Furnace Repair & Replacement

This master glossary defines the most common (and most misunderstood) heating and air conditioning terms in clear, homeowner-friendly language. It covers furnaces, heaters, heat pumps, and air conditioning systems — including efficiency ratings, diagnostics terms, refrigerant concepts, airflow measurements, and safety devices. Use it to understand your system, your estimate, and the difference between a real repair and a sales pitch.

Pro tip: If someone recommends a major repair or replacement, ask them to show you the measurements behind it (temperature rise, static pressure, superheat/subcooling, fault codes, combustion readings, etc.). Clear numbers beat vague fear tactics.

• A —
  • AC (Air Conditioning): A system that removes heat and humidity from indoor air to keep your home cool.
  • ACH (Air Changes per Hour): How many times the air inside a space is replaced in one hour (used in ventilation/IAQ).
  • AHU (Air Handling Unit / Air Handler): The indoor unit that moves air through the home; contains the blower and often the evaporator coil.
  • AFUE (Annual Fuel Utilization Efficiency): Furnace efficiency rating. Example: 90% AFUE means 90% of fuel becomes usable heat.
  • Ambient Temperature: The surrounding air temperature (indoors or outdoors).
  • Airflow: The amount of air moving through the HVAC system. Too little airflow causes overheating, freezing coils, poor comfort.
  • Airflow Restriction: Anything that blocks airflow (dirty filter, clogged coil, blocked ducts). Common cause of shutdowns and high bills.
  • Air Filter: Traps dust and debris before air reaches the blower and coil. A dirty filter is one of the most common causes of HVAC problems.
  • Air Balance: Adjusting airflow to different rooms so temperatures are more even.
  • Auxiliary Heat (Aux Heat): Backup heat used with heat pumps during cold weather (often electric heat strips or a gas furnace in dual-fuel systems).
  • ASHRAE: A major engineering organization that sets many HVAC ventilation and comfort standards.
• B —
  • BTU (British Thermal Unit): A measurement of heat energy. HVAC capacity is often rated in BTUs.
  • Blower: The fan that moves air through ducts and vents.
  • Blower Motor: Motor that powers the blower. Can be single-speed, multi-speed, or variable-speed (more efficient and quieter).
  • Burners: The components in a gas furnace where fuel burns to create heat.
  • Backdrafting: When combustion gases flow the wrong direction (back into the home). This is a serious safety concern.
  • Breaker: An electrical safety device in your panel that trips when current is too high.
• C —
  • Call for Heat: Signal from the thermostat telling the heating system to start because the home is below set temperature.
  • Call for Cooling: Signal from the thermostat telling the AC/heat pump to start because the home is above set temperature.
  • Capacitor: An electrical part that helps motors start and run. A failed capacitor is a very common AC/furnace issue.
  • CO (Carbon Monoxide): A dangerous gas produced by fuel-burning appliances. Proper venting and a safe heat exchanger prevent CO entry.
  • Combustion: The burning of gas/propane/oil to create heat.
  • Combustion Analysis: Testing furnace exhaust for safety and efficiency (oxygen, CO, draft, temperature, etc.).
  • Contact Relay / Contactor: An electrical switch in the outdoor unit that turns the compressor and condenser fan on/off.
  • Control Board (Circuit Board): The “brain” of the furnace or air handler that controls timing and safety checks.
  • Condensate: Water created when humid air meets a cold surface (evaporator coil) or during high-efficiency furnace operation.
  • Condensate Drain: Pipe/tube that removes condensate. A clog can cause leaks or safety shutdowns.
  • Condenser (Outdoor Unit): The outdoor part of an AC/heat pump that rejects heat outside.
  • Condenser Coil: Outdoor coil where refrigerant releases heat to the outside air.
  • Compressor: The pump-like component that moves refrigerant through the system. Often called the “heart” of the AC/heat pump.
  • CFM (Cubic Feet per Minute): A measurement of airflow amount.
  • Cycle: One on/off run period of the heating or cooling system.
• D —
  • Draft (Heating Sequence): The induced airflow that pulls combustion gases safely through the furnace and out the vent.
  • Draft (Cold Air Feeling): Cool air sensation from vents, commonly due to duct leakage, poor insulation, or blower running without heat.
  • Draft Inducer Motor: Motor/fan that starts the furnace heating sequence by creating draft and proving venting.
  • Ductwork: The supply and return air pathways that distribute air throughout the home.
  • Damper: A door/valve in ductwork that helps control airflow (common in zoning systems).
