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Can I use my chimney with my new furnace?

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 today¹s furnaces make specific demands many existing lined masonry chimneys simply can¹t 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. That¹s great news because it means you¹re 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, today¹s 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 today¹s 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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Custom VAC Ltd.
1567 Logan Avenue
Winnipeg, Manitoba
R3E 1S5
T. 204-775-7744
T. 204-772-5799
F. 204-774-8557


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