Negative pressure smoke issue

  • 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.

stoveliker

Minister of Fire
Nov 17, 2019
6,285
Long Island NY
You are not getting the point of the higher outside temps.

Draft makes the smoke go up. Draft is due to the physics of gases at different temperatures, pressures and heights.

An outside temperature that is higher results in less draft. The large horizontal section results in a need for higher draft to suck the smoke thru this section that has impedance but does not contribute to the draft. Evidently you had borderline draft to start with at lower outside temperature, and with higher temperature you've gone below the necessary draft for the stove to function.

Temperature differential, flue gas temperature, elbows, horizontal runs, weather, wind turbulence all matter. The last two are out of your control. The former not.
 
  • Like
Reactions: bholler

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Negative pressure and a reverse draft due to temperature differential are different things. Negative pressure isn't fixed with a hair dryer
Good point. I need to use the correct terminology. Turning on a kitchen or bathroom fan when the house is well sealed would be an example of negative pressure. I understand the physics of negative pressure, but I do not understand the physics of reverse draft due to temperature differential.

When I tested the venting with a match before installing the stove, on two different days, the match flame went up sometimes, and other times the flame wavered at first but it did go up after I ran the hair dryer up the pipe for less than a minute. When I would come back to check the air flow again a few minutes later, sometimes the air was still going up, and other times it wasn't. It was very close to being static.

This morning after I disassembled the stove pipes, I was able to check again for negative pressure, or reverse draft due to temperature differential, whichever should be called. It was a very mild downward flow like before. But a little while ago I checked it again and it is much stronger, probably due to doors and windows being open upstairs. It is strong enough now that I can actually feel the air moving without needing to light a match. Hmmmmm. Suppose at the end of a future burn, when it's down to just a few coals, the n.p. or reverse draft suddenly comes back. The house could be smoked again, maybe while I'm away or asleep.
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
I have seen glass blown out more than once from severe back puffs.
That is important information. I would have thought open to the sky venting with no obstructions would be the weak point that gives before steel and glass, but you found downward pressure sufficient to blow out a stove window. I've been underestimating the power of air flow.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
30,256
central pa
That is important information. I would have thought open to the sky venting with no obstructions would be the weak point that gives before steel and glass, but you found downward pressure sufficient to blow out a stove window. I've been underestimating the power of air flow.
Not downward pressure. The pressure of a firebox full of smoke exploding. Yes some of that pressure will go up the chimney but that may not be enough
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
You are not getting the point of the higher outside temps.

Draft makes the smoke go up. Draft is due to the physics of gases at different temperatures, pressures and heights.

An outside temperature that is higher results in less draft. The large horizontal section results in a need for higher draft to suck the smoke thru this section that has impedance but does not contribute to the draft. Evidently you had borderline draft to start with at lower outside temperature, and with higher temperature you've gone below the necessary draft for the stove to function.

Temperature differential, flue gas temperature, elbows, horizontal runs, weather, wind turbulence all matter. The last two are out of your control. The former not.
Re: "The large horizontal section results in a need for higher draft":

There is only one foot of horizontal, from the 90 degree turn to the 45 degree turn.

I will try to reassemble the stove piping as I had planned originally: with just one 45 degree turn and no 90 degree and no horizontal at all, but that will bring the thermostatic control assembly box up to the concrete wall instead of being ten inches away as it is now.

I was pleased to find no glazed creosote in the piping when I took it apart today. There was some flakey creosote in the horizontal and soot everywhere.
 

Highbeam

Minister of Fire
Dec 28, 2006
20,015
Mt. Rainier Foothills, WA
Just FYI, not every blaze king thermostat throttle blade has a hole in it. Some models have a hole hidden down the throat of the intake manifold directly into the intake ducting ahead of the thermostat.

The intake system between the OAK connection nipple on the bottom to the throttle blade is not super air tight but it's not super loose either. Plugging the OAK connection nipple should do a pretty good job of stopping air flow. Compared to some other stoves that have three or four air inlet holes scattered all over the stove body not even fed by the OAK connection, the BK design is pretty good.