  • Defrost Cycle: A heat pump function that melts ice on the outdoor coil in cold weather.
  • Delta T (ΔT): Temperature difference between return air and supply air. Helps verify performance in heating and cooling.
  • Diagnostic Code / Fault Code: A code displayed by a control board that helps pinpoint a failure (pressure switch, flame sense, limit, etc.).
  • Dryer Vent / Exhaust Interaction: Household exhaust fans can affect combustion appliances if the home becomes depressurized.
• E —
  • EER (Energy Efficiency Ratio): AC efficiency rating at a specific test condition; higher is better.
  • Energy Star: A program that labels energy-efficient products meeting specific standards.
  • Electric Furnace: A heating system that uses electric elements rather than burning fuel.
  • Electric Heat Strips: Electric resistance heaters used as backup heat in many heat pump systems.
  • Evaporator Coil: The indoor coil that absorbs heat from your home during cooling and removes humidity.
  • Expansion Valve (TXV/EEV): A device that meters refrigerant into the evaporator coil, controlling pressures/temperatures for proper performance.
  • ECM Motor: An electronically-commutated motor, often used for variable-speed blowers and higher efficiency.
• F —
  • Flame Sensor: Safety device that confirms the burners are lit. If it doesn’t sense flame, the furnace shuts off gas.
  • Forced Air: A system that uses a blower to push conditioned air through ductwork.
  • Furnace: A heating system that produces heat (gas, electric, or oil) and distributes it via airflow.
  • Fuel Type: The energy source used for heat (natural gas, propane, electric, oil).
  • Fan Relay: An electrical control that turns the blower fan on/off at the proper time.
  • Freeze-Up (Coil Icing): Ice formation on the evaporator coil caused by low airflow, low refrigerant, or system issues.
• G —
  • Gas Furnace: A furnace that burns natural gas or propane to create heat.
  • Gas Valve: Regulates gas flow to the burners and opens only when ignition conditions are proven safe.
  • Gas Pressure: Fuel pressure supplied to the furnace. Incorrect pressure can cause poor performance or unsafe operation.
  • Ground Fault: Electrical issue where current flows where it shouldn’t, often tripping a breaker.
  • Gauge Set (Manifold Gauges): Tools used to measure refrigerant pressures for AC/heat pump diagnosis.
• H —
  • Heat Exchanger: Sealed metal chamber that transfers heat from combustion to indoor air. Damage can be a safety issue.
  • Heat Pump: A system that heats and cools by moving heat using refrigerant (instead of burning fuel).
  • HSPF2 (Heating Seasonal Performance Factor 2): Modern efficiency rating for heat pumps in heating mode; higher is better.
  • High Limit Switch: Safety switch that shuts off heating if the furnace overheats (often from airflow restriction).
  • Humidity: Moisture in the air. AC removes humidity; heating season often dries air.
  • HVAC: Heating, Ventilation and Air Conditioning. An acronym used to describe the entire heating and cooling system of your house. 
  • Hydronic Heat: Heating that uses hot water (boilers/radiant systems) rather than forced air.
• I —
  • Ignitor (HSI): Hot surface ignitor that glows to light furnace burners.
  • Intermittent Spark Ignition: A spark system that lights burners (common on some furnaces/boilers).
  • Inducer Motor: Creates draft to vent combustion gases safely before ignition.
  • Inrush Current: The brief surge of electrical current when a motor starts (important in diagnosing compressors/motors).
  • IAQ (Indoor Air Quality): Overall cleanliness and comfort of indoor air (dust, humidity, odors, ventilation, filtration).
• J – K —
  • Manual J: A calculation used to properly size heating/cooling equipment for a home (based on insulation, windows, square footage, etc.).
  • Manual D: A duct design standard to size ducts correctly for airflow and comfort.
  • Manual S: Equipment selection method that matches HVAC equipment to the Manual J load.
  • Kilowatt (kW): A unit of electrical power. Electric heat strips and electric furnaces are often rated in kW.
  • kWh (Kilowatt-hour): A unit on your electric bill showing energy used over time.
• L —
  • Low Voltage Circuit: The 24-volt control wiring system that powers thermostat signals and controls relays/safeties.
  • Limit Switch: A safety switch that shuts down heating when unsafe temperatures are detected.
  • Latent Heat: Heat involved in moisture removal (dehumidification). AC doesn’t just cool air — it also removes humidity.
  • Liquid Line: Refrigerant line that carries high-pressure liquid refrigerant (typically the smaller copper line).
