Negative Pressure Draft - Better to Fix or Airtight Damper

rwhite

Minister of Fire
Nov 8, 2011
1,612
North Central Idaho
So it's a 3 level house? I assumed from the first post it was 2 level. Sounds like your heat is rising to the third level and allowing the cold air to dump into the main level. I'd try putting a space heater on the top floor and see if you can get the temp higher than the main floor then check your draft.
 

Areliven

New Member
Jan 26, 2020
14
Michigan
So it's a 3 level house? I assumed from the first post it was 2 level. Sounds like your heat is rising to the third level and allowing the cold air to dump into the main level. I'd try putting a space heater on the top floor and see if you can get the temp higher than the main floor then check your draft.
It's 3 levels including the basement yes. Fireplace on main level. I'll try the space heater idea.
 

rwhite

Minister of Fire
Nov 8, 2011
1,612
North Central Idaho
It's just a theory but worth a try. The way I see it is if your basement is colder than your main floor there's no replacement air rising there. If your main floor heat is rising the make up air needs to come from some where. Other than opening a door the chimney probably holds the coldest densest air. Worth a try anyway.
 

AlbergSteve

Minister of Fire
Dec 11, 2017
840
n
hot water heater vents through a separate flue right next to the chimney, and they terminate at the same height right next to each other above the house
THIS would concern me. There is the potential to pull CO from the HW heater back in to the house via the fire place.
 
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rwhite

Minister of Fire
Nov 8, 2011
1,612
North Central Idaho
No luck with the space heater. I'm gonna call some chimney places.
What is your current heating system? Is it ducted to all 3 levels? Are there cold air returns on all 3 levels? Does the current fire place have a damper above the fire box?
I had a somewhat similar situation. It was a 3 level house. Furnace in the basement. Two Open fireplaces above each other and a single chimney with two large open flues that were next to each other. No liners at all. Whenever I'd burn on the main floor the smoke would gets pulled down the adjacent chimney into the basement. Even after installing a insert and insulated liner it would happen. I ended up capping and completely sealing the basement flue and made the fireplace unusable to solve the problem.
 
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Areliven

New Member
Jan 26, 2020
14
Michigan
What is your current heating system? Is it ducted to all 3 levels? Are there cold air returns on all 3 levels? Does the current fire place have a damper above the fire box?
I had a somewhat similar situation. It was a 3 level house. Furnace in the basement. Two Open fireplaces above each other and a single chimney with two large open flues that were next to each other. No liners at all. Whenever I'd burn on the main floor the smoke would gets pulled down the adjacent chimney into the basement. Even after installing a insert and insulated liner it would happen. I ended up capping and completely sealing the basement flue and made the fireplace unusable to solve the problem.

So you made the basement fireplace unusable, but the main level one was still usable?

I have supply and return ducts on all three levels. Each room has it's own supplies and return (except the bathrooms with no return, only supply). The basement is partially finished, with the finished area containing two supplies in the ceiling and one return on the floor. The fireplace has a large throat damper, but nothing near the top of the chimney, other than a chimney cap. I've attached some pictures.
 

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rwhite

Minister of Fire
Nov 8, 2011
1,612
North Central Idaho
So you made the basement fireplace unusable, but the main level one was still usable?

I have supply and return ducts on all three levels. Each room has it's own supplies and return (except the bathrooms with no return, only supply). The basement is partially finished, with the finished area containing two supplies in the ceiling and one return on the floor. The fireplace has a large throat damper, but nothing near the top of the chimney, other than a chimney cap. I've attached some pictures.
Yes, the two fireplaces were on separate flues but within the same chimney. I would almost be certain there were cracks in the brick and mortar between the flues as well. In the basement I just removed the damper handle and sealed the damper shut. Actually put a electric fireplace in it. And then I capped and sealed the top. The reason I ask about the ducting is that it seems the chimney is showing the symptoms but is probably not the cause. With 3 levels and possibly an unbalanced hvac system seems more likely. Realizing that it's just circulating the same amount of air but if your taking more from one area than you are returning it can create negative pressure. Even turning off the hvac it will take time to stabilize so depending on the size of the house air will keep moving until it does. With the damper fully closed you can still feel air coming through the fireplace? It appear to seal pretty well.
 
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Starbrightsteve

New Member
Dec 29, 2019
23
Wellsboro, PA
Arelivin, here are more questions and some things to try. You said that you house was tight and I’m wondering what is the basis for that statement? I use to measure leakage in houses and regularly measured high leakage rates in houses thought to be tight. Did anyone ever test your house and if so , what was the measured CFM@50? Are there ice dams or big areas of snow melt on your roof??
You mentioned that every room has a supply and return except the bathrooms. Are any of the supply registers closed? Are any of the returns blocked? Is there a return grill on the ductwork in the basement?
Backdraft still occurs even with all mechanicals off? Furnace off, bath fans off, dryer off, range hood off?
What if when everything is off you cover the return grill in the room with the fireplace. Is there still flow down through the fireplace?
Now for outside conditions. Do you think the wind could be blowing down the chimney? Do other folks on the forum think this is a possibility? Is there a cap or shield of some kind above the chimney flues? Does the gas water heater backdraft?
I guess I have been (grasping at straws). I remember you saying the only way to stop the flow down thru the fireplace was to open a window or door to the outside. Is that correct?
 

