Damper and air supply experience and experiment - NC30

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

NickW

Minister of Fire
Oct 16, 2019
1,367
SE WI
Be forewarned, this is going to be long and potentially boring. All the long timers here have helped me immensely over the last few years and have heard much of this before. There has been a lot of discussion about dampers lately and I thought I'd share my experience.

The goal of these experiments was to see if I can run my NC30 with a stt higher than my flue temperature like some of the folks here are able to do with their stoves. Ultimately it was a failed experiment in relationship to the goal, but I did learn some things.

Disclaimer! Every system is different as are environmental factors. These exact settings are for my system with the environmental factors as they were at the time. I can hypothesize based on previous experiences along with the results of the experiments.

System is an NC30, damper, Duraplus double wall stove pipe and Duraliner oval inside a masonry chimney with appropriate connector tee. Overall height is about 24' from stove top to cap. Stove top to center of horizontal is 42", 2 adjustable 90's to make the turn and sidestep, 42" vertical center to center of liner (approx), 20' of Duraliner oval. Horizontal is pitched 3/4" in 1'. Temps are monitored with an Auber digital probe 18" above stove and Condar stove top thermometer. System installed January of 2020, flue probe added November 2021, damper added February 2023.
IMG_20240117_164800745.jpg

Wood is a variety stored inside. Wood is separated into shoulder season wood (pine, aspen, box elder, silver maple, etc), everyday hardwood (ash, cherry, birch, elm, walnut, etc), and premium hardwood (oak, sugar maple, beech, honey locust, hickory, ironwood). Everything tested between 14-18% although I have had a couple of light sizzlers. I season uncovered outside then move 4-5 cord into a storage area in the detached garage in July & August when it's been dry a minimum of 3 days a little at a time to allow air circulation to remove any surface moisture buried in the stacks. I try to stay 3 years ahead.

House is 2400' bi-level with electric baseboard heaters built in 1978. Insulation is pretty good, air sealing is poor. Wind is my enemy.

The difference between this stove and the old smoke dragon is night and day. I used 5-7 cord of all hardwood with the old smoke dragon and the baseboards were still kicking in by 2am when it was cold and windy. When I installed the NC30 it dropped to 4 cords of mixed and the baseboards rarely kicked in. When I installed the flue probe it heated overnight better. When I installed the damper it heated even better and wood usage dropped a little more.

With the original install I fought the wood supply the first winter. Poorly seasoned wood split that fall that burned OK in the old stove just wouldn't burn in the new one. With help here I learned to split it smaller and get the temps up fast to muddle my way through until spring. Overnight burns didn't last until morning. Went nuts on wood spring & summer of '20 focusing on ash as it seasons quick and burns well if not perfectly seasoned.

Winter of '20/'21 went much better but I often struggled with controlling the burn with only stt as a gauge. November of '21 installed the flue probe and learned that I had been probably running too hot of a flue temperature. Now I started shutting the air down sooner, but still struggled controlling temperature. Primary air open at all would run too hot. Even air fully shut would run too hot for awhile, and if I left it there overnight I would have a big pile of charcoal and few coals in the morning. It was running on secondaries only no primary. Started loading up earlier in the evening to let it get settled in and open the air a little before heading to bed, but the load still didn't last until morning. Fought this '21/'22 and most of '22/'23.

February '23 installed the damper. LOVE IT! Now I typically run with the air 1/4 open damper 3/4 closed, control the temperatures, have a healthier fire, and make the loads last longer. I aim to cruise 700-750 flue and 600-650 stt. Colder & windier I'll push it harder. These settings take me about 1 1/2 hours to get settled in where it isn't going either up or down too much, but I have a huge pile of coals in the morning and a warm house (unless it's really cold and windy).

I almost always run the blower on high and shut the door on loads/reloads as soon as I can get the fire to hold. My overall process goes faster if I char longer but I hate the thought of all that heat flying up the flue. Loads with a mix of species run best. All ash off gases all at once and doesn't last long enough, all premium can be slow to get going. The colder it is the more premium I use. More mild temperatures I use more everyday hardwood with one or two premium.

So my experiments were an attempt to get higher stt than flue temps like some of the members here have by closing the damper fully and having the air more open. Theoretically I should have gotten this, extended burn time more, plus have less coals in the morning. This was unsuccessful but I did learn some things...

Experiment #1 - burning down coals with small chunks of shoulder season wood.

On a huge pile of coals and needing more heat with outside temps below 0 and windy I put 2 small rounds on each side of the doghouse and a medium split over the top. Took off like a rocket. Left the air full open and incrementally shut the damper to full closed raising stt nicely, but there was a faint smoke smell. Flames started to die, opened damper to 3/4 closed. As it burned down I opened the damper back up. Was a better stt for a longer time than just letting it rip with everything full open.