This is a basement stove problem. Even strong chimney draft must overcome the stack effect of the house itself. Your home acts as a chimney too and competes with the cold stove chimney. Like a tug of war for the smoke. Air leaks up high in the home like windows can put a pretty good suck on the home and especially the basement because those leaks are acting on a huge diameter chimney. Like a huge piston in an engine can push harder because the same pressure differential is acting upon a larger surface.
 
Last edited:

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Not downward pressure. The pressure of a firebox full of smoke exploding. Yes some of that pressure will go up the chimney but that may not be enough

Just FYI, not every blaze king thermostat throttle blade has a hole in it. Some models have a hole hidden down the throat of the intake manifold directly into the intake ducting ahead of the thermostat.

The intake system between the OAK connection nipple on the bottom to the throttle blade is not super air tight but it's not super loose either. Plugging the OAK connection nipple should do a pretty good job of stopping air flow. Compared to some other stoves that have three or four air inlet holes scattered all over the stove body not even fed by the OAK connection, the BK design is pretty good.

This is a basement stove problem. Even strong chimney draft must overcome the stack effect of the house itself. Your home acts as a chimney too and competes with the cold stove chimney. Like a tug of war for the smoke. Air leaks up high in the home like windows can put a pretty good suck on the home and especially the basement because those leaks are acting on a huge diameter chimney. Like a huge piston in an engine can push harder because the same pressure differential is acting upon a larger surface.
That is very good information.

Thank you, and thanks to others here, for all your help. I think you are correct about everything. I was very surprised and upset by the "smoke out" yesterday, but beginning to understand the problem now and that it is not insurmountable.
 

stoveliker

Minister of Fire
Nov 17, 2019
6,285
Long Island NY
Or, if it's 50 outside, you don't run the stove. Basement wouldn't get too cold anyway. And if you need some heat, at this temperature the cost of conventional heat would not be much either. (No need to be a wood heating purist; ok to use oil/heat pump/propane if convenient...)
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Re: "a basement stove problem":

At least there is only one floor (and no attic) above the basement.

What downdraft happens to first-story stoves when there are two stories above it with some windows open and other air leaks, or even other stoves and fireplaces higher up pulling air?

The main advantage to a bottom-level heat source is that heat rises. The heat doesn't need to be forced down from above with fans and ductwork. That is my reason for having the main stove on the lower of the two floors.
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Or, if it's 50 outside, you don't run the stove. Basement wouldn't get too cold anyway. And if you need some heat, at this temperature the cost of conventional heat would not be much either. (No need to be a wood heating purist; ok to use oil/heat pump/propane if convenient...)
I had a different reason for making a little fire in it yesterday: There was some material that I needed to dry somewhere in a hurry, and it was raining outside.

Looking back now it was fortuitous that that I started a fire on such a mild weather day, or I would not have learned what can happen.

I do depend on the heat pump for heating in mild weather. The wood heat is for the cold weather and for power outage emergencies.

Now I'm looking into methods or machines to remove the smoke smell from the house. One thing leads to another.
 
  • Like
Reactions: stoveliker

Parallax

Minister of Fire
Dec 2, 2013
873
Bellingham, WA
The way this stove is designed, it doesn't seem to need much air and can draw enough air from the chimney, and pour its exhaust out of the thermostat assembly at the back and bottom of the stove, and there is no way to cut off the air supply to stop it. Is that where most of your smoke came from?
No. Mine was coming from the upper right corner of the door. Until the chimney sweep augmented the gasket. After that it didn't leak, though it was still hard to reload or start without getting smoke.
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
No. Mine was coming from the upper right corner of the door. Until the chimney sweep augmented the gasket. After that it didn't leak, though it was still hard to reload or start without getting smoke.
Thanks for explaining that. I never had smoke leaking here at all, never when starting or reloading, or otherwise, until this week, when I started the stove on a warm day (high in the fifties) instead of a cold day for the first time and the exhaust wouldn't go up the chimney and instead, poured out the bottom rear air supply entrance and the thermostat control rod holes and every crack and seam (except the door gasket which is very tight) on the stove and indoor stove pipe assembly. I shared the experience on your thread because I thought it might be relevant to your smoke experience.

The smell is still pretty bad. I'm researching air purifiers.

