down the primary air and if the secondary burners are working you should have white smoke or hardly no smoke at all coming out of the chimney. The other thing I check is seeing how much draw the secondary burners have by simply putting a lit match by the secondary burn intake on the outside of the stove. It should pull the flame into the intake. This way you can see if the stove is pulling in air through the secondary burners.

I would also suggest you doing some searches on Youtube such as “wood stoves, secondary burn” or similar searchers. Then do some similar searches on this site as there have been many discussions on primary and secondary air and some on this site have put in secondary burn tubes.
 
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Relivinmychildhood

New Member
Jul 31, 2021
4
Wisconsin
You have asked some good questions. As far as the ratio of primary air vs secondary air goes. The primary air is usually adjusted by the owner of the stove. If the stove has secondary burners you need to have the primary air open all the way when you first start the fire. Then add more wood until you get the outside temp of the stove up to about 350 to 450 degrees this is when the secondary burners start working by burning the hot gases in the stove. Then you start turning down the primary air as needed until you hit that sweet spot when you have the best combustion inside the stove from the secondary burners. Some newer wood stoves secondary air can operate when at the right temp without primary air.

As far as the secondary air or knowing how much you need can get very technical and over my head. I have just tried to copy what I have seen in other stoves and the secondary burners work great. There are many variables when it comes to primary and secondary air such as the height of the chimney how air tight your home is and your elevation. Some people who have put in secondary air tubes make them so they can adjust the air flow. Then you have control over both primary and secondary air which is a good idea when retrofitting.

You say your stove has no window then the only other option you have is to keep an eye on the smoke coming out of the chimney. If you can get your fire just at the right temp then you start turning down the primary air and if the secondary burners are working you should have white smoke or hardly no smoke at all coming out of the chimney. The other thing I check is seeing how much draw the secondary burners have by simply putting a lit match by the secondary burn intake on the outside of the stove. It should pull the flame into the intake. This way you can see if the stove is pulling in air through the secondary burners.

I would also suggest you doing some searches on Youtube such as “wood stoves, secondary burn” or similar searchers. Then do some similar searches on this site as there have been many discussions on primary and secondary air and some on this site have put in secondary burn tubes.
Do you think insulating the baffle helps? The hotter the baffle the better correct?
 

begreen

Mooderator
Staff member
Nov 18, 2005
88,996
South Puget Sound, WA
The objective is to get the firebox at or over 1100º for good secondary combustion. Firebrick walls, floor and an insulated baffle can all help get the firebox up to temperature quicker. As the primary air is reduced, the vacuum of draft will start pulling air from the secondary tubes or other source if provided. For example, the Jotul 602 introduces air at the base of the fire and at the turnaround point of the baffle. This is a basic form of secondary combustion without tubes.

Modern stoves do not completely shut off primary air as was possible in old stoves. There is a small percentage of primary air permitted to flow to maintain combustion and prevent smoldering. Typically this primary air is used to maintain the air wash that keeps the door glass clean.
 
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Relivinmychildhood

New Member
Jul 31, 2021
4
Wisconsin
The objective is to get the firebox at or over 1100º for good secondary combustion. Firebrick walls, floor and an insulated baffle can all help get the firebox up to temperature quicker. As the primary air is reduced, the vacuum of draft will start pulling air from the secondary tubes or other source if provided. For example, the Jotul 602 introduces air at the base of the fire and at the turnaround point of the baffle. This is a basic form of secondary combustion without tubes.

Modern stoves do not completely shut off primary air as was possible in old stoves. There is a small percentage of primary air permitted to flow to maintain combustion and prevent smoldering. Typically this primary air is used to maintain the air wash that keeps the door glass clean.
Is there anyone on here who might be considered the resident expert?
 

Corey

Minister of Fire
Nov 19, 2005
2,589
Midwest
Probably the three biggest keys are:

Getting control of the air supply - if you can't get control of the stove due to leaks or overdraft, then too much heat will go up the flue.

Baffle, as mentioned, helps keep the flu gasses in the stove longer to transfer more heat out. Can also help with getting control of the air.

