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by
Martin Zibauer
photos by Roger Yip |
AT THE HEARTH OF THINGS (part 2) |
Convection and Radiation
It’s not just heat output that counts, but how the fireplace
transfers heat—by convection or radiation—to you.
Convection is heat transfer when
air warmed by the fireplace insert mixes through the cooler
air in the room. It happens naturally because warm air is less
dense, so it rises and gets into a flow.
| A
CHAIR TO CURL UP IN |
| Some
furniture trivia: The wingback chair was developed
as the perfect complement to a roaring fireplace.
The wings absorb radiant heat from the fire while
sheltering you from draughts caused by the fire
pulling air out of the room. |
| TIP |
| To
help you determine the size of insert you need,
a heating contractor can calculate the “modified
heat loss” of your home |
|
Radiation is what you feel at
a campfire when your face is hot and your back is cold. A fireplace,
like any heat source, beams out infrared rays. The rays don’t
heat air much, but do heat the solid objects they “shine”
on. For that reason, you shouldn’t put a thermostat where
those infrared rays hit directly: the rays will warm the thermostat
and fool it into thinking the room is warmer than it is.
Manufacturers often hype one method
of heat transfer over the other. The inserts that are better
at convection are promoted as providing an even heat throughout
the room, like a forced-air furnace. Radiant inserts are promoted
as feeling more like a real fireplace and warming you faster.
In fact, it is not really an either-or choice between convection
and radiation. All fireplaces deliver both; different designs
just produce one more efficiently.
Look
for features that help heat transfer, such as the materials
in the glass front. Tempered glass doesn’t let infrared
rays pass through nearly as well as pyroceramic glass (similar
to Pyrex ovenware material). Channels around the firebox and
the exhaust system encourage heat exchange from the fireplace
to the air around it and convection into the room.
Fans
also speed convection by pushing that hot air out and, according
to Don Fuegler, senior researcher at Canada Mortgage and Housing
Corporation (CMHC), do tend to improve efficiency. CMHC suggests
you look for a quiet, “squirrel-cage” fan—the
kind that looks like a hamster’s exercise wheel and is
used in a forced-air furnace.
An
insulated outer casing is another of the CHMC’s recommended
features. It reduces heat transfer to the walls on the back
and sides, so that more heat goes out the front.
Opportunities
to Vent
Combustion needs an air supply, and almost all combustion produces
waste gases that need to be vented safely.
Natural
venting is like a traditional open fireplace—room air
is used for combustion, with exhaust vented outside. This simpler
venting system is fine for older, drafty homes, because there’s
enough fresh air coming in to replace air going up the chimney.
But in a tightly sealed, well-insulated home, combustion that
uses room air can create negative pressure, sucking dangerous
gases back down exhaust vents into the house.
The
safer choice in tightly sealed homes is direct venting, where
outside air goes into the firebox through one vent, while combustion
gases escape through a second. The fireplace insert has a dedicated
air supply, so it won’t suck air out of the home.
Ventless (or room-vented) fireplaces
use room air for combustion and vent exhaust back into the room.
They are not approved for use in Canada.
Startup Decisions
| BTUs
IN AND OUT |
On
the backs of those glossy insert brochures, you’ll
find numbers that will tell you how well each fireplace
works as a room heater.
First, mind your BTUs.
A BTU (British thermal unit) is a measure of the
heat needed to raise the temperature of one pound
of water by one degree Fahrenheit. For example,
lighting a wood match generates about one BTU. The
amount of gas a fireplace consumes can also be expressed
in BTUs per hour. (With a flame that can be adjusted,
you’ll see a range of numbers—from minimum
to maximum input.)
Even the most efficient
inserts lose some heat or unburnt gas out the vent.
How much? Look for efficiency ratings. An efficiency
of 76 per cent, for example, means the insert turns
76 per cent of the gas into heat in the room.
Exactly how this efficiency
is tested is an important “apples-to-apples”
consideration if you’re comparing brands.
“Steady-state” means efficiency when
the fireplace is burning at a constant rate. Since
you’re not likely to leave your fireplace
on 24 hours a day, fireplaces are also tested for
AFUE, or Annual Fuel Utilization Efficiency, which
measures the efficiency with all the usual stopping
and starting, heating up and cooling down. Steady-state
ratings can be significantly higher than AFUE numbers,
so be sure you know what you’re looking at.
The third measure
is the newer Annual Fireplace Efficiency (AFE) rating,
which uses a made-in-Canada fireplace test, similar
to AFUE. Ask the fireplace retailer for these numbers,
as they haven’t made it into many brochures
yet. |
|
How the burners ignite when you flick the switch affects efficiency,
but Fuegler suggests this might be one decision, depending on
your situation, where safety might outweigh efficiency. A traditional
pilot is reliable in a blackout, but burns constantly—not
very energy efficient. A two-stage pilot is better. It burns
at a very low level most of the time, and flares up when you
turn the fireplace insert on. When you’re not using the
insert regularly, extinguish the pilot to save gas.
Electronic ignition systems don’t
burn any gas, so they are the most energy efficient, but require
a battery backup to ignite if the power is out.
Comfort Levels
Without any heat control, a fireplace is just a decorative blast
furnace. Heat output can be controlled in three ways: by cycling
the flames on and off, by adjusting the flame height, and by
adjusting the fan speed. In some models, these adjustments are
manual. There’s a switch for a low or high flame, or there’s
a switch to control the fan speed. Other models come with a
thermostat that adjusts automatically as the room temperature
changes.
The range between
minimum and maximum gas inputs (see BTUs In and Out, right)
is a good indication of how much heat control an insert offers.
If you plan to use your fireplace year-round, a fireplace that
can be turned way down is a useful feature.
Steve
Haagmans has a final tip on getting more comfort out of your
fireplace, something he says people often forget: “Read
the instructions, or get the installer to show you how to use
your insert. Your fireplace can probably do a lot more than
you realize."
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| PART
1 | PART 2 | STEP-BY-STEP
GUIDE |
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