Why is my furnace not turning on?
A furnace that won't turn on is most often a power or thermostat problem rather than a dead furnace — a tripped breaker, a thermostat without batteries, or a service switch someone flipped by mistake. Beyond those, a clogged filter or a tripped safety limit will stop it from firing, and both need a technician to clear properly.
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What you can safely check
- Thermostat power and setting
- Blank screen? Replace the batteries. Then confirm it's set to HEAT and the target is a few degrees above the room temperature.
- The furnace switch
- There's a switch that looks like a light switch on or near the furnace, often at the top of the basement stairs. It gets flipped by accident constantly. Make sure it's on.
- The breaker
- Find the furnace breaker and reset it once — fully off, then on. If it trips again, stop. A breaker that won't hold is telling you something is wrong.
- The air filter
- If it's grey and matted, airflow is choked. A furnace that overheats on a blocked filter shuts itself down on a safety limit.
What’s probably going on
- Failed igniter
- The most common no-start on a modern furnace. The igniter glows to light the burners and they fail with age — the furnace tries, fails, and locks out.
- Flame sensor fouled
- A dirty sensor can't prove the flame is lit, so the furnace shuts the gas down within seconds as a safety measure. It lights, runs briefly, then quits.
- Tripped high limit
- Usually downstream of restricted airflow. The furnace is protecting its heat exchanger — clearing the lockout without fixing the cause just repeats it.
- Pressure switch or blocked vent
- Snow and ice over an exterior vent is a real New Brunswick failure mode. The furnace won't fire if it can't prove the vent is clear.
Stop here and call
If the breaker trips again, the furnace lights and quits repeatedly, or you smell anything burning — stop and call. Repeated ignition attempts on a fault are how small problems become expensive ones.
What we do about it
We read the fault code off the control board, test the igniter, flame sensor and limits, and tell you what failed and what it costs before any work starts.
Still stuck?
Tell us what the system is doing and we’ll tell you what it takes to fix — free quotes, no obligation, and a straight answer about whether it’s worth repairing.