Can I use my chimney with my new furnace?
We're glad you asked!
The answer is a definite maybe. Or more likely, maybe not. Truth
is, furnace technology has advanced significantly in recent years.
The higher efficiencies and enhanced performance achieved by todayıs
furnaces make specific demands many existing lined masonry chimneys
simply canıt meet. Your contractor can offer the best advice on
how to configure your new furnace to get the most comfort for
years to come.
Size can be an issue
There are several reasons why new furnaces and existing lined
masonry chimneys are not compatible. One area of concern is the
size, modern, higher efficiency furnaces transfer more heat into
your home and less heat up the chimney than older, less-efficient
units. Thatıs great news because it means youıre getting more
for your energy dollar. But it also means your chimney may be
too large for the new furnace, preventing proper venting of product
of combustion and causing condensation problems inside the chimney.
In addition, todayıs induced (fan assisted) furnaces often are
installed in combination with a natural draft water heater or
other appliance. Relining the masonry chimney with a properly
sized and approved metal vent can be done. Other areas for furnace-chimney
incompatibility could include the absence of a metal lined chimney.
Or if the chimney is located on the outside of the home a double
wall or B-vent may be required as a liner.
Is it possible to mix old and new?
There are installations where it is possible to marry a new furnace
to an existing chimney. Chimney height and location are key factors,
as are proper lining and physical condition of the chimney. Building
codes also must be taken into consideration. These requirements
must be met to ensure proper draft in the chimney for adequately
venting the products of combustion safely to the outdoors.
Why is Condensation a problem?
An older less efficient furnace produced a higher volume of flue
gas carrying more water vapor, a natural product of combustion,
up and out of the chimney. Flue gas from todayıs higher efficiency
furnaces contains less dilution air from inside the home, which
can increase the risk of condensation. This means the chimney
is cooler and water vapor will condense on the inside walls of
a cold chimney, just the reverse of taking a glass of ice water
outside on a hot summer day. Water drops condense on the glass
and run off quickly forming a puddle under the glass.
The problem is greatly magnified inside the chimney due to its
size. Condensate in your chimney is the source of two major problems.
The water combines with flue gases and forms corrosive acids,
which eat away at the chimney deteriorating tiles, bricks and
mortar.
Secondly, in winter conditions, moisture freezes and thaws breaking
away mortar and bricks. This can not only quickly destroy a chimney
and create leaks into the house, but moisture can damage interior
drywall near the chimney as well as running back into the furnace
causing corrosion there too.
Do all chimneys need to be lined?
In Manitoba, the code says ³Yes!² All residential homes and buildings
require a properly sized approved chimney to safely vent all products
of combustion to the outdoors. In addition, on a retrofit installation
the code does not require that the chimney be sized exactly. However,
the Manufacturers installation instructions may demand proper
sizing, they do take precedence if more stringent than the code
requirements and can void warranties if not followed.
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