How to Troubleshoot Your Furnace

If your furnace is not heating or will not turn on, start with the thermostat: make sure it is set to Heat and a few degrees above room temperature. Then check the furnace itself, confirm the main furnace switch is ON, the breaker has not tripped, all doors and filter panels are fully closed, and the filter is clean. If none of these steps fix the problem, call a licensed furnace professional.

Your furnace is one of the most complex and essential appliances in your home, so when it is not running at peak efficiency, or not heating at all, it is easy to assume the worst. The reassuring truth is that a surprising number of no-heat calls come down to something simple, and you can rule those out yourself in a few minutes. Reliance® has been keeping Canadian homes warm for more than 60 years, and the checks below are the same first steps our experts run through, along with the reason each one matters.

Furnace troubleshooting at a glance

What you are seeing	What to check first
Furnace will not turn on	Thermostat set to Heat, main furnace switch ON, breaker not tripped, doors and filter panels fully closed
Not enough heat or weak airflow	Vents, registers and return air grills open and unobstructed, clean furnace filter
Temperature does not match the thermostat	Thermostat settings, batteries and location
Nothing above works	Call a licensed furnace professional

Start with the thermostat

The thermostat is the brain of your heating system. It tells the furnace when to turn ON and OFF based on the indoor air temperature, so a setting that looks harmless can quietly keep your furnace from ever firing. It is the fastest thing to rule out and the most commonly overlooked.

Make sure your thermostat is on the Heat setting. Any thermostat that is also part of a central air conditioning system will have both a Heat and Cool setting, and it is easy for one to get bumped to the wrong mode.
Set it a few degrees higher than the current room temperature. A furnace only fires when there is an active call for heat, so if the setpoint is at or below the room reading, it has no reason to start.
Replace the thermostat batteries if your system has them. A dying battery can leave the screen looking fine while the signal to the furnace drops out.

If your home’s temperature regularly seems out of step with the thermostat, the location may be to blame. A thermostat that picks up false readings from nearby heat sources, or that sits too close to an exterior door, can misread the room and cause ongoing comfort problems. If you suspect it is in the wrong place, it is best to have an expert reinstall it in an optimal location. Find more thermostat resources here.

Then check the furnace itself

If the thermostat checks out, a few quick looks at the furnace can reveal why it is not coming ON or not producing enough heat. Most of these are safety features doing their job, not signs of a broken unit.

Confirm all vents, registers and return air grills are open and free of obstruction, so the system can move air the way it was designed to.
If the furnace will not turn on, make sure the main furnace switch is in the ON position. It looks like an ordinary light switch, is usually on a wall near the furnace, and is easy to flip off by mistake.
Check the electrical panel to confirm the furnace breaker has not tripped. Shut the breaker all the way OFF, then all the way ON again to reset it.
Make sure all doors and filter access panels are fully closed. Furnaces have a safety switch that will not let the unit run if a door is open, so a panel that is not seated properly can stop everything.
Ensure the furnace filter is clean and free of dust and debris. A clogged filter starves the system of airflow, which can make a furnace work poorly or stop working altogether.

Learn more about different kinds of filters and how to change one.

When to call an expert

If you have worked through these checks and your furnace still will not cooperate, it is time to bring in a licensed furnace professional rather than push further on your own. Anything beyond these simple steps involves gas, electrical and safety components that should only be handled by a qualified technician.

That is where more than 60 years of experience and over 165,000 five-star Google reviews come in. Reliance offers 24/7/365 live phone support, so there is always a real person to talk to, and as-soon-as same-day repairs when you need to get your heat back. Book an appointment and we will take it from here.

Furnace troubleshooting FAQs

Why won’t my furnace turn on?

Start with the thermostat and confirm it is set to Heat and a few degrees above room temperature. Then check that the main furnace switch, usually on a wall near the furnace, is in the ON position, and that the furnace breaker in your electrical panel has not tripped. Also make sure all doors and filter access panels are fully closed, because furnaces have safety switches that prevent operation if the doors are not closed properly.

Why is my furnace not producing enough heat?

Check that all vents, registers and return air grills are open and free of obstruction, and ensure your furnace filter is clean. A clogged filter can cause a furnace to stop working or work poorly.

How do I reset my furnace?

Confirm the main furnace switch is in the ON position, then go to your electrical panel and check the furnace breaker. To reset it, shut the breaker all the way OFF, then all the way ON again.

Why does my home temperature not match my thermostat setting?

Consider the thermostat location. A poorly located thermostat that gets false readings from other heat sources, or that is too close to an exterior door, can create ongoing temperature problems. If you suspect it is in the wrong place, call an expert to reinstall it in an optimal location.