Experiment #2 - shoulder season load on good coals. 6 aspen with 1 ash over the "tunnel of love". 5 degrees and windy out.

Load takes off, close door
Flue 500, air to 1/2, rise to 515 and paused
Flue 550, air to 3/8 open, dropped, paused, rose
Flue 600, stt 250, damper to 1/2
Paused @ flue 650, dropped, rose
Flue 700, stt 350, damper to 3/4 closed
Flue dropped to 630 & stalled, stt to 400 & rising
Flue 700, stt 500, fully closed damper
Temps dropped all flames died
Opened damper & air
Cracked the door for 1 second, flames re-ignite
Temps shot up, went through the process again
At damper 3/4 closed things died down
STT stayed well below flue temps
Tried air 1/2 open and damper shutdown
Unsuccessful
Went to normal shoulder season fire settings
(1/4 open air, damper 1/2)
Happy healthy fire

Assessment: Took a long time. To run damper full shut would require higher flue temps and stt still probably wouldn't get as high. Old settings of air 1/4 open and damper at 1/2 definitely better. Not surprising as crap wood needs more air to keep going and the damper fully shut wasn't pulling enough. Might try air fully open and damper fully shut, but for a shoulder season fire it seems pointless.

Experiment #3 - Full overnight hardwood load on good coals (2 ironwood, 1 honey locust, 1 elm, 1 hickory, 2 ash). 6 degrees and slight breeze.

500 flue, nowhere near ready to shut air
550 flue, 280 stt, air to 1/2
600 flue, 300 stt, air to 3/8
650 flue, 340 stt, damper to 1/2
700 flue, 390 stt, damper to 3/4 closed
Flue dropped to 595, stt to 440
650 flue, 460 stt, damper fully closed
Flue temp dropped fast, primary's died
Secondaries weak, damper back to 1/2
Flue temp rising, stt held
650 flue, 470 stt, damper to 3/4 closed
700 flue, 490 stt, damper fully closed
Same result as above but secondaries also died
Opened damper & air, secondaries reignited
600 flue, air to 1/2
700 flue, stt 480, damper 1/2
750 flue, stt 490, damper 3/4 closed
Temps rose, then flue dropped
Stt continued to rise, then flue rose
Levelled off 750 flue, 570 stt
Both rose slowly, closed air back to 3/8 open
Flue dropped back to 750, 590 stt
Never got back to damper fully closed

Assessment: Similar temps to how I normally run but a little lower and was more difficult. Wood burned faster. Nice coals in the morning but not a huge pile. I suppose I could have tried shutting the damper fully again with the air 1/2 open, but it seemed like the wood was burning faster without any more heat benefits. Maybe I will try that yet. It could work, but to have high enough flue temps to pull enough air with the damper fully closed would burn the wood faster and potentially run too high of a stt.

Overall the flue probe and damper helped my burning immensely. Just installing the damper and leaving it fully opened lessened my draft a little. Damper and air settings take a bit to get figured out. I don't think stt temp higher than flue temp is possible with my setup and possibly this stove (as @Highbeam would probably tell me).

Hopefully this will help some folks decide if a damper will help them and how to use it. Each setup will be different. Share your experiences or ask questions if you'd like. If/when I try anything else I will post results.
 
Do the flames die with the pipe damper fully open and the air shut down fully?
 
Following as I have almost the same set up and home(different stove but basically same size). I just need to add a damper.
 
  • Like
Reactions: NickW
Do the flames die with the pipe damper fully open and the air shut down fully?
At some point when the burn slows and flue starts to cool (before the damper was installed), but not for several hours.

Before the damper I had several times on full overnight loads where I sat up for hours after letting it get away from me waiting for it to settle down enough to be comfortable going to bed (flue temps high 800's, stt high 700's). Once I got to 850 flue and 750 stt and dropping I went to bed. After those events I usually had less charcoal and coals because it got burned up going wild. Once I hit a little over 1000 flue and probably 850-900 stt and almost messed myself.

Usually when I was good with my shutdown timing before the damper was installed air fully closed it wanted to cruise over 800 flue and stt about 100 less. Couldn't get it to settle in any lower no matter when I started shutting the air. It always at some point went over 800 flue or just died if I tried too low. That's why I had piles of charcoal and just a little coal in the mornings (and sometimes smoked glass), not enough air going to the wood/coals. Not a healthy fire with secondaries and light lazy primary.

