The Evil Outside Chimney
A fireside chat (rant) about chimneys for architects and builders
By John Gulland
If you design or build houses I need to have a serious talk with you about chimneys. I know that chimneys are hardly the most glamorous aspect of the building business and maybe your eyes glaze over when subject comes up, but I'm here to offer a different take on chimneys, one you probably haven't heard before. So respecting your busy schedule and your lack of interest in chimneys, here is the bottom line: chimneys belong inside houses.
The facts demonstrate without question that masonry chimneys built onto the sides of houses so their profile shows, or metal chimneys enclosed in framed chases, even though they might look alright, don't work well at all.
In fact, I suggest that a chimney hanging off the side of a house like an afterthought is an abomination, functionally and aesthetically.
Chimneys belong inside houses. I'm serious.
But as I look around, it's apparent that the majority of houses less than 50 years old were designed and built by people who don't share my views. Outside chimneys are rampant. They are everywhere, hordes of them in tract developments, and ones and twos stuck on big custom houses. It's not a class thing — the urban rich and the rural poor all seem to get outside chimneys these days.
Another thing I've noticed, a lot of people complain about their fireplaces and wood stoves being fussy and hard to light without getting a room full of smoke. And they complain because when it is not being used, the doors and the hearth are cold. If your houses have fireplaces, you've probably heard the complaints. Hey, you might be one of the complainers.
I know what you're thinking. You think I'm going to make a connection between outside chimneys and annoying fireplaces. Well, there is a connection and I can prove it, if you'll let me explain.
And it's not just fireplaces – wood stoves suffer the same problems when connected to outside chimneys. Although oil furnaces have fans that pump exhaust gases into the chimney, their outside chimneys spill a lot of cold air into basements between firing cycles. Conventional gas furnaces and hot water heaters are famous for spilling their exhaust gas as well as cold air from the chimney into basements. The common feature of all these failures to flow properly is the outside chimney.
I usually talk about fireplaces because they are the object of most complaints. People don't give a damn what their gas furnace is doing, unless chronic backdrafting leads to carbon monoxide poisoning. But when the male of the species has romance on his mind, or more serious still, is about to demonstrate his superior fire-building skills for the neighbors, and the room promptly fills with smoke, the air may be blue with more than smoke. Anyway, the science is the same for all chimney vented combustion equipment and the science says put the chimney inside.
You think I'm stalling. Okay, here's the proof. A chimney is an essentially vertical structure enclosing a space full of air and/or exhaust gas. When it is operating, the contents of the chimney flue are warmer than the outdoor air. Because of its buoyancy, the warm air and/or exhaust gas rises, creating the desired upward flow in the chimney. The flow and the force that cause it are referred to as draft.
Chimneys are in the business of expelling air and/or exhaust gas outside. It is no trivial matter when outside air comes down a chimney into a house. Backdrafting, as it is called by those in the know, is roughly like the wings falling off an AirBus. It is precisely the opposite of the desired behavior. It is a catastrophic event in the life of the chimney.
Most builders and maybe even some architects working in moderate-to-cold climates have heard of the "house as a system" principle which suggests that the house functions as a system rather than as a number of unrelated parts and that its various sub-systems, particularly those that move or contain air, behave in an interactive way — one might say they influence each other. You probably knew that already.
And this: When it's cold outside, the warm air inside makes the house act sort of like a chimney. The warm air in the house wants to rise because it is less dense, more buoyant, than the cold air outside. So, when it is cold out the air pressure high in the house is positive, slightly higher than the atmospheric pressure outside. And the air pressure low in the house is negative, slightly lower than atmospheric pressure. This phenomenon is called stack effect. Somewhere between the high pressure high in the house and low pressure low in the house is a zone of neutral pressure which is called, rather cleverly, the neutral pressure plane.
Now that we have the ingredients assembled, we'll build a truly lousy fireplace just to examine the backdraft phenomenon. This particular one we'll build out of bricks although it could just as easily be a factory-built fireplace and metal chimney enclosed by a framed enclosure or chase.
The fireplace is located on the first floor of a two story house. The first thing we decide is to have the back of the fireplace and its chimney project out from the brick veneer wall of the house. The projection is wide at the bottom and tapers above the fireplace to the outline of the chimney as the brickwork rises. It's a nice architectural element, don't you think, adding interest to an otherwise blank wall? As is normal in this type of construction, there is insulation in the walls of the rooms upstairs between the chimney brick and the drywall.
