HVAC●FAULTS

Furnace not heating

When the house will not reach the setpoint, first decide whether the furnace is not running at all, running without enough heat, or leaving only some rooms cold. Check thermostat settings, power status, the filter and room grilles. If rooms stay cold after that, a technician needs to find the cause.

No heat is a starting point, not a diagnosis. Wrong thermostat settings, a power problem or an unmet safety condition can keep a furnace from responding. A restricted filter or blocked grilles can reduce the heat that reaches rooms. If air is moving but feels cold, or the burner never lights, the more specific pages fit better.

This page is about rooms or the whole house not reaching the temperature you set. It does not cover cold air from the vents or ignition failures in detail.

Safe checks before calling

  1. Thermostat settings

    Check that the thermostat is on HEAT, the setpoint is above the current room temperature, and no schedule or hold is lowering it. Replace batteries if it uses them and its manual allows. Leave wiring alone.

  2. Whether anything runs

    Note whether the furnace does nothing, runs without warming the house, or leaves only some rooms cold. Feel for airflow at a few supply registers. This tells the technician which part of the system to look at first.

  3. Power status

    Look at the furnace service switch and the labeled breaker, and note what you see. A working thermostat display does not prove the furnace has power. Do not reset a tripped breaker, open panels or touch fuses.

  4. Filter and grilles

    With the furnace off, inspect an external filter you can reach easily and replace a dirty disposable one with the specified size. Make sure supply and return grilles are open and unblocked. A clean filter helps airflow but does not clear a safety lockout.

  5. Record the code

    Write down the full model and serial number and any LED pattern or display you can see through the viewing window, with the time it appeared. Do not remove panels, press door switches or start test modes.

Common reasons for no heat

Settings or power

The wrong mode, a low setpoint, a schedule, or a loss of power can leave the furnace idle. These are the easiest to rule out, but a power problem inside the furnace still needs a technician.

A safety control not satisfied

Pressure, limit and flame-proving checks can stop the heating sequence. On Heil G80CTL Series A, for example, codes 3, 4 and 6 each point to a different step, and none names a failed part.

Restricted airflow

A clogged filter or blocked supply and return grilles reduce airflow, which can cut the heat delivered and, in some cases, trip overheat protection. Fixing airflow is basic maintenance, not proof every cause is gone.

Running but not keeping up

If the furnace runs but rooms stay short of the setpoint, airflow distribution, controls, combustion, or a mismatch between the equipment and the house can be involved. Room temperature alone does not justify replacing the furnace.

When to call a technician

Call a technician if the house stays cold after the thermostat, power, filter and grille checks, or if any fault code appears. They separate a missing heat call, no power, no burner, no blower and low temperature rise, then check controls, combustion and airflow. Equipment sizing and ductwork may be assessed, but only after measurement.

Repair quotes

Common questions

Is no heat the same as the furnace blowing cold air?

Not quite. No heat means rooms are not reaching the setpoint. Cold air means the blower is moving air that does not feel warm. Both can happen together, and the cold-air page covers fan settings and delays.

The thermostat screen is blank. What can I do?

If it runs on batteries and its manual allows, replace them. A hard-wired thermostat that stays blank, or any electrical supply question, belongs with a technician rather than a homeowner check.

Can a dirty filter stop my furnace from heating?

It can restrict airflow enough to reduce heat or trip overheat protection. Replacing it with the specified size is basic maintenance. If heat does not return, the filter was not the whole story.

Sources

Reviewed 2026-10-07 · How we verify codes

Other symptoms