Furnace and heating service in Oshawa
Metropolitan Heating and Air Conditioning has repaired, serviced and replaced furnaces since 1979, and serves Oshawa households, on all makes and models. Almost everything people describe as "the furnace won't come on" is one step of a fixed start-up sequence that failed to complete, and knowing which step narrows the fault to a handful of parts before anyone opens a panel.
Call (416) 283-5698, Monday to Friday, 8:00 AM to 4:00 PM.
A gas furnace lights in a fixed order
Every modern gas furnace runs the same sequence, and it will not skip a step. The thermostat closes a circuit and calls for heat. The control board starts the inducer, a small fan that pulls combustion air across the burners and pushes exhaust out the flue. A pressure switch senses that draft and closes, proving the flue is clear. Only then does the board energise the igniter — a hot surface element that glows, or a spark electrode. The gas valve opens, the burners light, and a flame sensor confirms that flame actually exists. Thirty to sixty seconds later, once the heat exchanger has warmed, the blower starts and air reaches the registers.
Each step is an interlock: if one does not report back, the board stops and never opens the gas valve. That is the system working correctly, not failing.
Where the sequence usually breaks
- Nothing happens at all. No call for heat is reaching the board: the thermostat, its batteries or wiring, a tripped breaker, the service switch beside the furnace left off, or a blown low-voltage fuse on the board.
- The inducer runs, then everything stops. The pressure switch did not prove. Either the draft is obstructed — a plugged flue, a blocked intake, a condensate trap full of water on a high-efficiency unit — or the switch or its sensing hose has failed.
- The igniter glows but nothing lights. A gas supply problem, a gas valve that is not opening, or burners fouled enough that flame will not carry across them.
- It lights, runs a few seconds, shuts down, and repeats. The classic flame-sensor fault, described below.
- It heats, then quits before the thermostat is satisfied. A high limit opening on temperature, which is nearly always airflow.
- Burners run but the blower never starts. A failed blower motor or run capacitor, or the fan control on the board.
Reading the flame sensor
The flame sensor is a single metal rod sitting in the burner flame. It works by rectification: flame conducts a tiny current in one direction only, and the board reads it in microamps as proof that gas is burning rather than accumulating. A film of oxide builds on the rod over ordinary seasons of use, dropping the signal below the board's threshold. The furnace lights, the board cannot see the flame it just made, and it closes the gas valve within seconds.
The signature is unmistakable: it fires, runs briefly, shuts off, tries twice more, then locks out. Cleaning or replacing that rod is a small job, and it is one of the most common winter calls there is.
What a limit trip is actually telling you
The high limit switch opens when the plenum above the heat exchanger gets hotter than it should. Heat is produced faster than air carries it away, and the cause is almost always restricted airflow: a loaded filter, blocked or closed return grilles, undersized return ducting, a blower wheel caked with dust, or a weak capacitor letting the motor turn slower than its rating.
Resetting a limit addresses none of that. Repeated overheating cycles stress the heat exchanger, which is how an airflow problem nobody corrected becomes a replacement a few seasons later.
The heat exchanger ends the conversation
The heat exchanger is the metal wall separating combustion gases from the air circulating through your house. Expansion and contraction over years of cycling eventually cracks it. When that happens it is not a repair — the furnace is condemned, and the path forward is replacement.
Shut the system down at the thermostat and the service switch and call us if you smell gas, see soot or scorching around the burner compartment or flue, find a yellow and lazy burner flame where it should be crisp and blue, or a carbon monoxide alarm sounds.
Heating services we bring to Oshawa
Alongside furnace repair and annual maintenance, we cover cooling, heat pumps, ductless systems, water heaters and indoor air quality — the full range of services, across our service areas. We are licensed and insured, we work on all makes and models, and we install Carrier, Lennox, Napoleon, Trane, Bosch and Daikin. Financing is available through Vista.
Book furnace service in Oshawa
Call (416) 283-5698 weekdays between 8:00 AM and 4:00 PM, or send us the details. Have the make, model and rough age from the label inside the furnace door, and — the useful one — which step of the sequence above it reaches before it stops. If the control board is flashing a code, count the flashes.