Haven't bothered to try it with the damper installed because I know the air is already restricted a little more just having the damper installed. I can only assume it will be more controlled but still leave a pile of charcoal unless I loaded early enough to open the air some later.

With the damper 3/4 closed and air 1/4 open I have a nice healthy controllable fire that keeps more of the heat in the stove instead of going up the flue unrestricted.
 
  • Like
Reactions: Todd
I wonder if your long horizontal run and elbows are contributing to the high flue temps slowing the hot gas down a bit? Sounds like you’re doing ok though. Chimney pretty clean?
 
  • Like
Reactions: stoveliker
. I don't think stt temp higher than flue temp is possible with my setup and possibly this stove (as @Highbeam would probably tell me).

I don't know if it's impossible with the nc30 but I sure haven't been able to get it to happen. But perhaps consider that flue temperature doesn't tell the whole story. There's also flow rate. A little tiny bit of really hot air running up your chimney can be less wasted wood than a crap ton of less hot air.

The sweet spot for me seems to be to reduce the flow rate through the stove by closing the damper as much as possible while keeping a vigorous clean fire burning. An overnight burn is not my priority though.
 
  • Like
Reactions: NickW
I wonder if your long horizontal run and elbows are contributing to the high flue temps slowing the hot gas down a bit? Sounds like you’re doing ok though. Chimney pretty clean?
I assume the same. Cold it drafts OK, don't get a lot of smoke inside; once it gets heated up it pulls like crazy. The primary air from the doghouse just can't be shut completely (for me at least). Other owners of the same stove also run similar temps to mine. Just can't run stt the same as or higher than flue temp. Many other stoves seem to be similar where flue temp is always higher than stt. I thought it would be more efficient to run higher stt than flue which is why I wanted to play with it.

Chimney cap & chimney are cleaner now than before the damper was installed. I usually go up on the roof in late December or early January to knock buildup off the screen and rub the little bit of fluff off the top foot of pipe, but this year I can see the cap screen is clear from the ground. Usually I can see when the screen is clogging. If/when the snow clears and I can safely go on the roof I'll check it. Running these flue temps it has to be hard to get buildup unless it smolders (like running air fully closed and waking up to charcoal).
 
I never have had the stove temp above the flue temp with a conventional secondary tube stove except when the flames have died down after the wood stops outgassing. I could see this possible in a cat stove though.
 
  • Like
Reactions: Highbeam
The sweet spot for me seems to be to reduce the flow rate through the stove by closing the damper as much as possible while keeping a vigorous clean fire burning. An overnight burn is not my priority though.
I try to walk that line between good clean burning and getting it to last all night. I'm fairly successful but when it's cold and windy and I need more BTU's it's definitely tougher to go all night.
 
  • Like
Reactions: Highbeam
Thanks to @begreen for merging my double post of this thread. I was getting all kinds of confused...
 
I never have had the stove temp above the flue temp with a conventional secondary tube stove except when the flames have died down after the wood stops outgassing. I could see this possible in a cat stove though.
Nope, I don't think I've ever had that either. Might be able to if I turned the blower off...? I suppose with the blower off running full tilt I might get closer to flue temp with stt, but that would be defeating the purpose... I try like heck to heat a lot of square footage with this stove. I want the heat off the stove and moving around the house.
 
Yes, with the blower running the STT will be below the flue temp.
 
In reviewing my notes I forgot to mention that with the damper fully closed I noticed a slight smoke smell. Nothing real bad or visible smoke, just a little whiff. I can only assume that might have gotten worse as flue temperature dropped causing draft to slow.
 
In reviewing my notes I forgot to mention that with the damper fully closed I noticed a slight smoke smell. Nothing real bad or visible smoke, just a little whiff. I can only assume that might have gotten worse as flue temperature dropped causing draft to slow.
Been running some loads and just to add some complexity, this isn’t a steady state thing. When a fresh load of fuel gets going there are lots of volatiles that feed the secondary fire and during that time you’re going to have higher flue temperatures. After that “bloom” things chill out and I get cooler flue temperatures and about the same stove temperature. In fact, post bloom I can match flue temperature to stove top temperature on an nc30. Even get a bit hotter stove temp than flue by like 50 degrees.

The noncat combustion system requires that the bloom of volatiles be burned up or else it would go up the chimney as pollution. I do believe that some stove designs can convert more of that bloom burn into stove temperature increase than others.
 
I have about the same problem with my regency f2400. I did add a flue damper and with the damper closed and air supply all the way off stoll no difference. Stove is not controllable with more the three pieces of wood. With less than three I can keep it under control.
Following as I have almost the same set up and home(different stove but basically same size). I just need to add a damper.
 
