Brick Chimneys With Multiple Flues

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Brick Chimneys With Multiple Flues

Wood-burning appliances are incompatible with a modern airtight home — especially if the home has a brick chimney with multiple flues

Posted on Jan 26 2018 by Martin Holladay

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 massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. ; 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 flue, you can install a duct that brings outdoor air to the room with the wood-burning appliance. But in a house with three wood-burning flues, it’s hard to compete with those three flues. The flues are big holes in the home’s thermal envelope, and are likely to be the easiest way for makeup air to enter the house. Even when the flues have some type of damper, the dampers are unlikely to be tight enough to prevent downward air flow.

These holes in the home’s thermal envelope are the likely path for many types of makeup air — not just the combustion air required for the wood stoves. The three flues will also be the path of choice for any makeup air entering the house when the kitchen's range hood fan is operating. Of course, the makeup air is likely to pick up a sooty smell on the way into the house.

Possible remedies are all imperfect

Q. Can you suggest any possible ways to improve the situation?

A. All solutions are imperfect, and all solutions include the risk of homeowner complaints. One option is to open a window before lighting a wood-fired appliance.

Another option is to install a 6-inch-round outdoor air duct to each room that includes a wood-fired appliance. This type of duct requires some type of relatively tight damper — possibilities include (a) a motorized damper, (b) a large ball valve usually used for plumbing pipes, or (c) the type of blast gate usually installed in duct collection systems — and should terminate in a wall-mounted register. (Note that some homeowners complain that this type of makeup air system can cause cold drafts when open.)

Another option worth considering is to install three chimney-top dampers (one for each flue). Note that most chimney-top dampers are controlled by stainless-steel cables that dangle down the flue — an operating system that works better for fireplace flues than for wood stove flues. The disadvantage of this solution (besides the obvious point that these dampers are made for fireplaces, not stoves) is that the homeowner has to remember to open and close the damper each time the flue is used.

Still another option is to install a powered makeup air unit controlled by a wall switch. To prevent makeup air from entering the house through the unused chimney flues, the homeowner could turn on the powered makeup air unit before lighting any of the wood stoves. For more information on powered makeup air units, see Makeup Air for Range Hoods. [Author's note: I'd like to thank reader David Baerg for this suggestion; see Comment #6, below.]

Q. Couldn't I just drill a bunch of holes in the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. in a variety of inconspicuous locations — maybe in closet walls or the basement rim joist?

A. That's a possible solution, but one that guarantees that the house will always be leaky, and that the homeowners will be burdened by high energy bills for the life of the building.

Q. Masons are still being hired to build this type of chimney. Don’t masons understand that these flues aren’t working as well as they used to?

A. Some masons understand the building science principles that explain why these flues have problems. But most don’t.

What if the customer insists on this type of chimney?

Q. Building codes require that new homes pass a blower door test. Yet some owners who are planning a new custom home still insist on a massive brick chimney with multiple flues for wood-burning appliances. What do I tell these customers?

A. The best approach is to explain that wood-burning appliances are incompatible with a modern airtight home. The second-best approach is to explain that there may be a way to include a single wood-burning flue for one wood stove — as long as the homeowner accepts certain conditions that must be thoroughly explained and understood before proceeding — but that multiple wood-burning flues are impossible. Whenever a tight house includes a wood stove, a signed legal waiver would be a good idea.

If an owner insists on including features that are highly likely to cause problems, the best way to proceed is to say, “I’m sorry, but I can’t build that. Good luck finding another builder.”

Martin Holladay’s previous blog: “All About Air Purifiers.”

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Image Credits:

  1. Fine Homebuilding

Jan 26, 2018 8:34 AM ET

Nice article Martin, Just
by Randy Williams

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.

Jan 26, 2018 9:23 PM ET

Large thermal mass chimneys
by Malcolm Taylor

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.

Jan 29, 2018 2:01 PM ET

What about brick/stone clad
by Ryan Magladry Ottawa, Ontario

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...

Jan 29, 2018 2:16 PM ET

Response to Ryan Magladry
by Martin Holladay

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.

Jan 29, 2018 10:35 PM ET

by Malcolm Taylor

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.

Jan 31, 2018 12:27 PM ET

Powered Make-Up air
by David Baerg

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.

Jan 31, 2018 1:53 PM ET

Response to David Baerg
by Martin Holladay

Thanks for your useful comments. I have edited my article to include your suggestion. I appreciate it.

Jan 31, 2018 5:53 PM ET

by Scott Wynn

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.

Jan 31, 2018 6:15 PM ET

Response to Scott Wynn
by Martin Holladay

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.

Jan 31, 2018 10:19 PM ET

Edited Jan 31, 2018 10:19 PM ET.

Outside Air Damper Options
by Kohta Ueno

Another option is to install a 6-inch-round outdoor air duct to each room that includes a wood-fired appliance. This type of duct requires some type of relatively tight damper — anything from a motorized damper to a large ball valve usually used for plumbing pipes — and should terminate in a wall-mounted register.

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.

Feb 1, 2018 12:12 AM ET

Blast gates
by Malcolm Taylor

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.

Feb 1, 2018 7:29 AM ET

Edited Feb 1, 2018 7:32 AM ET.

Response to Kohta Ueno (Comment #10)
by Martin Holladay

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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