One of my first construction jobs in Vermont, back in the late 1970s, was at an architect-designed home with a massive brick chimney with four flues: one flue for the oil-fired boiler, and three flues for the home’s three wood stoves. The chimney worked fine — mostly because the house had so many air leaks that the wood stoves were never starved for combustion air.
Massive chimneys like the one I remember from that job are expensive to build, but they are often a source of pride for the owner. They provide interior thermal mass; they are durable; and they are handsome to behold.
Older cold-climate homes often included similar large brick chimneys with multiple flues. This type of chimney is usually located near the center of the house, so that the bricks and the flues stay as warm as possible.
These days, however, this type of chimney is responsible for a variety of problems. It turns out that massive brick chimneys perform poorly in a modern airtight house.
I recently had a conversation with a builder about problems in a new home with a three-flue brick chimney. Each flue served a separate wood-burning appliance. I’m going to present the case in the form of a dialogue. (Full disclosure: a few details have been changed to protect the privacy of the people involved.)
Makeup air is pulled down one of the flues
Q. When one of the wood stoves in this home is operating, makeup air seems to enter the house through one of the unused flues of the chimney. The entering air has a sooty smell. I realize that the problem is due to the fact that the house is relatively airtight. What’s the solution?
A. That’s a tough one. If a house has a single wood-burning…
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Nice article Martin, Just
Nice article Martin, Just wanted to point out, as you well know, the problem also exists in new manufactured, sealed combustion wood stoves, fireplaces, and chimney systems. Air sealing a class A chimney, which requires a 2 inch air space, is tough, and back drafting is sometimes not realized until after the home is finished. All fireplace and wood stove installations are going to become more difficult, regardless if manufactured or custom built, if future blower door requirements are reduced.
Large thermal mass chimneys
My next door neighbour's brother-in-law is a mason, so when they built their house it included a huge brick chimney that ascents through three stories. When they start a fire the smoke hangs heavy in the air for the first half hour or so until the flue warms. I would imagine it also represents a big thermal bridge at the roof, and a way for moisture to descend into the structure (although perhaps the constant fires negate this problem, at least in the winter). Smoke from our metal insulated chimney becomes clear and almost transparent within five minutes.
A long way of saying that even without multiple flues, large brick chimneys are problematic dinosaurs.
What about brick/stone clad
What about brick/stone clad chimney chases with insulated metal chimneys?
This allow the "look" of a brick/stone fireplace...
I plan on installing this type of system, with the chimney top damper, and a regency single door wood insert with outdoor combustion air kit. While we are still in getting permits, I'm fairly confident this will reduce air infiltration, but still allow for occasional burns on really cold -35C or colder days (or for ambiance).
Recirculating range hood, fully ducted ERV with no bath fans, and a ventless dryer. Aiming for very good airtightness.
The stove is rated at 55Kbtu , which is more thna double my design heat load of 23Kbtu, however it is the smallest firebox in the style we like...
Response to Ryan Magladry
If you are installing a single wood-burning stove hooked up to a single-flue chimney -- and if your chimney has a chimney-top damper -- you should be fine, especially if you have an outdoor combustion air kit.
I think you will be pleased with the Regency insert. I installed one several years ago and it has worked extremely well. It didn't look as nice as the current model you linked to either.
Powered Make-Up air
When I do ventilation designs on houses with wood burning appliances, I am required by code to assess the potential for backdrafting. Assuming the house will be about 1.5 ACH50 and will include a range hood and dryer, all these houses require some method of preventing the house from being depressurized by more than 5 Pa. The most common method here is to install a powered make-up air system interlocked with the large exhaust devices. If the occupants are finding that they are drawing air down the other flues by starting a fire even with no exhaust appliances running, the make-up air unit could also be manually activated using a switch near the stove.
Response to David Baerg
Thanks for your useful comments. I have edited my article to include your suggestion. I appreciate it.
Most Codes require that the firebox be supplied directly with outside air (not room air); coupled with an airtight glass firescreen (also often code) a new fireplace is required to not draw air through the room. This is easily accomplished when the fireplace is located on the outside wall, more complicated for an interior fireplace or an existing one, but it can be done: most masonry is easily cut and an exterior air supply can often be retrofitted. The Codes are pretty specific with size, location, materials, screening, etc., and there are details available as well.
Response to Scott Wynn
The topic of this article is homes with multiple wood stoves connected to a brick chimney with multiple flues.
You raise an entirely different issue, namely: If you want to install a wood-burning fireplace, what are the best details? I agree that a glass door on the fireplace, coupled with ducted outdoor combustion air, make sense.
It's worth noting, however, that the outdoor combustion duct can in some cases turn into an alternative flue -- the fire doesn't always know which duct is for combustion air, and which duct is the intended flue -- with potentially catastrophic consequences (a house fire). So the details of the outdoor air duct need to be carefully considered.
Outside Air Damper Options
A very elegant solution suggested by Marc Rosenbaum was to use blast gates for dust collection systems as a very airtight damper. I believe he actually used them to ensure that the dryer vent wasn't acting as an unintentional hole while in operation, but it is likely to seal better than most motorized dampers... albeit requiring manual operation.
That's a good solution. The PVC ball valves I've seen on wood stove air-intakes wouldn't fly here. The components all have to be non-combustable, which seems sensible to me.
Response to Kohta Ueno (Comment #10)
Thanks for your comment, and for the reminder about blast gates that are usually used for dust collection systems. (Another GBA reader has posted the same suggestion a few times on our Q&A pages; I'm sorry that I have forgotten the name of this other reader, who deserves credit for the suggestion.)
I will edit the article to include the suggestion.
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