Did you ever figure out yo
Be forewarned, this is going to be long and potentially boring. All the long timers here have helped me immensely over the last few years and have heard much of this before. There has been a lot of discussion about dampers lately and I thought I'd share my experience.

The goal of these experiments was to see if I can run my NC30 with a stt higher than my flue temperature like some of the folks here are able to do with their stoves. Ultimately it was a failed experiment in relationship to the goal, but I did learn some things.

Disclaimer! Every system is different as are environmental factors. These exact settings are for my system with the environmental factors as they were at the time. I can hypothesize based on previous experiences along with the results of the experiments.

System is an NC30, damper, Duraplus double wall stove pipe and Duraliner oval inside a masonry chimney with appropriate connector tee. Overall height is about 24' from stove top to cap. Stove top to center of horizontal is 42", 2 adjustable 90's to make the turn and sidestep, 42" vertical center to center of liner (approx), 20' of Duraliner oval. Horizontal is pitched 3/4" in 1'. Temps are monitored with an Auber digital probe 18" above stove and Condar stove top thermometer. System installed January of 2020, flue probe added November 2021, damper added February 2023.
View attachment 323140

Wood is a variety stored inside. Wood is separated into shoulder season wood (pine, aspen, box elder, silver maple, etc), everyday hardwood (ash, cherry, birch, elm, walnut, etc), and premium hardwood (oak, sugar maple, beech, honey locust, hickory, ironwood). Everything tested between 14-18% although I have had a couple of light sizzlers. I season uncovered outside then move 4-5 cord into a storage area in the detached garage in July & August when it's been dry a minimum of 3 days a little at a time to allow air circulation to remove any surface moisture buried in the stacks. I try to stay 3 years ahead.

House is 2400' bi-level with electric baseboard heaters built in 1978. Insulation is pretty good, air sealing is poor. Wind is my enemy.

The difference between this stove and the old smoke dragon is night and day. I used 5-7 cord of all hardwood with the old smoke dragon and the baseboards were still kicking in by 2am when it was cold and windy. When I installed the NC30 it dropped to 4 cords of mixed and the baseboards rarely kicked in. When I installed the flue probe it heated overnight better. When I installed the damper it heated even better and wood usage dropped a little more.

With the original install I fought the wood supply the first winter. Poorly seasoned wood split that fall that burned OK in the old stove just wouldn't burn in the new one. With help here I learned to split it smaller and get the temps up fast to muddle my way through until spring. Overnight burns didn't last until morning. Went nuts on wood spring & summer of '20 focusing on ash as it seasons quick and burns well if not perfectly seasoned.

Winter of '20/'21 went much better but I often struggled with controlling the burn with only stt as a gauge. November of '21 installed the flue probe and learned that I had been probably running too hot of a flue temperature. Now I started shutting the air down sooner, but still struggled controlling temperature. Primary air open at all would run too hot. Even air fully shut would run too hot for awhile, and if I left it there overnight I would have a big pile of charcoal and few coals in the morning. It was running on secondaries only no primary. Started loading up earlier in the evening to let it get settled in and open the air a little before heading to bed, but the load still didn't last until morning. Fought this '21/'22 and most of '22/'23.

February '23 installed the damper. LOVE IT! Now I typically run with the air 1/4 open damper 3/4 closed, control the temperatures, have a healthier fire, and make the loads last longer. I aim to cruise 700-750 flue and 600-650 stt. Colder & windier I'll push it harder. These settings take me about 1 1/2 hours to get settled in where it isn't going either up or down too much, but I have a huge pile of coals in the morning and a warm house (unless it's really cold and windy).

I almost always run the blower on high and shut the door on loads/reloads as soon as I can get the fire to hold. My overall process goes faster if I char longer but I hate the thought of all that heat flying up the flue. Loads with a mix of species run best. All ash off gases all at once and doesn't last long enough, all premium can be slow to get going. The colder it is the more premium I use. More mild temperatures I use more everyday hardwood with one or two premium.

So my experiments were an attempt to get higher stt than flue temps like some of the members here have by closing the damper fully and having the air more open. Theoretically I should have gotten this, extended burn time more, plus have less coals in the morning. This was unsuccessful but I did learn some things...

Experiment #1 - burning down coals with small chunks of shoulder season wood.

On a huge pile of coals and needing more heat with outside temps below 0 and windy I put 2 small rounds on each side of the doghouse and a medium split over the top. Took off like a rocket. Left the air full open and incrementally shut the damper to full closed raising stt nicely, but there was a faint smoke smell. Flames started to die, opened damper to 3/4 closed. As it burned down I opened the damper back up. Was a better stt for a longer time than just letting it rip with everything full open.

