Basement woodstove, negative pressure

[ajs349]

This may be more of an HVAC/house pressure question but I know there's a lot of people in the same boat here:

So we had a wood stove installed in our finished basement earlier this year which is piped to an existing, exterior masonry chimney going up above our two story house. We didn't do any pipe in the chimney so its the terracotta 8x8 build. Home was build in 2000. Natural gas furnace and natural gas hot water heater are both in the basement and both direct vent and exhaust. Wood stove shares a chimney with a fireplace directly above which is not used and currently has a natural gas insert. Chimney had never been used before and we just had a sweep check everything.

Typically when I open the stove there is a strong back draft of cold air coming down the chimney into the room which from my research is pretty typical in a newer home with a basement chimney. On certain days, especially windy days there is a draft into the stove, which I've started to track to find the specific characteristics. With enough heat (and time) I can reverse the draft and the stove works great. But when the fire goes out and stove cools the smell of the fire comes back into the room along with the back draft.

So I guess my question is around creating more of a neutral or positive pressure in the basement. I have a 4" fresh air intake which exits a few feet from the stove and also added an inline fan which reports to move about 195 CFM. When the fan is running I could barely notice a difference in the back draft but when I open the exterior bilco door from the basement there is no longer a back draft. (Obviously coming into an open 36" door is less restrictive for air to enter the basement then the chimney.)

That being said would it be worth installing a second fresh air intake with another fan, possibly a 6" moving more CFM? Or would there be another option like better windows upstairs or more insulation in the attic?

There is no difference to the back draft when the furnace or hot water heater are running and we don't use exhaust fans when the stove has a fire.

 

[jatoxico]

From your post it sounds like you have a pretty good understanding of the problem. May be that you can minimize the stack effect with insulation in the attic or sealing other sources of air leakage esp in the attic area (e.g. pull downstairs). Think it just going to be trial and error as to what, if anything, works. Install a CO detector. Worst case scenario is the draft reverses on a full coal bed and you dump fumes into the basement.

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[Ludlow]

Upstairs FP is creating stack effect maybe.

 

[bholler]

It also may have nothing to do with house pressure. If the chimney is cold the air is naturally going to want to drop

 

[ajs349]

Thanks for all the replies. The upstairs fireplace damper is closed and the only attic access is a hole in a closet. I’m going to check the attic insulation next though most cold comes from bedroom windows. I’ll have to check if they’re drafty are all. Not sure if there’s anything else I’m missing. Thanks.

 

[SpaceBus]

I think an insulated liner would go a long ways in keeping draft established on a coal bed. My cookstove (exterior class A chimney) will reverse once the whole thing cools down to room temp, usually overnight. As long as even an ember is present in the firebox the draft is still going the right way.

 

[ajs349]

It also may have nothing to do with house pressure. If the chimney is cold the air is naturally going to want to drop

The only reason I was thinking that it was negative pressure is because when I open an exterior door there is no longer a back draft from the stove. Actually when I have the exterior door open the stove pulls air in and up the flue.

 

[ajs349]

I think an insulated liner would go a long ways in keeping draft established on a coal bed. My cookstove (exterior class A chimney) will reverse once the whole thing cools down to room temp, usually overnight. As long as even an ember is present in the firebox the draft is still going the right way.

Once I get the draft going the right way it stays until the fire is completely out but after it cools it reverses and I get the wood stove smell throughout the house.

 

[coaly]

Are there other vented heating appliances such as boiler, water heater, furnace? Anything power vented?
I see where you mentioned them, are they on the same level, or in a utility room? Any barometric dampers into another chimney?

 

[Woodfire2019]

What you are describing is definitely a bummer for some homes with poor drafting. Like you said, you can overcome the draft issue when the stove is hot, but when it cools down the draft reverses and brings the smell back into your house. The best solution I can think of is to add a stove pipe damper that you shut after a fire is out. Not a great solution since you have to be there to make this change to damper position, but it could help....