It is 0°C or 32°F outside and the basement furnace is keeping the house at a comfortable 21°C or 72°F. There is no fire in the fireplace, and hasn't been for days. The couple who bought the house are sitting in the living room near the fireplace and she comments that her ankles are cold. He reaches down to the carpet and verifies that it's cool there. They trace it to the fireplace and start to gripe about the jerk that built the house or the mason, or whoever it is they feel comfortable blaming.
Let's just stop here and take stock. The chimney is brick with a clay tile liner, no insulation. For much of its length there is an insulation barrier preventing the chimney from gaining heat from the house. The chimney gives up its heat to the outside and as the average temperature of the air in the chimney falls, the draft declines and the upward flow in the chimney becomes less stable.
Meanwhile, the house is at a stable temperature from top to bottom which is higher than the average temperature in the chimney now that it has cooled. The negative pressure low in the house due to stack effect is more powerful than the draft being developed in the chimney and the chimney backdrafts. Remember the AirBus? The couple who bought the house are suffering the cold hearth syndrome and are ticked off as cold outside air gushes down the chimney onto the hearth and into the low pressure zone caused by stack effect in the house.
The cold hearth syndrome is caused when the house acts as a better chimney than the chimney. You might think that's a trite little saying and actually that's the reason I like it so much — and the fact that it's true and accurate in every way. The house works better as a chimney because the air inside it stays warm, buoyant and wants to rise, unlike the air in the outside chimney that gives up its heat to the great outdoors.
Another thing worthy of note is that a cold backdraft like this is quite stable. Once the air starts flowing down, the chimney really cools off fast. That is why when you light a fire in a backdrafting fireplace, there's a good chance you'll get a face full of smoke.
Although our example uses a brick fireplace, note that a factory-built fireplace with its backside hanging off the side of the house in a flimsy frame chase is every bit as likely to spill cold air, odors and smoke into the room as is a masonry fireplace with its back showing from the outside. The common cause of their failure is their outside location. Bring the same systems inside and they'll work fine.
Here is the harsh reality: When you combine an outside chimney with an appliance installation below the neutral pressure plane of the house, the system will suffer the cold hearth syndrome during cold weather. Period. The result is just as certain for furnaces and water heaters, only it's not called the cold hearth syndrome, it's called a cold basement.
Now, I don't know about you, but I find this astounding. In many areas of North America the majority of chimneys run up outside the building envelope, outside the heated space. And I just showed that if you do this, an appliance installed low in the house will screw up when it is cold outside. Don't you think we should have talked about this sooner? Don't you think someone should have said something?
It's fashionable lately to talk about houses that are so tight that the stove or fireplace — or whatever — "can't get enough air". Meanwhile, the chimney is out in the cold, crippled from the start by its location. The not-enough-air claim is mostly nonsense. Few houses are so tight that a healthy chimney can't pull enough air to run a heating appliance. Open fireplaces, having a huge appetite for house air, are an entirely different matter.
Let's be clear – take the same chimney and move it inside the house envelope, to the warm side of the insulation, and it will be transformed. It will make draft, lots of it, and quick as kindling. This chimney will always perform better than the house and even when there is no fire burning, it will gently tug the air at each leak in the fireplace. When you open the doors to light a fire, air from the room rushes in and up the chimney. When you light the kindling fire the smoke goes up the flue immediately and you'll have a hot bright fire very soon. It's a fine chimney, you'll say with satisfaction.
If you work at it, you can overcome even a good (read inside) chimney, by, for example, turning on a large exhaust system like one of those downdraft kitchen range exhausts for indoor barbecuing. Some of these suckers are powerful enough to make your ears pop, or at least to backdraft a fireplace chimney. Here's my advice for people with chimneys: Barbecue outside. You don't have to like my advice.
If you can't do without the monster kitchen exhaust, you could hire an engineer and have an equally monster fan-forced make-up air system designed and installed, one that is interlocked to turn on when the range fan is switched on.
There are builders who tell me they won't give up the expensive floor space that the fireplace would occupy if it wasn't hanging off the side of the house in a chase. To them I say, fine, then start building chases that are truly inside the building envelope, part of the heated space. Run the insulated chase to the top of the house envelope, seal it properly and do not isolate the chase from the house with insulation. Of course that would be fussy and expensive detail work to do properly. But if you see yourself as a quality builder, you are kidding yourself to do less. You can either act on my advice or you can listen to complaints. Pick one.
One last thing, I'd prefer you didn't shoot the messenger. I've just given you a brief physics lesson on how gravity and temperature affect air flow. You learned that chimneys belong inside houses. Now quit fighting it and quit complaining. Start putting chimneys inside. You'll be a better, more successful builder for it.