Experiment #2 - shoulder season load on good coals. 6 aspen with 1 ash over the "tunnel of love". 5 degrees and windy out.

Load takes off, close door
Flue 500, air to 1/2, rise to 515 and paused
Flue 550, air to 3/8 open, dropped, paused, rose
Flue 600, stt 250, damper to 1/2
Paused @ flue 650, dropped, rose
Flue 700, stt 350, damper to 3/4 closed
Flue dropped to 630 & stalled, stt to 400 & rising
Flue 700, stt 500, fully closed damper
Temps dropped all flames died
Opened damper & air
Cracked the door for 1 second, flames re-ignite
Temps shot up, went through the process again
At damper 3/4 closed things died down
STT stayed well below flue temps
Tried air 1/2 open and damper shutdown
Unsuccessful
Went to normal shoulder season fire settings
(1/4 open air, damper 1/2)
Happy healthy fire

Assessment: Took a long time. To run damper full shut would require higher flue temps and stt still probably wouldn't get as high. Old settings of air 1/4 open and damper at 1/2 definitely better. Not surprising as crap wood needs more air to keep going and the damper fully shut wasn't pulling enough. Might try air fully open and damper fully shut, but for a shoulder season fire it seems pointless.

Experiment #3 - Full overnight hardwood load on good coals (2 ironwood, 1 honey locust, 1 elm, 1 hickory, 2 ash). 6 degrees and slight breeze.

500 flue, nowhere near ready to shut air
550 flue, 280 stt, air to 1/2
600 flue, 300 stt, air to 3/8
650 flue, 340 stt, damper to 1/2
700 flue, 390 stt, damper to 3/4 closed
Flue dropped to 595, stt to 440
650 flue, 460 stt, damper fully closed
Flue temp dropped fast, primary's died
Secondaries weak, damper back to 1/2
Flue temp rising, stt held
650 flue, 470 stt, damper to 3/4 closed
700 flue, 490 stt, damper fully closed
Same result as above but secondaries also died
Opened damper & air, secondaries reignited
600 flue, air to 1/2
700 flue, stt 480, damper 1/2
750 flue, stt 490, damper 3/4 closed
Temps rose, then flue dropped
Stt continued to rise, then flue rose
Levelled off 750 flue, 570 stt
Both rose slowly, closed air back to 3/8 open
Flue dropped back to 750, 590 stt
Never got back to damper fully closed

Assessment: Similar temps to how I normally run but a little lower and was more difficult. Wood burned faster. Nice coals in the morning but not a huge pile. I suppose I could have tried shutting the damper fully again with the air 1/2 open, but it seemed like the wood was burning faster without any more heat benefits. Maybe I will try that yet. It could work, but to have high enough flue temps to pull enough air with the damper fully closed would burn the wood faster and potentially run too high of a stt.

Overall the flue probe and damper helped my burning immensely. Just installing the damper and leaving it fully opened lessened my draft a little. Damper and air settings take a bit to get figured out. I don't think stt temp higher than flue temp is possible with my setup and possibly this stove (as @Highbeam would probably tell me).

Hopefully this will help some folks decide if a damper will help them and how to use it. Each setup will be different. Share your experiences or ask questions if you'd like. If/when I try anything else I will post results.
 
Doesn't seem to really be a "problem". Just some setups aren't going to run higher stt than flue temps, particularly with the blower running. My best setting seems to be air 1/4 open, damper 3/4 closed. Adding the damper greatly improved my burns.
 
Finally got the damper installed and got some experience using it. Stove is more controllable and easier to dial in after a reload. STT is higher also.
After a full reload, I close damper incrementally until its about 3/4 closed, then I will close primary air control. Find it to take less time to get to cruising. Very pleased with this addition.
 
  • Like
Reactions: NickW and Todd
I don't think I've ever started with closing the damper first. Will have to give it a try...
 
  • Like
Reactions: Gearhead660
Just a note on The 30 there are basically 4 combustion air intakes, primary 3" this has the control on it. Secondary apx 1.5" x 2.5" on rear of stove no control on this one and 2 air wash intakes bottom front corners again no control on these. adding some sort of restriction to the secondary intake helps immensely in high wind conditions, as does the flue damper. Some have gone as far as blocking the air wash intakes off also. Prior to the epa getting involved most stoves had a a control for the secondaries combustion air ( sadly missed by me ).
 
I find that I am in agreement with @Gearhead660. Closing the damper first (incrementally) before starting to shut down the air supply gets to cruising faster.
 
  • Like
Reactions: Gearhead660
I’ll have to try this out in my 32 to see how it works for me