Why Does My Furnace Keeps Turning On and Off? A Guide for Ontario Homeowners
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At LG Mechanical, we know how brutal Ontario winters can be, and the last thing you need is an unreliable furnace when temperatures drop below freezing. If your furnace keeps shutting off unexpectedly, let’s explore the common causes behind furnace short cycling and what you can do to get your home warm and comfortable again.
The Role of Commercial HVAC: Beyond Comfort
Heating, Ventilation, and Air Conditioning (HVAC) systems are the backbone of commercial spaces, ensuring optimal indoor conditions for employees, customers, and sensitive equipment. But traditional systems often come with high operational costs and significant environmental impact. At LG Mechanical, we’re helping businesses embrace smarter, greener solutions tailored for the modern workplace.
One of the most common culprits behind a furnace turning on and off is a dirty air filter. Your furnace relies on a steady airflow to function properly, but when the filter becomes clogged with dust, pet hair, and debris, it restricts airflow to the heat exchanger, causing it to overheat. To prevent a fire hazard or damage, your furnace’s high limit switch automatically shuts off the system.
- Turn off the furnace and inspect the air filter.
- If it looks clogged with dirt and dust, replace it if it’s a disposable filter or wash it if it’s reusable.
- Going forward, check your air filter once a month and replace or clean it every 90 days.
- Consider upgrading to high-efficiency filters to improve indoor air quality and protect your heating system.
In Ontario’s harsh winters, where heating systems run for extended periods, a clean filter ensures your furnace keeps running efficiently works efficiently while preventing unnecessary strain on its components.
Your thermostat is the control center of your heating system. If it’s not functioning properly, your furnace keeps kicking on and off too frequently.
Common Thermostat Issues:
- Incorrect Temperature Setting: Ensure your thermostat is set to “HEAT” and adjusted to a temperature that provides consistent warmth.
- Poor Placement: If your thermostat is near a heat source (such as direct sunlight, fireplaces, or kitchen appliances), it may register incorrect temperatures and turn off your furnace prematurely. It should be mounted about 1.5 meters (5 feet) from the floor and away from heating or cooling influences.
- Wiring Problems or Battery Issues: If the thermostat isn’t communicating properly with the furnace, it may need new batteries or a technician’s inspection.
- Quick DIY Test:
- Have one person stand by the thermostat and another near the furnace.
- Slowly increase the temperature setting.
- If the furnace doesn’t respond within a few seconds, your thermostat may be faulty and require professional servicing.
If your thermostat is outdated, consider upgrading to a smart thermostat, which provides better temperature regulation, energy savings, and remote control via your smartphone.
3. A Dirty Flame Sensor is Causing Your Furnace to Shut Off
Gas furnaces use a flame sensor to confirm that the burners have ignited properly. If this sensor is covered in soot, dirt, or debris, it may fail to detect the flame, causing your furnace to turn on but no heat then turns off.
Signs of a Dirty Flame Sensor:
- Your furnace turns on but shuts off after a few seconds.
- You hear the burners ignite briefly before the system powers down.
- The furnace keeps short cycling with no consistent heating.
How to Fix It:
- Turn off the furnace and gas supply.
- Locate the flame sensor (it’s a small metal rod near the burners).
- Gently clean it using fine-grit sandpaper or a soft cloth to remove buildup.
- Reinstall it and restart your furnace.
If cleaning doesn’t resolve the issue, LG Mechanical’s experienced technicians can replace your sensor and perform a full furnace inspection to prevent further interruptions.
4. Your Furnace is Oversized for Your Home
Bigger isn’t always better when it comes to heating systems. If your furnace is too powerful for the size of your home, it will heat the space too quickly, shut off, and then restart shortly after. This cycle repeats frequently, leading to wasted energy, uneven temperatures, and unnecessary wear and tear on the unit.
Signs Your Furnace is Too Large:
- Rooms heat up too fast but cool down just as quickly.
- The furnace keeps turning on and off within a short time frame.
- High energy bills despite limited heating periods.
Solution:
If your furnace is oversized, the best long-term fix is to replace it with a properly sized, energy-efficient unit. A professional heat load calculation from LG Mechanical ensures you get a furnace that’s the right fit for your home’s square footage, insulation, and climate.
5. A Blocked Exhaust Vent is Triggering Safety Shutdowns
Your furnace expels combustion gases, including carbon monoxide, through an exhaust vent. If this vent becomes blocked by snow, ice, leaves, or animal nests, it can trigger a safety shutoff to prevent dangerous gas buildup.
How to Prevent Vent Blockages:
- Inspect the outdoor exhaust pipe regularly, especially after snowstorms.
- Remove any obstructions and clear snow around vent openings.
- Install vent covers to prevent animals from nesting inside.
If you suspect a blocked vent but can’t safely access it, call LG Mechanical to inspect and clear any obstructions before restarting your furnace.
6. A Faulty Blower Motor is Disrupting Airflow
Your blower motor is responsible for pushing heated air through your home’s ductwork. If it’s malfunctioning or clogged with debris, your furnace may struggle to maintain proper airflow, causing it to short cycle.
Symptoms of a Blower Motor Issue:
- Weak airflow from vents
- Unusual noises coming from the furnace
- A burning smell when the furnace runs
- Higher energy bills due to inefficient operation
If you notice any of these signs, it’s time to schedule a furnace maintenance service with LG Mechanical to inspect and repair the blower motor before it leads to a system failure.
7. The High Limit Switch is Preventing Overheating
The high limit switch is a built-in safety mechanism that shuts down your furnace turning on and off if it gets too hot. This can be triggered by:
- Restricted airflow due to a dirty filter
- Blocked vents or registers
- A failing heat exchanger
Fixing High Limit Switch Issues:
- Replace the air filter regularly to maintain airflow.
- Ensure all vents and registers are open and unblocked.
- Schedule an HVAC inspection to check for deeper issues, such as a cracked heat exchanger.
If your furnace keeps turning on and off, it’s time to call LG Mechanical for a professional assessment.
Read More: The Future of Commercial HVAC
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To prevent short cycling:
- Replace or clean the air filter
- Ensure the thermostat is working properly
- Clean the flame sensor
- Keep vents open and unblocked
- Schedule professional maintenance if needed
