Why furnaces fail here
A furnace in a cold climate runs most of the year and stays warm and dry. Ours sits idle through months of heat and humidity. Flame sensors build an oxide film that keeps the board from proving flame, burners collect dust, and electrical contacts corrode.
Airflow is the other half. The blower has run all summer for cooling, so by December the filter is loaded, and a restricted furnace overheats, trips the high limit, and shuts the burners down again and again. From inside the house that looks like a broken furnace. It is usually a starved one.
How we diagnose it
A gas furnace runs a fixed sequence, and a fault stops it at a specific point: thermostat call, inducer, pressure switch, igniter, gas valve, burners, flame sensor, blower. We watch that sequence run and find where it stops instead of guessing at parts.
Then we measure. Flame signal in microamps, manifold gas pressure, temperature rise against the range on the data plate, duct static pressure, blower amp draw. Those numbers separate a failed part from a furnace being starved by its own ductwork.
Combustion safety, handled plainly
A furnace burns fuel inside a sealed heat exchanger and vents the products of combustion outdoors, while your house air passes over the outside of it and never mixes with them. That separation is the entire safety design, which is why it is worth verifying once a year.
We look for soot, flame that lifts when the blower starts, tripped rollout switches, and a blocked or separated flue, then take a carbon monoxide reading. If we find a cracked heat exchanger we show you what we saw and shut the furnace down. It is not a part you patch.
What the repair looks like
You get the price in writing before anything comes apart, and where the failed part is one we carry, the repair happens on that visit. Afterward we cycle the furnace from cold, confirm the temperature rise lands where the manufacturer says it should, and take a carbon monoxide reading before we leave.
What we check on a no-heat call
- Thermostat and control powerWe confirm the thermostat is calling for heat and that 24 volt power reaches the board.
- Draft and pressure switchThe inducer has to pull a proper draft before the pressure switch allows ignition, so we test both.
- Ignition, gas pressure, flame signalWe watch the burners light, then measure manifold pressure and the flame signal the board reads.
- Airflow and filter restrictionA loaded filter or closed returns will trip the high limit and make a healthy furnace look broken.
- Heat exchanger and carbon monoxideWe inspect the exchanger and take a carbon monoxide reading before calling the job finished.