Troubleshooting guide

Why Is My Furnace Running but Not Heating?

A furnace blower can run with no heat being made. Separate blower operation from the ignition sequence, airflow limits and fault codes.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

The fan that moves air through the ducts and the burner that makes the heat are controlled separately. A furnace can run its blower, for example on a fan-only setting, and make no heat at all. The first step is to find out how far through the heating sequence the furnace gets.

Gas-fired equipment can leak gas and produce carbon monoxide. If you smell gas, leave the building and call the gas utility or emergency services from outside. Diagnosis of gas valves, ignition, flue and safety circuits belongs with qualified technicians. Never bypass a safety control.

What you observe	Where in the sequence it stops	Likely cause
Blower runs, nothing else happens	No call for heat reaches the furnace	Thermostat set to fan only or not calling, wrong mode, dead batteries, or no control voltage
Inducer fan starts, then stops, no flame	Pressure switch or vent proving fails	Blocked flue or intake, blocked condensate drain, or a faulty switch
Inducer runs, igniter glows, no flame	Gas valve does not open or gas supply is off	Gas supply and valve (technician)
Flame starts, then goes out after a few seconds	Flame sensing fails	Dirty or worn flame sensor, poor grounding, or unstable flame
Burners run, blower starts late or warm air is short-lived	Limit switch opens because the furnace overheats	Dirty filter, blocked return or closed registers, or a failing blower
Repeated starts, then no further attempts	Control board lockout after failed ignition tries	Fault code on the board. Reset only after the cause is understood

Why airflow shows up as a heat problem

The temperature rise across the furnace is the heat output divided by 1.08 times the airflow. For a furnace with a 60,000 Btu/h output:

Airflow	Temperature rise	Effect
1,200 CFM	46 °F	Within a typical rated range. Check the furnace nameplate
800 CFM (dirty filter)	69 °F	Likely above the rated rise, so the high-limit switch opens and shuts the burners off

The limit switch is a protection, so a furnace that heats for a few minutes and then blows cool air often has an airflow problem rather than a heat problem. The nameplate shows the permitted temperature-rise range.

Fault indications

Most modern furnaces show a flashing light or a code on the control board, and the meaning is given on a label in the furnace or in the manual. The code names the stage that failed, which saves time.

Write a useful problem statement

Furnace type, age and fuel. Thermostat type and settings.
Which parts run (blower, inducer, igniter, flame) and for how long.
The fault code or the number of flashes on the control board.
Last filter change and any recent work on the vent, ducts or gas supply.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Is it safe to reset the furnace?

One reset is generally acceptable. Repeated resets without finding the cause are not, because a lockout means a safety control stopped the furnace for a reason.

Why does a dirty filter stop the heat?

Less airflow raises the temperature inside the furnace, and the high-limit switch shuts the burners off to prevent damage.

What is a flame sensor?

A rod in the burner flame that proves a flame exists. If it is dirty, it cannot detect the flame, and the gas valve closes after a few seconds.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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