Outside Combustion Air
#1
02-28-19, 01:34 PM
Outside Combustion Air
Hi,
A few months ago I had new gas furnace installed. 96% efficency / 80,000 btu.
The installer added a new direct vent pipe to the outside of the house for the combustion air intake. Since the installation we notice firewood smoke inside the home. Many homes in our neighborhood use wood burning fireplaces during the winter months and the smoke can be rather thick outside during low wind days.
Before the new intall, my previous furnace was drawing combustion air from wihin the basement and never did I smell wood smoke fumes inside the house over a 10 year period.
Is it possible smoke fumes from outside are entering the house through the combustion air intake pipe and eventually being forced out the air vents into each room?
I thought combustion air after ignition was immediately vented back outside through the exhaust pipe?
The smoke inside our home is a problem so I'm contemplating the best solution would be to eliminate the outside air intake pipe, and instead draw air from inside the basement. The furnace manual list indoor air an exceptable option if "outside air is not practical".
I read and understand all the benifits of drawing outside air into a furnace, but eliminating smoke inside the house is my main concern.
If I change the intake to use indoor combustion air, do I need to make any further adjustments to the furnace? My basement is not finnished, about 1200 sq/ft
Any help is appreciated.
A few months ago I had new gas furnace installed. 96% efficency / 80,000 btu.
The installer added a new direct vent pipe to the outside of the house for the combustion air intake. Since the installation we notice firewood smoke inside the home. Many homes in our neighborhood use wood burning fireplaces during the winter months and the smoke can be rather thick outside during low wind days.
Before the new intall, my previous furnace was drawing combustion air from wihin the basement and never did I smell wood smoke fumes inside the house over a 10 year period.
Is it possible smoke fumes from outside are entering the house through the combustion air intake pipe and eventually being forced out the air vents into each room?
I thought combustion air after ignition was immediately vented back outside through the exhaust pipe?
The smoke inside our home is a problem so I'm contemplating the best solution would be to eliminate the outside air intake pipe, and instead draw air from inside the basement. The furnace manual list indoor air an exceptable option if "outside air is not practical".
I read and understand all the benifits of drawing outside air into a furnace, but eliminating smoke inside the house is my main concern.
If I change the intake to use indoor combustion air, do I need to make any further adjustments to the furnace? My basement is not finnished, about 1200 sq/ft
Any help is appreciated.
#2
02-28-19, 04:03 PM
Group Moderator
Is it possible smoke fumes from outside are entering the house through the combustion air intake pipe and eventually being forced out the air vents into each room?
Take the burner service door off and see if the fresh air in line just lets air into the furnace area..... rather than being directly connected to the burner.
You be less likely to draw smoke into the house with a fresh air intake on the furnace than if you were just using inside air. If the fresh air line was the cause of the issue..... it would smell the worse right at the furnace.
#3
02-28-19, 05:06 PM
There is an NFPA Code for venting of makeup air for a gas-fired appliance. It's rather complicated, but it tends to boil down to having ventilation from the boiler-room to the rest of the house. Unless your furnace is enclosed in a separate room, without ventilation to the rest of the house, then an outside combustion air vent is most likely not required.
If you have an outside air intake, and your outside air is smoky, then it stands to reason that smoke will be drawn inside the house.
Best to call back your installing contractor.
If you have an outside air intake, and your outside air is smoky, then it stands to reason that smoke will be drawn inside the house.
Best to call back your installing contractor.
#4
03-01-19, 01:11 PM
I took a picture of the burner area - the combustion air intake is just above the burners.
Is this considered a sealed system? Is this entire chamber area part of the sealed system?
Is this considered a sealed system? Is this entire chamber area part of the sealed system?
#5
03-01-19, 01:55 PM
Group Moderator
Yes. That area is where the combustion happens. The lower section is where the house air is circulated. The two don't mix.
On a side note, natural gas or propane do not produce any smoke. Only water vapor and CO2
On a side note, natural gas or propane do not produce any smoke. Only water vapor and CO2
#6
03-01-19, 02:06 PM
Group Moderator
Correct..... the two don't mix. If the furnace is drawing in smoky air..... it should be very strong at the front of the furnace. It that was true.... there could be leakage into the system that supplies the house. It wouldn't be a y exhaust fumes from the furnace but the smokey smell could be drawn in.
#7
03-01-19, 03:18 PM
As was mentioned, nothing should be getting into the ducts from bringing in outside air unless you have a leak in the heat exchanger.
Did they happen to by chance install a return vent in the basement when the put the new furnace in. If so, then when the smokey fresh air was drawn into the basement which would also be sucked into the heat exchanger through the return opening then that would give you your smokey smell through the vents.
If that were the case you can block off the basement return to see what happens.
Just a thought.
Did they happen to by chance install a return vent in the basement when the put the new furnace in. If so, then when the smokey fresh air was drawn into the basement which would also be sucked into the heat exchanger through the return opening then that would give you your smokey smell through the vents.
If that were the case you can block off the basement return to see what happens.
Just a thought.
#8
03-02-19, 04:56 AM
Didn't see it mentioned but the outside smoky air is constantly being circulated through the house by way of stack effect. Not uncommon during cold weather to have all the air inside a home replaced by outside air every 3 hours, that is a testing number but illustrates the potential air flow. When the system was changed to burning outside air then the purging on inside air stopped, the old system used to consume some of that basement air.
Note, a fireplace also increases the amount of outside air that is brought into the house, assuming a traditional wood FP.
Bud
Note, a fireplace also increases the amount of outside air that is brought into the house, assuming a traditional wood FP.
Bud
#9
03-02-19, 06:43 AM
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If that rectangular area (where the arrow is pointing) is open to the basement the furnace is NOT sealed combustion. For a furnace to be sealed, it must draw all of it's combustion air from outside.
