My oil furnace keeps cycling on and off during cold snaps in Miramichi — is the heat exchanger cracked or something simpler?

Your furnace is likely short-cycling due to restricted airflow or an oversized unit rather than a cracked heat exchanger — cracked heat exchangers typically cause continuous operation as they struggle to reach temperature, not frequent cycling.

During Miramichi's cold snaps (often hitting -25°C to -30°C), your oil furnace is working harder than usual, which can expose underlying issues that don't show up in milder weather. The most common culprit is a dirty or clogged air filter — when airflow is restricted, the furnace overheats quickly, triggers the safety limit switch, shuts down to cool off, then restarts. This creates the annoying on-off cycle you're experiencing.

Check your air filter first — if it's grey or brown instead of white, replace it immediately. Standard 1-inch filters should be changed monthly during heating season in New Brunswick's dusty conditions. Also inspect your return air vents — furniture, curtains, or accumulated dust can block airflow. Each return vent should have at least 6 inches of clearance.

Another possibility is an oversized furnace — common in older Miramichi homes where previous owners upgraded the unit without proper load calculations. An oversized furnace heats the space too quickly, satisfies the thermostat, shuts down, then repeats the cycle. This wastes oil and creates uneven heating.

The heat exchanger issue you're worried about is more serious but less likely. A cracked heat exchanger typically causes the furnace to run continuously without reaching temperature, plus you'd smell combustion gases or see rust flakes near the unit. If you suspect a cracked heat exchanger, shut down the furnace immediately and call a licensed technician — this is a carbon monoxide risk that requires professional diagnosis.

For immediate troubleshooting, replace the filter and check that all supply and return vents are unobstructed. If the cycling continues, you'll need a licensed oil burner technician to check the flame sensor, limit switches, and combustion chamber. In the Miramichi area, expect to pay $150-$250 for a diagnostic service call, with repairs typically running $200-$600 depending on the issue.

Given that many Miramichi homes are 30-50 years old with original oil furnaces, this might be the perfect time to consider upgrading to a cold-climate heat pump with your existing oil furnace as backup. NB Power offers rebates for heat pump installations, and you'd see significant savings on heating costs while improving comfort during these cold snaps.