HVAC System Not Heating: Diagnostic Checklist

A heating failure can stem from a thermostat misconfiguration, a failed igniter, a tripped safety switch, or a refrigerant-side fault — each requiring a different diagnostic path. This page covers the structured diagnostic process for identifying why an HVAC system is not producing heat, organized by system type, failure category, and escalation threshold. Understanding the distinction between user-serviceable checks and licensed-technician territory is essential for both safety and regulatory compliance.

Definition and scope

An HVAC heating failure is defined as any condition in which the system fails to raise indoor air temperature to the setpoint within the expected operating window, typically 15–30 minutes under normal load conditions. The scope of this checklist covers the four primary residential heating system architectures common in the United States: gas furnaces, electric furnaces, heat pumps, and packaged HVAC units. For system-type background, the HVAC System Types Overview page classifies these architectures in detail.

Heating failures fall into two broad regulatory categories. The first involves fuel-burning equipment — gas and oil furnaces — which are governed under NFPA 54 (National Fuel Gas Code, 2024 edition) and require licensed technicians for any work on gas trains, heat exchangers, or venting. The second category covers electrical and refrigerant-side faults, which fall under NFPA 70 (National Electrical Code, 2023 edition) and EPA Section 608 of the Clean Air Act for refrigerant handling, respectively. The HVAC Repair Licensing Requirements by State page covers jurisdiction-specific contractor credential requirements.

How it works

Heating in an HVAC system follows a demand-response sequence initiated by the thermostat and executed through fuel combustion, electric resistance, or refrigerant-cycle heat transfer. The diagnostic process maps each step in that sequence to identify where the chain breaks.

Diagnostic sequence — numbered by priority:

1. Thermostat verification — Confirm the thermostat is set to HEAT mode with a setpoint at least 3°F above the measured room temperature. Check battery status on battery-powered units and verify the schedule is not overriding manual settings. The HVAC Thermostat Repair and Calibration page covers calibration drift and wiring fault identification.

2. Power and breaker check — Inspect the system's dedicated circuit breaker in the main panel. Furnaces typically draw 15–20 amps; air handlers in heat pump configurations may draw 20–60 amps depending on strip heat capacity. A tripped breaker that resets and immediately trips again indicates a downstream fault requiring licensed evaluation.

3. Filter inspection — A clogged air filter restricts airflow to the point where high-limit safety switches shut the system down after overheating. Filters rated MERV 13 or higher impose higher static pressure than MERV 8 filters and can trigger limit trips in systems not designed for that resistance level. Replace if visibly occluded.

4. Ignition system check (gas furnaces) — The inducer motor, draft pressure switch, hot surface igniter (HSI), and flame sensor form a sequential chain. If the inducer runs but the burner does not light, the draft switch or igniter is the probable fault. Cracked hot surface igniters, which are silicon nitride or silicon carbide components, account for a high proportion of no-heat calls on furnaces over 7 years old.

5. Heat pump reversing valve verification — On heat pump systems, a stuck or failed reversing valve leaves the refrigerant circuit in cooling mode regardless of thermostat demand. This is identifiable when the system runs but produces air near ambient outdoor temperature rather than heated air. See the Heat Pump Systems Repair page for reversing valve fault isolation.

6. Limit switch and safety control reset — Manual-reset high-limit switches on furnaces will lockout the system after an overtemperature event. These are located on the heat exchanger plenum and require physical reset after the root cause (typically a blocked filter or failed blower motor) is corrected. A limit that trips repeatedly indicates an ongoing airflow or combustion fault.

7. Blower motor confirmation — The HVAC Blower Motor Repair page details fault modes; in a heating failure context, a seized or failed blower allows the heat exchanger to overheat and trip the limit switch within 30–90 seconds of ignition.

Common scenarios

Gas furnace — no ignition: Inducer runs, pressure switch clicks, but no flame is established. Probable causes are a failed HSI (measurable by continuity test, nominal resistance 40–75 ohms for silicon nitride igniters), a contaminated flame sensor (carbon deposits reduce microamp signal below the control board's threshold, typically 0.5–1.0 µA), or gas valve failure. Gas valve and heat exchanger work requires licensed gas fitter involvement under NFPA 54 (2024 edition).

Heat pump — insufficient heat at low outdoor temperatures: Heat pumps lose heating capacity as outdoor temperatures drop. At approximately 35–40°F, a properly functioning system relies on supplemental electric strip heat to maintain setpoint. If the strip heat is not energizing, the fault lies in the sequencer, control board, or a failed contactor. The HVAC Contactor Repair and HVAC Control Board Repair pages address those component-level faults.

Electric furnace — partial heat: If only 1 of 2 electric heating elements is functioning, the system produces heat but cannot reach setpoint. This is a sequencer or element failure and requires voltage testing at each element terminal.

Packaged unit — no heat: Packaged systems locate all components in a single outdoor cabinet. Duct connections, gas supplies, and electrical feeds must all be verified at the unit. The Packaged HVAC Systems Repair page covers access and isolation procedures.

Decision boundaries

The distinction between user-addressable checks and technician-required repairs follows clear boundaries defined by fuel type, component access, and regulatory requirements.

Permitting requirements apply when heating equipment is replaced rather than repaired. Most jurisdictions require a mechanical permit for furnace or air handler replacement, and inspections are governed by local amendments to the International Mechanical Code (IMC) published by the International Code Council. Repair-level work on existing equipment generally does not trigger permit requirements, though local ordinances vary. The HVAC Repair vs. Replacement Decision page covers the cost and code thresholds that move a repair into replacement territory.

For safety classification, the U.S. Consumer Product Safety Commission (CPSC) categorizes carbon monoxide hazards from failed heat exchangers as a Category 1 priority risk. Any system exhibiting CO detection above 9 parts per million at a register (the EPA secondary standard) warrants immediate shutdown and licensed inspection before operation resumes.

References

• NFPA 54: National Fuel Gas Code, 2024 edition — National Fire Protection Association
• NFPA 70: National Electrical Code, 2023 edition — National Fire Protection Association
• EPA Section 608 – Stationary Refrigeration Equipment — U.S. Environmental Protection Agency
• International Mechanical Code (IMC) — International Code Council
• U.S. Consumer Product Safety Commission – Carbon Monoxide — CPSC
• EPA National Ambient Air Quality Standards – Carbon Monoxide — U.S. Environmental Protection Agency