How do I extend my existing ductwork into a home addition in Ottawa?

Extending your existing ductwork into an addition is often the simplest way to heat and cool the new space, but it takes careful planning to avoid comfort problems throughout the house. Ottawa's heating season is long and cold, and a poorly designed extension can leave both the new and the existing rooms with hot and cold spots.

The basic challenge is that your duct system was designed as a balanced network, delivering a certain amount of air to each room. When you tap into it for the addition, you take air from the existing rooms unless the system can move more air overall. How many supply registers and returns the addition needs, and how much air each must deliver, should come from a room-by-room heat loss and duct design, not a rule of thumb.

Start with an assessment of the existing system. An HVAC technician can measure static pressure in the supply and return sides with a manometer. If the system is already working hard against restrictive ducts, which is common in older homes with undersized returns, adding more ductwork without changes will reduce airflow everywhere. Ask for this measurement before anyone designs the extension.

The trunk line that feeds the addition is the key component. It usually branches off the main trunk and runs through the basement or crawl space below the addition, which avoids opening up finished rooms. Its size depends on the airflow the addition needs and the length of the run, and the designer should size it rather than the installer guessing on the day.

Return air is where many duct extensions fall short. Air pushed into the addition has to find its way back to the furnace. Without adequate returns, the addition becomes pressurized, pushing conditioned air out through gaps and pulling cold air in elsewhere in the house, and doors may whistle or swing. Ask how the returns will be handled and where the grilles will go.

Insulation and sealing matter in a cold climate. Ducts that run through unheated spaces such as a vented crawl space or attic lose heat and can form condensation if they aren't insulated. All joints should be sealed with mastic or a suitable foil tape, not ordinary cloth duct tape, which tends to dry out and fail. Better still, keep new ducts inside the heated envelope wherever the design allows.

If the furnace doesn't have the capacity, or the route to the addition is awkward, compare the extension with a separate heat source for the addition. A heat loss calculation answers both questions at once.

Ask the City's Building Code Services whether your heating and duct changes need to be included in the permit. Then compare itemized quotes from HVAC contractors that show the static pressure reading, the new trunk and returns, and the registers as separate items.

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