Like my high school custodian... smoking in the boiler room

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Nelsonba

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Hello! We have radiant heat with a gas boiler. We also have a wood burning fireplace and have noticed that when we use it, the utility room where our boiler is located will start to smell like smoke. The boiler is in the basement. The fireplace is above it on the main floor. I suspect we might be getting a downdraft through the boiler flue. What would be the fix for this? Do I need an air make-up system in the basement? There currently isn't a vent for combustion air either. Would that do the trick? Thanks!
 

Dana

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When the gas-burner isn't running, if the house is super tight, and atmospheric drafted gas flue could be the lowest impedance path for the huge air volumes being drawn out of the house by the fireplace, as you suspect. Even if there is sufficient make-up air for the gas burner, there may not be sufficient make-up air for the fireplace. A temporary solution (and not a great one) would be to crack a window an inch or so on the first floor whenever you are using the fireplace.

Houses in MN tend to be tighter than the national average, and a big gas-burner can sometimes depressurize the basement sufficiently to pull air/smoke through any air leakage in the firebox of the fireplace, even if it's not actively backdrafting the fireplace flue, and even if there is a reasonable amount of makeup air via the whole house for the gas burner. The fireplace cleanout trap is the most likely air leak between the fireplace & boiler room, but there could also be cracks (large or small) in the masonry.

An open hearth fireplace (even one with glass doors) is a grossly inefficient thing, that sucks more cold air into the house than you would ever believe. The best you'll ever get out of them is 15% steady-state efficiency, most run less than 10% steady-state, and if you light fires then let them burn out it's usually energy-negative in locations at cold as MN.

The best solution (but not very cheap) would be to install an air-tight woodburning insert into the fireplace, one with a big viewing window door. Better still would be an insert that accommodates ducted combustion air kit. That isolates all air-currents between the wood burner and the gas burner, as well as from the conditioned space air. EPA rated woodburning fireplace inserts run north of 75% steady state efficiency (many north of 80%), produce far less smoke while delivering substantially more heat, and without creating drafts/backdrafts by pulling large volumes of air out of the house.

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Tom Sawyer

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The best thing would be to install a fresh air supply duct to the boiler
 

Dana

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Even a pretty big boiler draws less than 25% of the air that open hearth fireplace does.

Adding more air supply to the boiler is an oblique way to attack the problem, which is the make-up air for the fireplace, not the boiler.
 

Tom Sawyer

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Yea, but I don't care about he fireplace. I'm a boiler man. Lol. No doubt though that the fireplace is the culprit. OTOH, I doubt it gets run all that much so cost wise it's probably going to be cheaper to run a 4"'flex duct to the burner housing.
 

Dana

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The flex duct to the burner housing is fine for the combustion air for the boiler, but it won't stop the fireplace from sucking smoke back down the boiler's flue when the fireplace is going, unless his is in fact a sealed-combustion boiler (like a modulating-condensing boiler?) If it IS a sealed combustion boiler the flex duct solution could work, and rightly should have been installed on day-1.

If it isn't, the house needs to stay at the outdoor air pressure when the fireplace is running rather than being depressurized, which means opening up a window, or going with a lower-air-volume wood burner, preferably with an outdoor air supply to keep it from depressurizing the house.

Or, buy a direct-vented sealed combustion boiler that offers no path for air to enter the house via the exhaust venting. That's a reliable solution to a fireplace problem that a boiler-man can get behind, eh? :)

Seriously, open hearth fireplaces are an air-polluting inefficient stinking abomination, only made worse by sharing a chimney that also houses the boiler flue so that it sucks the air pollution indoors directly. (If the boiler flue wasn't in close proximity to the fireplace flue you probably wouldn't smell smoke in the boiler room no matter how much the fireplace was depressurizing the house.) If you're going to burn wood, it's better to do it in a way that yields the heat & aesthetic benefit without causing ridiculously high particulate counts and stinking up your boiler room, which means a wood burning insert (or a wood stove) with a big viewing window. It's not the best way to burn wood for roasting marshmallows, but you get just about everything else you'd want out of wood burning. It's not a cheap solution, but it's the right solution if you are burning wood on a regular basis.
 

Nelsonba

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Thanks for the info! Regardless of what we do with the fireplace, does it make sense to go ahead with the fresh air intake for the boiler? (It’s not a direct vent system) Had a tech out and he tested the draft, checked to make sure the flue wasn’t clogged, and took CO readings. Everything checked out fine (without a fire going). I will say that an open window in the boiler room solves the smoke problem, but also freezes that room pretty quickly on even a moderately cold day. An open window on the main floor by the fire does not solve the smoke issue, at least not with it open to a point where the room doesn’t become colder by having it open. I would assume that the open window on the main floor maybe wasn’t open far enough to depressurize the basement when a fire was going? We don’t have fires often, but enjoy doing so every once in a while, which makes it hard to justify the cost of an insert. We may choose to put one in at some point. Point taken on the inefficiency and pollution of an open fireplace. Thanks again. I will also say that since we’ve been in the house we’ve put in new windows, and insulation (spray foam in some areas). So back to the original question… Should I go ahead with the fresh air intake? Thanks!
 

Dana

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An open window at just about any place in the house should fix the smoke problem if it's open enough, but if just cracking a window in the boiler room while the fireplace is going it may be enough to stall the back draft, and that would be a reasonable solution.

Unless your house is exceptionally tight or the boiler is unusually large you probably have more than enough makeup air for the boiler. Unless it's a direct-vented version or it doesn't have sufficient draw when firing there won't be a need for a dedicated make up air solution. Try firing up the boiler when the fireplace has a good fire going, while running the clothes dryer, bath fan, and kitchen fan to see if there is any chance in hell that the boiler can actually backdraft while firing.

Draft/backdraft testing is wise to do any time you have done another round of house-tightening, such as new windows + spray foam insulation (or even blown cellulose insulation in wall cavities.) At some level of tightness you WILL have to install dedicated make-up air. But since a make up air port will increase infiltration, it increases the heat loss of the house, so you don't want to go there until it's necessary.
 
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