For builders . . .
The Evil Outside Chimney
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 being fussy and hard to light
without getting a room full of smoke. And they complain
because when the fireplace 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.
JG
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