Secondary air. This can actually burn some of the smoke and increase efficiency dramatically... provided that the first two points are already met. The downside is that you really need to be able to see inside to see what is going on and control secondary air. If you open the door to take a look, all secondary combustion instantly goes away. So with solid steel doors, you either need to mod those for new ceramic 'glass' or you're limited to guessing.

Quite a few discussions and examples have been posted, so a good search should turn up lots of reading material.
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,235
NE PA
Do you think insulating the baffle helps? The hotter the baffle the better correct?
Insulating the baffle allows it to operate at a higher temperature, but not always better for overall operation of stove.

Raising the combustion chamber temperature is the primary concern by adding firebrick and baffle. The flue gas temperature is also critical since that is what makes the stove go. You can only retain so much heat in the firebox before decreasing the flue gas temperature below the critical limit. The basic operating principal is the heated gasses rising up the chimney create the low pressure area needed in the stove to allow the higher atmospheric air pressure to PUSH oxygen into the stove intake. Without secondary air above the fire, the intake air must mix with the flammable gasses expelled from the fuel. The higher the velocity, and changing direction, the better the mix. Insulating the baffle can retain too much heat in the firebox, slowing the draft, reducing the intake air. Many factors are involved in each vent system, firebox design, baffle angle and “smoke space” the exhaust travels through. The smaller the firebox, the more critical this becomes since you don’t have as much waste heat to allow to escape up the chimney.

Properly sizing the chimney, Insulating the chimney flue, using double wall connector pipe, and baffle plate adjustment will do more than insulating the baffle plate.

If adding secondary burn tubes, the correct heat lost up chimney becomes more critical since the temperature differential (difference between inside and outside of chimney flue)= pressure differential (of atmosphere and inside connector pipe and firebox) which is what allows atmospheric air pressure to push the oxygen into the secondary burner intake. The amount of air, its temperature and velocity are critical.

The chimney is the engine that runs the stove. Heat is the fuel that makes the chimney work. You can see how retaining too much heat in the stove slows the burn by decreasing the strength of chimney (draft). Chimney height is another factor that determines how much heat must be left up chimney. The object is to maintain 250* f. all the way to the top of chimney flue. Adjust baffle exhaust area and angle to maintain this critical flue temperature at top whenever smoke is present to avoid creosote formation.
 

Relivinmychildhood

New Member
Jul 31, 2021
4
Wisconsin
Insulating the baffle allows it to operate at a higher temperature, but not always better for overall operation of stove.

Raising the combustion chamber temperature is the primary concern by adding firebrick and baffle. The flue gas temperature is also critical since that is what makes the stove go. You can only retain so much heat in the firebox before decreasing the flue gas temperature below the critical limit. The basic operating principal is the heated gasses rising up the chimney create the low pressure area needed in the stove to allow the higher atmospheric air pressure to PUSH oxygen into the stove intake. Without secondary air above the fire, the intake air must mix with the flammable gasses expelled from the fuel. The higher the velocity, and changing direction, the better the mix. Insulating the baffle can retain too much heat in the firebox, slowing the draft, reducing the intake air. Many factors are involved in each vent system, firebox design, baffle angle and “smoke space” the exhaust travels through. The smaller the firebox, the more critical this becomes since you don’t have as much waste heat to allow to escape up the chimney.

Properly sizing the chimney, Insulating the chimney flue, using double wall connector pipe, and baffle plate adjustment will do more than insulating the baffle plate.

If adding secondary burn tubes, the correct heat lost up chimney becomes more critical since the temperature differential (difference between inside and outside of chimney flue)= pressure differential (of atmosphere and inside connector pipe and firebox) which is what allows atmospheric air pressure to push the oxygen into the secondary burner intake. The amount of air, its temperature and velocity are critical.

The chimney is the engine that runs the stove. Heat is the fuel that makes the chimney work. You can see how retaining too much heat in the stove slows the burn by decreasing the strength of chimney (draft). Chimney height is another factor that determines how much heat must be left up chimney. The object is to maintain 250* f. all the way to the top of chimney flue. Adjust baffle exhaust area and angle to maintain this critical flue temperature at top whenever smoke is present to avoid creosote formation.
Can any of this be accomplished with a forced draft wood furnace or is it natural draft only?