Areliven

New Member
Jan 26, 2020
14
Michigan
Yes, the two fireplaces were on separate flues but within the same chimney. I would almost be certain there were cracks in the brick and mortar between the flues as well. In the basement I just removed the damper handle and sealed the damper shut. Actually put a electric fireplace in it. And then I capped and sealed the top. The reason I ask about the ducting is that it seems the chimney is showing the symptoms but is probably not the cause. With 3 levels and possibly an unbalanced hvac system seems more likely. Realizing that it's just circulating the same amount of air but if your taking more from one area than you are returning it can create negative pressure. Even turning off the hvac it will take time to stabilize so depending on the size of the house air will keep moving until it does. With the damper fully closed you can still feel air coming through the fireplace? It appear to seal pretty well.
With the damper closed I can still feel air coming through, at a couple of spots along the edges, and there's a small hole under the handle (not sure what it's for but you can see it in the picture) where most of the air is coming in. Would an HVAC company know what I mean if describe to them this issue and bring up "balancing"?

Arelivin, here are more questions and some things to try. You said that you house was tight and I’m wondering what is the basis for that statement? I use to measure leakage in houses and regularly measured high leakage rates in houses thought to be tight. Did anyone ever test your house and if so , what was the measured CFM@50? Are there ice dams or big areas of snow melt on your roof??
You mentioned that every room has a supply and return except the bathrooms. Are any of the supply registers closed? Are any of the returns blocked? Is there a return grill on the ductwork in the basement?
Backdraft still occurs even with all mechanicals off? Furnace off, bath fans off, dryer off, range hood off?
What if when everything is off you cover the return grill in the room with the fireplace. Is there still flow down through the fireplace?
Now for outside conditions. Do you think the wind could be blowing down the chimney? Do other folks on the forum think this is a possibility? Is there a cap or shield of some kind above the chimney flues? Does the gas water heater backdraft?
I guess I have been (grasping at straws). I remember you saying the only way to stop the flow down thru the fireplace was to open a window or door to the outside. Is that correct?
By tight I mean I just think it is. I couldn't feel any cold air coming in from any windows or baseboards, everything is insulated, etc. Never had it tested. All supply's are open. There is one cold air return in the basement at the bottom of a wall, with two supply's on the ceiling. No returns are blocked. When the sliding glass door in my living room is open, the backdraft goes away, so I don't think it's a wind issue. I've been monitoring it for over a week now so I'm quite certain it is not wind.

I did turn off the furnace and cover the cold air return in the living room (where FP is), and backdrafting still occurred. I tried this after having the furnace shut off for maybe only 5 minutes. However maybe I should try again but leave the furnace off longer? There is a typical cap on the chimney.

On another note, the flue next to the fireplace flue is for the hot water heater. I did notice today there is a small backdraft there as well. Also, the trap door to the ash pit (underneath the FP) backdrafts as well, which isn't surprising but I thought I'd mention it anyway. I have since covered this up with tape and plastic.
 

begreen

Mooderator
Staff member
Nov 18, 2005
83,648
South Puget Sound, WA
I did turn off the furnace and cover the cold air return in the living room (where FP is), and backdrafting still occurred. I tried this after having the furnace shut off for maybe only 5 minutes. However maybe I should try again but leave the furnace off longer? There is a typical cap on the chimney.
As noted earlier in this thread, the issue is not with the heating system nor its balance. The area is a negative pressure zone.
 

rwhite

Minister of Fire
Nov 8, 2011
1,612
North Central Idaho
I guess I missed that part. I'm not sure that shutting off the furnace for 5 min is long enough for the entire house to stabilize.
 

Areliven

New Member
Jan 26, 2020
14
Michigan
As noted earlier in this thread, the issue is not with the heating system nor its balance. The area is a negative pressure zone.
After reading your attached article, it is basically saying I am out of luck. My chimney is on an outside wall, and it's on a section of my house that is single story (no floor above the living room or attached garage, but second floor is above rest of house).

I do eventually want to convert this to a gas fireplace. I'm thinking my best course of action to deal with the negative pressure issue when I do this, will be to have an air tight door installed on it, so when we are not using the gas fireplace, we just close it up. No backdraft, no damper we have to remember to open and close.

What do you think?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,901
central pa
After reading your attached article, it is basically saying I am out of luck. My chimney is on an outside wall, and it's on a section of my house that is single story (no floor above the living room or attached garage, but second floor is above rest of house).

I do eventually want to convert this to a gas fireplace. I'm thinking my best course of action to deal with the negative pressure issue when I do this, will be to have an air tight door installed on it, so when we are not using the gas fireplace, we just close it up. No backdraft, no damper we have to remember to open and close.

What do you think?
I would go with a gas insert that has its combustion air supplied by a pipe through the chimney. Then everything else can be sealed. That type of unit will also be unaffected by the negative pressure. Air tight doors are extremely expensive and won't prevent flue gas spilage into the house due to negative pressure.