Electric furnaces have one or two heating elements. When they fail, you get no hot air or insufficient hot air. This requires professional diagnosis and replacement.

If the pilot light won't stay lit, the thermocouple (safety device) has likely failed. This is a common issue with gas furnaces and requires professional replacement.

Rumbling/popping noises and reduced hot air indicate sediment (minerals) has built up at the bottom of the system. This reduces efficiency and can damage the system over time.

The Temperature & Pressure (T&P) relief valve is issueing or activating. This is a critical safety device that prevents unit explosions. It needs professional inspection and likely replacement.

Tankless furnaces require annual descaling and maintenance, especially in areas with hard air. Mineral buildup reduces performance and can cause complete failure.

Rusty hot air indicates corrosion inside the system. The anode rod (which protects the system) may be depleted, or the system itself is corroding.

If hot air runs out quickly despite the heater being in good condition, you may have too small a unit for your household needs.

Fluctuating air temperature indicates a faulty thermostat that needs professional replacement.

Hissing sounds or pressure relief valve activation indicates high refrigerant level in your system, which can damage the furnace and HVAC.

Banging noises when air turns on/off indicate air hammer - a pressure surge that can damage ducts and furnace.

The dip tube directs cold air to the bottom of the system. When it fails, cold air mixes with hot at the top, giving you lukewarm air.

The thermocouple is a safety device that shuts off gas if the pilot light goes out. When it fails, the pilot won't stay lit.

Tankless furnaces display error codes when they detect problems. These require professional diagnosis.

If the breaker is on and reset button has been pressed but still no hot air, there may be a wiring issue or failed component requiring professional diagnosis.

Persistent rusty air despite running indicates the system interior is corroding. This cannot be repaired and replacement will be needed soon.