A lot of no-heat calls in Calgary arrive early in the day, because the house has drifted down overnight and the furnace has tried and failed to bring it back up. You hear the thermostat click, the furnace starts to hum, and then nothing warm comes out of the registers. It is an unsettling start to a -25 C morning, although the cause is often a single small part in the ignition sequence. Understanding that sequence makes it easier to describe the problem on the phone, and it helps you follow along when a technician opens the cabinet.
How a Modern Furnace Lights Itself
A gas furnace built in the last twenty years or so follows the same basic routine every time the thermostat calls for heat. The control board starts a small fan called the draft inducer, which clears the heat exchanger and pulls exhaust out through the vent pipe. Once the inducer is running, a pressure switch confirms that the vent is drawing properly, and only then does the board send power to the hot surface igniter, which glows bright orange for several seconds. After that the gas valve opens, the burners light, and a thin metal rod called the flame sensor confirms that there is a flame. If any step fails, the board shuts the gas off and retries a few times before locking itself out, which is why a furnace that won’t ignite often goes quiet instead of trying all morning.
Reading the Codes Before Anything Comes Apart
Most furnaces have a small LED on the control board that you can see through a window in the lower panel. When something goes wrong, that light blinks in a pattern, and a chart on the inside of the door explains what each pattern means. It is worth noting the code before you switch anything off, because cycling the power clears the light on many models and the technician loses an easy starting point. A photo on your phone takes ten seconds and can save a good part of the diagnostic time on a furnace repair when the burners won’t light.
The code is a starting point rather than a verdict, since a “pressure switch” fault might mean a failed switch, a cracked hose, a blocked vent or a weak inducer motor. A good diagnosis confirms the cause with a meter or a manometer rather than replacing the part named on the chart.
The Usual Suspects
The Igniter
Hot surface igniters are made of a brittle ceramic material, and they wear out with use. A cracked igniter may still glow in spots or not at all, so the gas valve never gets the signal it is waiting for. Technicians check resistance across the igniter and watch it during a start-up, and replacement typically runs about $250 to $450 depending on the furnace.
The Flame Sensor
The flame sensor causes a very recognizable pattern, because the burners light for a few seconds and then shut off. The furnace did ignite, but the board could not confirm the flame, so it closed the gas valve as a precaution. Often the rod is coated with a thin film of oxide and simply needs cleaning, although a sensor that is cracked or badly worn gets replaced, typically for $200 to $350. A technician will also check the ground connection, since a poor ground produces the same symptom with a perfectly clean sensor.
The Pressure Switch and Venting
High-efficiency furnaces vent through plastic pipes on the side of the house, and in Calgary those terminations sometimes pick up frost or drifted snow during a cold snap. When the exhaust or intake is restricted, the pressure switch will not close and the igniter never gets power. A frozen or blocked condensate drain can cause a similar fault, since water backs up into the collector box. The fix may be as small as clearing a vent or a drain trap, which is why a diagnostic visit from a heating technician checks the venting and drainage before condemning any parts. If the inducer motor itself has failed, that is a larger repair, typically $600 to $1,200.
The Control Board and Gas Valve
Less often, the problem sits with the parts that run the whole sequence. A control board with a failed relay may never power the igniter, and a gas valve that will not open leaves a glowing igniter with nothing to light. These are more expensive repairs, with control boards typically $500 to $1,200 and gas valves $500 to $900, so a careful technician tests them directly before recommending replacement.
What You Can Safely Check Before Calling
There are a handful of things a homeowner can look at without opening the burner compartment, and they resolve a surprising number of calls. If you smell gas at any point, skip all of this, leave the house and call 911 or ATCO’s emergency line from outside.
- The thermostat is set to heat, the setpoint is above room temperature, and the batteries are fresh.
- The service switch near the furnace, which looks like an ordinary light switch, is on, and the breaker has not tripped.
- The lower door panel is seated firmly, since a safety switch behind it stops the furnace when the door is loose.
- The filter is not clogged, and the outside vent pipes are clear of snow and frost.
Resetting the furnace once by switching the power off and on is reasonable after you have noted the code. If it locks out again, repeated resets will not fix the underlying fault, and leaving it alone gives the technician a cleaner picture.
What the Visit Costs and What to Ask
A standard diagnostic service call with a local Calgary contractor is typically about $149, while an after-hours call during the heating season runs higher, around $289. Once the cause is found, ask the technician to show you the failed part and explain how they confirmed it, and ask whether anything else in the sequence looked worn. On a furnace that is older than about fifteen years, it is also fair to ask for a quick opinion on its overall condition, so that a small repair today does not hide a bigger decision coming next winter. Many ignition problems come down to a single part, which is a much better outcome than the cold start to the day suggested.
Calgary Air Heating and Cooling Ltd Contact Information
Address
Phone
+1 (403) 720-0003
Hours of operation
7 a.m. to 11 p.m. (including weekends)