I disassembled and swept out the indoor stove pipe, cleaned the stove, removed all the firebricks, vacuumed the combustor, etc. For days I've been testing for "negative pressure" and "downdraft caused by temperature differential" using matches and smoldering paper. Almost every time I find the draft going up, but a few times it was coming down but it was reversible with a hair dryer and then continued to go up, so you could say that it was really almost static. I don't understand what happened the day of the smoke-out, but it might have been the warmer outdoor temperature causing a strong downdraft. The weather has been colder since then.

I changed the venting on the stove so that now there is only one 45 degree turn and no horizontal and no 90 degree. The first vertical rise is a little shorter though, and the thermostatic control box is very close to the wall. I may light the stove again, on a cold day.
 
  • Like
Reactions: MR. GLO

MR. GLO

Feeling the Heat
Jan 26, 2021
355
Massachusetts
Thanks for explaining that. I never had smoke leaking here at all, never when starting or reloading, or otherwise, until this week, when I started the stove on a warm day (high in the fifties) instead of a cold day for the first time and the exhaust wouldn't go up the chimney and instead, poured out the bottom rear air supply entrance and the thermostat control rod holes and every crack and seam (except the door gasket which is very tight) on the stove and indoor stove pipe assembly. I shared the experience on your thread because I thought it might be relevant to your smoke experience.

The smell is still pretty bad. I'm researching air purifiers.

I disassembled and swept out the indoor stove pipe, cleaned the stove, removed all the firebricks, vacuumed the combustor, etc. For days I've been testing for "negative pressure" and "downdraft caused by temperature differential" using matches and smoldering paper. Almost every time I find the draft going up, but a few times it was coming down but it was reversible with a hair dryer and then continued to go up, so you could say that it was really almost static. I don't understand what happened the day of the smoke-out, but it might have been the warmer outdoor temperature causing a strong downdraft. The weather has been colder since then.

I changed the venting on the stove so that now there is only one 45 degree turn and no horizontal and no 90 degree. The first vertical rise is a little shorter though, and the thermostatic control box is very close to the wall. I may light the stove again, on a cold day.
Curious to see if that helps...pe claims you need a foot starter pipe feom stove top for good lighting.

I had a 6 inch damper section then a 45. Now I'm testing 6 inch damper with 12 pipe to 90 run..

But I think 45 might be better for reloads but the 90 way helps lower my draft.....01 to .02 ....

please keep us posted on your results.
 

ratsrepus

Minister of Fire
Jan 5, 2018
778
Howell, Mi
You can't close off the air completely; there is a hole in the flapper that is run by the Tstat. This is to ensure a minimum burn rate, compatible with the cat. To keep things clean.

But when one wants to stop a fire, it's a pain, I agree.
we call that runnin on the hole
 
  • Like
Reactions: stoveliker

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Curious to see if that helps...pe claims you need a foot starter pipe feom stove top for good lighting.

I had a 6 inch damper section then a 45. Now I'm testing 6 inch damper with 12 pipe to 90 run..

But I think 45 might be better for reloads but the 90 way helps lower my draft.....01 to .02 ....

please keep us posted on your results.
Yes I'll keep you posted. It's re-assembled now with a 28 to 30 inch (depending where you measure from the stove top) vertical rise to the 45 degree turn; the 45 angled run goes about 30 inches more to another 45 inside the wall and then vertical (up the chimney) about 20 feet, all rigid stainless insulated with vermiculite except the last three feet above the chimney that is uninsulated.

I'm waiting for a very cold day to try it again because I don't want to chance having another downdraft smoke-out on a warm day.
 
  • Like
Reactions: MR. GLO

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Yes I'll keep you posted. It's re-assembled now with a 28 to 30 inch (depending where you measure from the stove top) vertical rise to the 45 degree turn; the 45 angled run goes about 30 inches more to another 45 inside the wall and then vertical (up the chimney) about 20 feet, all rigid stainless insulated with vermiculite except the last three feet above the chimney that is uninsulated.

I'm waiting for a very cold day to try it again because I don't want to chance having another downdraft smoke-out on a warm day.
I said I'd get back, but I haven't tried the BK40 again because in the meantime I had a similar smoke-out with the Princess insert upstairs. See comment below.
 

