Furnace or Heat Pump Not Heating

No heat is an urgent problem in cold weather. The right diagnostic path depends on your system type: gas furnace, heat pump, or electric furnace each fail in distinct ways.

Quick Diagnosis

  • Difficulty: intermediate
  • Estimated time: 30–60 min
  • Estimated cost: $0–50

Likely Causes

Thermostat Set Incorrectly

Likelihood: Common first-check

Mode must be HEAT and setpoint must be above the current temperature.

Fix: Set to HEAT mode, set temperature 3–5 degrees above current room temp.

Furnace Filter Clogged (Gas/Electric)

Likelihood: Very common

A completely blocked filter causes the furnace to overheat and shut down on its high-limit switch.

Fix: Replace the filter immediately.

Pilot Light Out / Ignition Failure (Gas)

Likelihood: Common in older gas furnaces

Modern furnaces use electronic ignition. A failed igniter or dirty flame sensor causes no-heat with a blinking error code on the furnace board.

Fix: Read the blinking light error code (pattern is in the furnace manual or door label). Clean or replace the flame sensor.

Heat Pump in Defrost Mode

Likelihood: Normal operation usually

Heat pumps defrost themselves periodically. During defrost, they blow cool air briefly. This is normal. If the unit seems stuck in defrost (steam, no heat for 20+ min), the defrost board may be faulty.

Fix: Wait 10–15 minutes. If heat returns, it was normal defrost cycle.

Tripped Breaker

Likelihood: Common

Furnace or air handler circuits can trip.

Fix: Check and reset the furnace breaker.

How to Fix It

  1. Check thermostat and breaker

    Set to HEAT, set 5 degrees above current temp. Check the furnace breaker (reset if tripped). Check for a secondary breaker near the air handler.

  2. Replace the filter

    A severely clogged filter causes furnace lockout via the high-limit switch. Install a new filter, then reset the furnace by turning it off for 30 seconds and back on.

  3. Read the furnace error code

    Most modern gas furnaces have a small LED on the control board inside the furnace cabinet. Count the blink pattern (e.g., 3 short + 1 long) and look up the code on the chart printed inside the access panel door. Common codes: pressure switch fault, flame sensor, igniter failure.

  4. Clean the flame sensor

    The flame sensor is a small metal rod that proves the burner has lit. When it gets coated with oxide, it can't detect flame and shuts down after ignition. Turn off power, remove the sensor (one screw), gently rub the metal rod with fine steel wool or light sandpaper, reinstall.

Tools

  • Fine steel wool or emery cloth
  • Screwdriver

Materials

  • Flame sensor (if needed) - $15–30
  • Igniter (if needed) - $25–50

Common Questions

My furnace turns on, runs briefly, then shuts off. What's wrong?

Short-cycling (turning on and off rapidly) usually means: (1) dirty filter restricting airflow → high limit switch trips, (2) flame sensor failure, or (3) oversized furnace. Replace the filter first. If that doesn't help, clean the flame sensor.

My heat pump's outdoor unit is covered in ice. Is that normal?

Light frost on the coil is normal in cold, humid weather. Ice covering the entire unit and persisting for more than 2 hours is not. It usually indicates low refrigerant or a faulty defrost control. Call an HVAC tech.

When to Call a Pro

Call an HVAC technician if: error codes indicate heat exchanger or pressure switch issues (safety-critical), if you smell gas near the furnace (call the gas company first), or if the heat pump refrigerant needs service.

Related Symptoms

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