A gas furnace does not simply switch on. It works through a fixed sequence, and each step has to be proven before the next one is allowed. When a furnace runs but the house stays cold, one of those steps has failed. Knowing the order helps you tell a thermostat problem from a venting problem, and it tells you what is worth checking before you call anyone.
A furnace lights in a set order
On a modern gas furnace the sequence runs like this:
- The thermostat calls for heat and the control board receives the signal.
- The inducer motor, the small fan on the side of the burner box, starts and pulls air through the heat exchanger and out the vent.
- The pressure switch confirms that the inducer has created enough draft. If the vent or the condensate drain is blocked, the switch stays open and the sequence stops here.
- The hot-surface igniter glows, and the gas valve opens.
- The flame sensor, a thin metal rod in the flame, confirms that the burners have lit. If it cannot detect the flame, the gas valve closes within moments.
- After a short delay to warm the heat exchanger, the blower starts and pushes warm air through the ducts.
- High-limit switches watch the temperature throughout and shut the burners off if the heat exchanger overheats.
Most control boards have a small light that flashes a code when the sequence stops. The code chart is usually printed on the inside of the furnace door. Write the code down before you do anything else; it narrows the cause considerably.
When the furnace will not kick on at all
If nothing happens when the thermostat calls for heat, start with the simple causes:
- The thermostat is set to cool or off, the setpoint is below room temperature, or its batteries are flat.
- The furnace power switch, which looks like a light switch on or near the unit, has been turned off.
- The breaker for the furnace has tripped.
- The blower compartment door is not fully closed. A door switch cuts power to the furnace when the panel is off.
If all of these are fine and the inducer still does not start, the fault is usually in the control board, the transformer that supplies the thermostat circuit, or the thermostat wiring. Those need testing with a meter.
When it runs but the air coming out is cold
This is the most common complaint, and it has several distinct causes.
The thermostat fan is set to on rather than auto. The blower then runs all the time, and between heating cycles it moves unheated air. Set it to auto and see whether the cold air stops.
The furnace tries to light, fails and locks out. Most furnaces retry a few times and then go into lockout, while the blower may keep running to cool the heat exchanger. The usual reasons are:
- A flame sensor coated with a thin film of oxide, so it cannot prove the flame. The burners light, then shut off.
- A cracked or worn hot-surface igniter that no longer reaches lighting temperature.
- A pressure switch that will not close because the vent is restricted or the condensate cannot drain.
The furnace is overheating and tripping the high limit. A clogged filter or too little return air means not enough air crosses the heat exchanger. It overheats, the limit switch shuts the burners off, and the blower keeps running with cooling air until the limit resets.
Why the cause differs from house to house in Grimsby
The age of the house changes which of these faults is most likely.
In the newer homes and townhouses of the west end, around the Winston Road Neighbourhood and the GO Station Secondary Plan Area, most furnaces are high-efficiency condensing units with plastic vents through the side wall. These furnaces make water as they run. When the condensate trap clogs, or the drain line freezes where it runs near an outside wall, water backs up and the pressure switch will not close. Snow drifts or frost over the vent terminations do the same thing. The furnace starts, the inducer hums, and nothing lights.
In the post-war houses of the Lakemount Terrace Subdivision in the north end, the original ductwork was sized for an older oil or coal furnace. Undersized ducts and weak return air are common, and a newer furnace pushed through them can run hot and trip its limit switch. The furnace seems to be running, but the burners keep shutting off.
In the older cottages of Grimsby Beach and the houses along Main Street East and Main Street West, the ducts were often fitted in after the house was built, and the walls usually have little insulation. A furnace can be working correctly and still not keep up, especially if a duct has come apart in a crawlspace or a wall cavity and is heating the space under the floor.
The lake moderates Grimsby's winters, so a furnace that is marginal for the house may get through most of the season and only show the problem on the coldest nights. That timing does not mean the fault appeared suddenly.
How to reset a furnace safely
If the furnace has locked out, one reset is reasonable:
- Check that the filter is not clogged and replace it if it is.
- Go outside and clear snow or frost from the intake and exhaust pipes, by hand, without damaging them.
- Turn the furnace power switch off, wait a moment, and turn it back on. This clears the lockout on most control boards.
- Set the thermostat to heat, a few degrees above room temperature, with the fan on auto.
If it lights and keeps running, watch it over the next day. If it locks out again, stop resetting it. Repeated resets do not fix a flame sensor, a blocked trap or an overheating heat exchanger, and running a furnace through repeated limit trips adds wear to the parts that are most costly to replace.
Do not remove the burner cover, bypass a switch or try to light the furnace by hand. If you smell gas, leave the house and call your gas utility from outside.
The most common problem and the most common failure
The most common problem is a dirty filter. It causes overheating, limit trips, short cycling and weak airflow, and it is the one fix every homeowner can do.
The most common part failures are the flame sensor and the hot-surface igniter, followed by pressure switch faults on condensing furnaces, which are more often caused by a blocked condensate trap or vent than by a failed switch. Inducer motors and blower motors wear out later in a furnace's life. Control boards fail less often but cost more. A cracked heat exchanger is the most serious failure; on an older furnace it often settles the question of repair or replacement, which the guide on whether a 25-year-old furnace should be repaired or replaced covers in detail.
Carbon monoxide comes first
A heat exchanger crack or a blocked vent can let combustion gases, including carbon monoxide, into the house. The Ontario Fire Code requires a working carbon monoxide alarm next to each sleeping area of a home with a fuel-burning appliance, a fireplace or an attached garage. Test the alarms when the furnace starts acting up. If one sounds, get everyone outside and call for help before looking at the furnace.
What a technician measures before pricing a repair
A proper diagnosis reads the system rather than swapping parts. Work is performed by TSSA-certified gas technicians, and in Ontario gas appliances may only be serviced by registered contractors using certified gas technicians, under O. Reg. 212/01 and the CSA B149.1 installation code. On a no-heat call the technician typically checks:
- The flame sensor signal, in microamps, to see whether it is proving the flame.
- The igniter's resistance and the voltage reaching it.
- The draft the inducer creates against what the pressure switch needs, and whether the condensate trap and drain are clear.
- Inlet and manifold gas pressure at the gas valve.
- Temperature rise across the heat exchanger and static pressure in the ducts, which show whether airflow is the real problem.
Only once the cause is found is the repair priced, in writing, so you can decide before any part is fitted.
If your furnace is still running cold after one reset, see our furnace repair service.