MR. GLO

Feeling the Heat
Jan 26, 2021
355
Massachusetts
I said I'd get back, but I haven't tried the BK40 again because in the meantime I had a similar smoke-out with the Princess insert upstairs. See comment below.
And you have tried preheating the flue? and using kindlin to get the draft started instead of logs?

yesterday it was warm mid 40's and the stove did not want to draft quick...even with a short preheat of flue.
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Now the BK Princess did the same thing. Can't blame this on being in the basement, because it's upstairs. Can't blame it on the venting being "barely adequate" because the venting is PERFECT according to the manufacturer's specifications. Yet when I tried to light it a few days ago, the kindling started nicely but suddenly the flame disappeared because the smoke wouldn't go up. Instead it leaked out of the air intake at the bottom and back of the stove. I immediately set the thermostat to the very bottom of the range, kept the door shut, and hoped for the best, and yes, the smoldering kindling finally did go out.

I had not used it for weeks because there is not enough very dry (15% or less, tested on inside of splits) wood and I was saving the remaining very dry wood for use in a power outage. But a few days ago we had a guest in the house, and I decided to make an "entertainment fire" for the guest. The outdoor temperature was about 40 degrees that evening, down from almost 50 degrees F. that afternoon. The masonry chimney containing the venting is on the north side of the house and was quite cold after days of average temperature 40 and below. If this stove can't push air up the venting inside a chimney at 40 degrees, that means I can use it only in VERY cold weather.

My old wood stove NEVER had this problem, nor the open fireplace that we had before I installed this BK insert. I just can't believe it, after spending all this money and time.

This last smoke-out can't be blamed on "being in the basement" or on the venting. The venting of the insert is all 6 inch hybrid pre-insulated, all straight-up vertical except the exhaust angle design of the stove itself, all clean as a whistle, and the perfect length according to installation directions . Doors and windows were all closed, and the only exhaust fan in the house was not turned on. It was not even windy outside.
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
And you have tried preheating the flue? and using kindlin to get the draft started instead of logs?

yesterday it was warm mid 40's and the stove did not want to draft quick...even with a short preheat of flue.
Yes I used very good kindling. I set it up for a hot start. It started vigorously until the oxygen ran low due to smoke not rising.
 

MR. GLO

Feeling the Heat
Jan 26, 2021
355
Massachusetts
I'm going to guess your issues are at startup and the old stove put more heat up chimney and faster...these problems can be solved with preheat flue and get draft going fast with dry kindlin.
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
Yes I used very good kindling. I set it up for a hot start. It started vigorously until the oxygen ran low due to smoke not rising.
When I think about it, the old open fireplace flue was much larger and the bricks and clay gained heat from the house because the damper always leaked some air. There was always a tendency for air to rise in it. Using it since 1964 we never had smoke backing up except the few times that someone forgot to open the damper before lighting, and those times it was fixed immediately by opening the damper.

But this insulated steel venting is a "column of air" (I don't know how else to express it) and the stove needs to have enough strength (again I don't know how to express it properly) to push the column of air through the pipe to get started.

Also, my old inefficient wood stove that burns any kind of wood to ashes has never had this problem.
 

Slate Dale

Member
Dec 27, 2021
161
Slatington, Pennsylvania
I'm going to guess your issues are at startup and the old stove put more heat up chimney and faster...these problems can be solved with preheat flue and get draft going fast with dry kindlin.
How do I preheat the venting? I remember reading a post on this forum by someone who says he always starts his BK by shooting a propane torch up the vent.

I'm fortunate that the kindling didn't keep smoldering forever like it did in the BK40 that really smoked up the house until it finally went out.
 

MR. GLO

Feeling the Heat
Jan 26, 2021
355
Massachusetts
torch doesn't work as good....Try preheating the chimney with a space heater or heat gun. 10-15 min. then start with kindlin only and not wood logs ...then use the logs after its going good. if that doesn't work at least you will know its the house or the chimney and not operator issue. clean out ash before heater or point heat gun up. Also use a fire starter that helps too...