• M —
  • MERV Rating: A filter rating showing how well it captures particles. Higher MERV filters can restrict airflow if the system isn’t designed for them.
  • Manifold Gauges: Tools for measuring AC/heat pump refrigerant pressures.
  • Metering Device: The part that controls refrigerant flow into the evaporator coil (TXV/EEV/piston/orifice).
  • Motor (PSC/ECM): Powers fans/blowers/inducers. ECM motors are generally more efficient and adjustable.
• N —
  • NATE: A nationally recognized HVAC technician certification program.
  • Nitrogen: An inert gas used to pressure-test and purge refrigerant lines to prevent contamination and leaks during installation/repair.
  • Normal Temperature Rise: The expected increase in air temperature through a furnace (return to supply). Used to verify safe performance.
• O – P —
  • Ohms (Resistance): A measurement used in electrical testing (for motors, sensors, ignitors, etc.).
  • Overcurrent: Too much electrical current, often causing breaker trips or component damage.
  • Plenum: The main air box attached to the furnace/air handler where air is distributed into ducts.
  • Pressure Switch: A furnace safety device that proves draft/venting before ignition can occur.
  • PSI (Pounds per Square Inch): Pressure measurement used for refrigerant and gas systems.
  • Package Unit: A system where heating and cooling components are combined in one outdoor cabinet (common on rooftops or some homes).
• Q – R —
  • R-Value: A measure of insulation effectiveness. Higher R-value means better insulation.
  • Refrigerant: The fluid in AC/heat pumps that carries heat. Common types include R-410A and newer refrigerants depending on equipment.
  • Recovery (Refrigerant Recovery): Safely removing refrigerant into a recovery tank during repairs (required by law).
  • Register: The vent where conditioned air enters the room (supply register). Return grilles pull air back.
  • Return Air: Air pulled back into the system to be reheated or recooled.
  • Reversing Valve: Heat pump component that switches the system between heating and cooling modes.
• S —
  • SEER2 (Seasonal Energy Efficiency Ratio 2): Modern AC efficiency rating for cooling season. Higher SEER2 generally means lower cooling cost.
  • Superheat: A refrigerant measurement used to ensure the evaporator coil is fed correctly. Too high/low can indicate airflow issues, low charge, or metering problems.
  • Subcooling: A refrigerant measurement used to verify proper charge and condenser performance. Helps confirm the system has the right refrigerant amount.
  • Static Pressure: The “pushback” resistance air faces in ductwork. High static pressure often means restrictions and can damage equipment.
  • Sensible Heat: Temperature change you can feel on a thermometer (different from moisture removal).
  • Short Cycling: System turns on/off rapidly. Causes include airflow restriction, thermostat problems, oversizing, or refrigerant issues.
  • Suction Line: The larger refrigerant line (typically insulated) that carries cool, low-pressure vapor back to the compressor.
  • Startup Amps: The surge of amperage when a motor/compressor starts; used for electrical diagnosis.
  • Safety Switch Trips (Limit / Rollout): A shutdown caused by overheating or unsafe flame conditions. The fix is finding the cause — not repeatedly resetting.
• T —
  • Thermostat: The wall control that tells your system when to heat or cool based on temperature settings.
  • Transformer: Converts household voltage down to 24 volts for the low-voltage circuit.
  • Temperature Rise: The amount supply air warms up compared to return air when the furnace runs. Used to confirm safe operation.
  • TXV (Thermostatic Expansion Valve): A common metering device that regulates refrigerant flow into the evaporator coil.
  • Ton (Cooling Capacity): AC capacity measurement. 1 ton = 12,000 BTU of cooling per hour.
• U – V —
  • UV Light (UV-C): A light used in some systems to help reduce microbial growth on coils or in air handlers.
  • VAC (Volts AC): Electrical voltage measurement used in diagnosis.
  • Vent Pipe / Venting: The pathway that carries furnace exhaust gases safely outside.
  • Variable-Speed Motor: A motor that adjusts speed to match demand, often improving comfort and efficiency.
  • Vacuum Pump / Evacuation: Tool/process used to remove air and moisture from refrigerant lines before charging a system.
• W – Z —
  • Warranty: Coverage that protects you against certain parts failures and/or labor costs for a time period.
  • W.C. (Water Column): A pressure measurement commonly used for gas pressure (inches of water column).
  • Zoning: A system that allows different areas of the home to be controlled independently, often using dampers and multiple thermostats.
  • Zone Damper: A duct damper that opens/closes to direct air to a specific zone.