 

[Dieselbreath]

Another thing you can try is to close down any vent thats feeding the basement. I built my house in 1994 and I didn't realize there was a code that covered how close to the basement fireplace that return air vents were allowed, 3 feet ln my case. I had less than that. Closing it partially off did the trick. I had the same problem you had, when it went out I got the stinky woodburning smell throughout the house

 

[ajs349]

Are there other vented heating appliances such as boiler, water heater, furnace? Anything power vented?
I see where you mentioned them, are they on the same level, or in a utility room? Any barometric dampers into another chimney?

There is a utility closet in the basement which houses a hot water heater and a furnace. Both natural gas. The furnace has a pvc intake and exhaust going to the back yard and the hot water heater seems to suck air locally from the top but exhausts outside through pvc.

 

[ajs349]

Another thing you can try is to close down any vent thats feeding the basement. I built my house in 1994 and I didn't realize there was a code that covered how close to the basement fireplace that return air vents were allowed, 3 feet ln my case. I had less than that. Closing it partially off did the trick. I had the same problem you had, when it went out I got the stinky woodburning smell throughout the house

That’s something I found on an hvac site. I have two air returns in the basement and about 6 air vents in the basement which blow furnace air out. The air returns are about 15 and 20’ away but I believe closing them and allowing air to flow from the vents INTO the basement will create a more positive pressure. How did you close off the air returns in the basement? Do you do that in the summer as well?

 

[coaly]

If that water heater opening on top is a diverter, that allows indoor cooler air to mix with and cool the exhaust. By code the door (if any) on that utility room would need to be vented to the rest of the house. If the water heater is not directly connected to outside air, that room needs its own intake from outside, and close the room off to the rest of the house. Any leaks in the furnace returns will also depressurize that room, pulling from the basement. Any doors that slam shut are a sign of depressurization or return air leaks.
With the stove cold, I'd walk around that utility room checking air movement with an incense stick. With the water heater burning, you'll probably find the smoke being pulled into that utility room big time. If there is a vent into that room, put a piece of newspaper over it with appliances running to see if it is sucked to the vent.
The pressure in that room with a gas appliance using indoor air must be atmospheric pressure for the burner to get the proper air mix. Natural gas will have a set pressure about 1/4 psi. When the gas valve opens, gas vapor pushes the air out of the burner tube, allowing atmospheric air pressure to PUSH in. (this is called the mixing tube on the burner) Decreasing that atmospheric air pressure at the intake of the burner makes the mixture rich, which is why the room needs to be vented to the rest of the house by code if it doesn't have it's own outside air intake.
I normally installed what I called an "elephant trunk" intake into any utility room housing gas equipment. I'm retired from my own propane business that did installs and service of LP and Natural gas equipment. A hole is bored through outside wall for 3 inch PVC pipe. Install an elbow facing upwards on the inside, downwards on the outside with screen over the end. This prevents warm indoor air from dropping down the pipe. It will only allow air to move in when the indoor air pressure drops, balancing the room with any fuel burning equipment. I installed many behind stoves and next to fireplaces as well.
Keep the inside air inlet away from any water pipes, and the outside high enough to prevent snow from covering the inlet.

To see if the return air is depressurizing the basement, you can measure the pressure with a draft gauge. really handy for any stove installation, and recommended for coal stove chimney control setting. (barometric damper) Drill a small hole through an outside wall to insert tube. The outside end is open to atmospheric pressure, the inside end is open to indoor pressure. With equipment on, it will show the unequal pressure.

