Why does my furnace keep turning on and off

Short cycling is when your furnace turns on and off frequently without completing full heating cycles, and it's one of the most common HVAC issues Ottawa homeowners face during our cold winters.

The most likely culprit is a dirty air filter. When your filter is clogged with dust, pet hair, and debris, it restricts airflow over the heat exchanger. This causes the furnace to overheat and shut down as a safety measure, then restart once it cools down. Check your filter right away - if it's gray or brown instead of white, replace it immediately. During Ottawa's heating season, filters typically need changing every 1-3 months depending on your home and pets.

Thermostat issues can also cause short cycling. If your thermostat is located near a heat source (like a sunny window, fireplace, or heat register), it may be getting false temperature readings. The thermostat thinks the house is warm enough and shuts off the furnace, but the rest of your home is still cold. Older thermostats can also malfunction and send incorrect signals to your furnace.

More serious causes include an oversized furnace for your home, blocked vents restricting airflow, or internal component failures like a faulty flame sensor or heat exchanger problems. In Ottawa's climate, short cycling not only wastes energy and increases your heating bills, but it also prevents your home from reaching comfortable temperatures during those -20°C winter days.

Safety note: If your furnace is short cycling and you smell gas or notice yellow flames instead of blue, turn off your system immediately and call for emergency service. These could indicate dangerous combustion issues that require immediate attention from a licensed G2 technician.

Start by checking and replacing your air filter, then ensure all vents are open and unblocked. If the problem persists after these steps, it's time for professional diagnosis. For a thorough inspection of your furnace's performance, Ottawa HVAC Pro offers comprehensive system evaluations to identify and resolve short cycling issues.

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