#10
03-02-19, 10:15 PM
Now that I understand how a sealed combustion furnace functions, I think the problem of smoke entering the house is more likely coming from drafty windows, doors or bathroom exhaust vents. I'll verify if air leaks are present and causing the problem.
The rectangular area is a sealed glass panel to view the burners when ignited, Im not sure if this is simply a marketing feature or a benefit.
No, they did not install a return vent during installation, they only added the outside air combustion intake and exhaust pipes.
If that rectangular area (where the arrow is pointing) is open to the basement the furnace is NOT sealed combustion. For a furnace to be sealed, it must draw all of it's combustion air from outside.
Did they happen to by chance install a return vent in the basement when the put the new furnace in. If so, then when the smokey fresh air was drawn into the basement which would also be sucked into the heat exchanger through the return opening then that would give you your smokey smell through the vents.
#11
03-03-19, 03:52 AM
Hi Pluto
When the temps are cold the infiltration occurs through leaks in the lower areas of the house while the exfiltration is concentrated in the upper areas. One of the primary sources of incoming cold air (infiltration) is the top of the foundation where the house sits. I believe you said it is an unfinished basement which makes the search much easier.
Turn on your bath and kitchen exhaust fans while you move your hand around at the top of the foundation. Increasing the exfiltration increases the infiltration making leaks easier to find.
They also rent infrared cameras to make the process super easy.
Bud
When the temps are cold the infiltration occurs through leaks in the lower areas of the house while the exfiltration is concentrated in the upper areas. One of the primary sources of incoming cold air (infiltration) is the top of the foundation where the house sits. I believe you said it is an unfinished basement which makes the search much easier.
Turn on your bath and kitchen exhaust fans while you move your hand around at the top of the foundation. Increasing the exfiltration increases the infiltration making leaks easier to find.
They also rent infrared cameras to make the process super easy.
Bud
#12
03-03-19, 05:01 AM
Are you using a wood stove or fireplace of your own?
If the amount of forced air exhaust from the house is greater now than it was before then a wood stove in your own house might be letting more of its own combustion chamber contents (smoke) directly into the living space. Air could even be pulled down the chimney of a fireplace not being used, and pick up smoke oders before coming out into the living space.
If the amount of forced air exhaust from the house is greater now than it was before then a wood stove in your own house might be letting more of its own combustion chamber contents (smoke) directly into the living space. Air could even be pulled down the chimney of a fireplace not being used, and pick up smoke oders before coming out into the living space.
Last edited by AllanJ; 03-03-19 at 05:22 AM.
#13
03-03-19, 11:44 AM
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Pluto9, Thanks for the info on the glass window. It wasn't clear in the picture (sorry, no pun intended). Now we know the furnace is sealed, I would agree the source of the odor is likely around windows, doors, sill plate, etc.
#14
03-04-19, 10:19 AM
Last night was cold, -15c / 5 degrees. I checked the basement windows and noticed cold drafts around the sliding frames. As a temporary measure I sealed the windows with tape, five windows in total. I'll see if it helps reduce outside smoke entering the house.
There is a Cold Air Return vent in the middle of the basement but at the top of the ceiling. It's located about 12ft from the furnace. If I block off the vent, would this reduce any possible negative pressure in the basement and reduce drafts from the windows?
No, I do not have a wood stove or fireplace. I do have a gas burning fireplace on the main floor but I never notice cold air or outside smoke in that area.
When the temps are cold the infiltration occurs through leaks in the lower areas of the house while the exfiltration is concentrated in the upper areas. One of the primary sources of incoming cold air (infiltration) is the top of the foundation where the house sits.
Are you using a wood stove or fireplace of your own?
#15
03-04-19, 11:09 AM
If the smoke is getting into the basement blocking the return would change how much of that smoke id circulated throughout the house, but blocking either supply or return vents is something to avoid, longer topic.
The negative pressure in the basement is due to stack effect and is always present when there is a difference in temperature between inside and outside, regardless whether the furnace is running. The only way to reduce its effects is to seal as many leaks as possible.
When it is cold it is a good time to feel for cold air at the top of the foundation, just like the leaks you found around the windows.
Bud
The negative pressure in the basement is due to stack effect and is always present when there is a difference in temperature between inside and outside, regardless whether the furnace is running. The only way to reduce its effects is to seal as many leaks as possible.
When it is cold it is a good time to feel for cold air at the top of the foundation, just like the leaks you found around the windows.
Bud
#16
03-06-19, 09:21 AM
The past few days have been cold in my area and so the wood fireplace smoke has picked up in the neighborhood, however, we are noticing much less smoke enterinng the house.
As mentioned in my previous post, I sealed all basement window air leaks with tape and yesterday I added additional pink insulation to a few areas of the foundation near the ceiling wood joist. I also increased the furnace themostat overnight temperature from the usual 17c / 62F setting up to 20c / 68F ...although I'm not sure if this has an impact on drafts/smoke entering the house it feels more comfortable at night.
Thanks to everyone who replied to this thread, your input helped a great deal.
As mentioned in my previous post, I sealed all basement window air leaks with tape and yesterday I added additional pink insulation to a few areas of the foundation near the ceiling wood joist. I also increased the furnace themostat overnight temperature from the usual 17c / 62F setting up to 20c / 68F ...although I'm not sure if this has an impact on drafts/smoke entering the house it feels more comfortable at night.
Thanks to everyone who replied to this thread, your input helped a great deal.
#17
03-06-19, 12:35 PM
The air sealing you did around the windows was a good step towards reducing infiltration. Adding fiberglass insulation to the loist cavities will not stop air leakage, possibly reduce the heat loss. More air sealing when you can but sounds like you have made progress.
Bud
Bud
