Question 1

Why does my furnace turn on and blow air but there's no heat?

Accepted Answer

This usually indicates the burner isn't igniting. Common causes are a failed ignitor, a dirty flame sensor, a gas supply issue, or a safety control that has locked the system out due to repeated failed ignition attempts.

Question 2

What should I check if my furnace isn't heating?

Accepted Answer

Check that the thermostat is set to heat and above room temperature, the filter isn't severely clogged, the gas supply is on, and the breaker hasn't tripped. If all looks fine, call for professional diagnosis.

Question 3

Is it safe to run my furnace if it's not heating properly?

Accepted Answer

If the furnace is cycling on and off but producing no heat, it's generally safe to leave running while you wait for service. If you smell gas or the CO detector alarms, leave immediately and call your gas utility.

Question 4

Why is my furnace blowing cold air?

Accepted Answer

The most common causes are a dirty air filter restricting airflow, a malfunctioning flame sensor that shuts down the burner after ignition, or a thermostat set to 'ON' instead of 'AUTO' so the blower runs even when the burners aren't firing. If you've checked the filter and thermostat setting and still get cold air, the issue is likely a failed ignitor, gas valve problem, or cracked heat exchanger — all of which need professional diagnosis.

Question 5

My furnace runs but doesn't produce heat — what's wrong?

Accepted Answer

When the blower runs but there's no heat, the burners aren't igniting or the gas supply is interrupted. Check that your gas valve is open and your thermostat is set to heat mode. If everything looks right, the ignitor or flame sensor has likely failed — the control board fires the ignitor, doesn't detect flame, and locks out the burners while leaving the blower running for safety.

Question 6

Why does my furnace blow warm air for a few minutes then switch to cold?

Accepted Answer

This usually points to a dirty flame sensor — the burners light initially but the sensor can't detect the flame, so the control board shuts them off as a safety measure while the blower continues running. A technician can clean or replace the flame sensor in about 15 minutes. Less commonly, a cracked heat exchanger can cause the high-limit switch to trip, which also shuts down the burners mid-cycle.

Question 7

Can a clogged filter cause my furnace to blow cold air?

Accepted Answer

Absolutely. A severely clogged filter restricts airflow across the heat exchanger, causing it to overheat and trip the high-limit safety switch. When that happens, the burners shut off but the blower keeps running, pushing unheated air through your vents. Replacing the filter every 1–3 months is the single easiest thing you can do to prevent furnace problems.

Question 8

Why won't my furnace turn on at all?

Accepted Answer

Start with the basics: check that the thermostat is set to heat and the temperature is set above room temperature, then verify the furnace power switch (usually a light switch on or near the unit) is in the ON position. Also check your breaker panel for a tripped breaker. If all of those check out, the issue is typically a failed control board, blown fuse on the board, or a bad transformer — you'll need a technician at that point.

Question 9

My thermostat is on but the furnace won't start — what should I check?

Accepted Answer

First, make sure the thermostat display is lit and the batteries aren't dead — a blank screen means no signal is reaching the furnace. Next, check that the furnace door panel is fully seated, because most furnaces have a safety switch that kills power if the panel is even slightly ajar. If neither of those is the issue, the problem is usually in the control board, wiring, or a failed component like the inducer motor or pressure switch.

Question 10

Why won't my furnace shut off?

Accepted Answer

If the burners are running continuously, the furnace can't satisfy the thermostat — this usually means the unit is undersized for your home, there's significant heat loss through poor insulation or duct leaks, or it's extremely cold outside and the system has reached its capacity. If the blower fan runs nonstop but the burners cycle normally, check that your thermostat fan setting is on 'AUTO' rather than 'ON.'

Question 11

My furnace runs all day but the house never gets warm enough — why?

Accepted Answer

In Asheville's mountain climate, this often comes down to undersized equipment, leaky ductwork losing heated air in unconditioned spaces like crawlspaces or attics, or a furnace that's lost efficiency due to age and wear. We see 15–25% heat loss through duct leaks alone in many older homes. A technician can measure your system's temperature rise and static pressure to pinpoint exactly where the problem is.

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