 

[Jackfre]

I think you have two problems. Yes, a negative pressure in the place as well as the very cold exterior chimney. We have pretty much taken the technology of the thermos bottle and applied it to home construction. We then put assorted devises in to pump air out of a building, bath fans, etc, but rarely in. You have added the in-line fan, but how do you control it? I have used Tjernlund (TJ is pronounced CH) Combustion air Enforcers on furnaces, boilers and water heaters to good effect. It interlocks with the units and runs only on a call for heat. Exterior chimneys of any sort are hard. You have all of that mass at or close to outdoor temperatures and it take a pile of heat to get it to want to draw properly. An insulated liner is the best solution, but costly. I suspect that for the balance of this season you will want to continue to experiment with it. Prior to building a fire, neutralize the pressure. Build the fire and deal with the slow start, and some smoke until things get going. Please keep us posted on your progress.

 

[MR. GLO]

It also may have nothing to do with house pressure. If the chimney is cold the air is naturally going to want to drop

So how do you stop low readings co readings 7 to 42 from the coals burning down in the am? So far only way is cracking/opening a door and only happens once or twice a season.

 

[bholler]

So how do you stop low readings co readings 7 to 42 from the coals burning down in the am? So far only way is cracking/opening a door and only happens once or twice a season.

Fresh air supply to the appliance. Increase chimney height eliminate 90s lots of different options depending upon the situation

 

[MR. GLO]

Fresh air supply to the appliance. Increase chimney height eliminate 90s lots of different options depending upon the situation

Thanks for info. I did the fresh air part (with 6 inch passive air) and vacu-stack now going to try the chimney height, then test house between floors vs basement vs outside. Then last resort a draw collar. It definitely the cold air coming down stopping the draft. I can feel it with my hand in the stove outlet area.

 

[bholler]

Thanks for info. I did the fresh air part (with 6 inch passive air) and vacu-stack now going to try the chimney height, then test house between floors vs basement vs outside. Then last resort a draw collar. It definitely the cold air coming down stopping the draft. I can feel it with my hand in the stove outlet area.

What type of chimney? Interior or exterior?

 

[MR. GLO]

exterior stainless steel but most people probably don't know they have problems because they don't have an ultra low Co meter

 

[bholler]

exterior stainless steel but most people probably don't know they have problems because they don't have an ultra low Co meter

42 is far from ultra low.

 

[MR. GLO]

agree i dont want any but for how long? most units need a 30 60 min window or even 8 hours. .i caught it at 7 ppm on the defender model in seconds.

most likely it would have stayed around 42 then dropped as coals died out . but all depends on fresh air entering home.
if you have a standard off the shelf co alarm you would most likely never know your coals are puting out co or not. they can still be putting co out into room and get a zero reading if you have lots of fresh air at the time. appliances and conditions etc make a perfect storm sometimes.

 

[bholler]

agree i dont want any but for how long? most units need a 30 60 min window or even 8 hours. .i caught it at 7 ppm on the defender model in seconds.

most likely it would have stayed around 42 then dropped as coals died out . but all depends on fresh air entering home.
if you have a standard off the shelf co alarm you would most likely never know your coals are puting out co or not. they can still be putting co out into room and get a zero reading if you have lots of fresh air at the time. appliances and conditions etc make a perfect storm sometimes.

Yes most alarms are pretty worthless. You need a monitor not an alarm

 

[stoveliker]

Thanks for all the replies. The upstairs fireplace damper is closed and the only attic access is a hole in a closet. I’m going to check the attic insulation next though most cold comes from bedroom windows. I’ll have to check if they’re drafty are all. Not sure if there’s anything else I’m missing. Thanks.

Not having read farther down yet, I'd like to point out that it's not necessarily the attic insulation, but the air sealing that you should look at.
Seal all boards on top of interior and exterior walls, all fixtures thru the ceilings, check for pipe or duct chases etc. Make sure air can't get out up there.

Is your OAK inlet outside the home above the stove as it's in a basement? That would be a problem (for other reasons).

 

[MR. GLO]

Can you explain how an OAK above the stove would be a problem and how much of a difference you think is passive air opening vs OAK?
If OAK brings in cold air wouldn't that make the chimney pipe colder when the coals go down?