- For the homeowner
- Gas furnace
- Some error codes for gas furnaces
- Service sheet for the gas furnace
- gas furnace design
- The gas heat exchanger
- Dangerous conditions in gas furnaces
- Annual service of the gas furnace
- Repair procedures for gas furnaces
- Gas fireplace millivolt systems
- Oil furnace
- Setting gas input
- Quick tips for troubleshooting furnaces
- Operation and troubleshoot furnace by manufacturer
- HVAC war stories blog
- Annual service of an oil furnace
- Oil furnace design
- Oil furnace troubleshoot
- Repair procedures for oil furnaces
- Gas code training
- Piping and connections
- FAG w pilot no fire
- Combustion analysis
- Electric furnace
- Air conditioner
- Heat Pump
- Ductwork design and troubleshoot
- Diagnostic problems
- Electric test meters
- Electrical diagram training
- Electrical symbols
- Single and 3 phase power systems
- Electric wiring solutions
- Transformer design and troubleshoot
- Electronic air cleaner
- Blowers and fans design & troubleshoot
- Humidity and humidifiers
- Furnace, Air Conditioner and part manuals
- Electric motors
- Run Capacitors
- Start capacitors
- Troubleshooting the capacitor
- Gas furnace short cycling
Definitions of terms used in the HVAC industry
These definitions should help you to understand the terminology used to describe parts used and operations done by those in the industry.
80% efficient furnace: This term applies to gas furnaces mostly built in 1990 or newer that are rated with an AFUE rating of a minimum of 78% efficient or higher. These furnaces use inducers to extract the exhaust gasses and are vented with metal pipe.
Adjust burners: Older furnaces had air adjustments on their burners. They should be adjusted to just get the yellow off the flame.
Alternating current: An electric current that changes its polarity from positive to negative usually at a rate of 60 cycles per second. Common source would be power supplied to homes and businesses by a utility.
Ambient Temperature: This means the temperature of the surrounding area, such as the inside temperature of the structure.
Analog gauge: Essentially this means a gauge that uses a needle and a dial to indicate pressure, voltage amperage ohms etc.
Analog meter: An electric test meter that uses a needle to show the reading.
Back EMF: This is a voltage that is produced by the the motor windings as a result of the rotation of the rotor. This voltage is essentially "stacked on top of" the supply voltage.
Bimetal: 2 metals (usually copper and steel) bonded together whose expansion rates with temperature changes are different. This causes the metal to bend or warp with temperature changes. Normally used in all types of thermostats.
BTUH: British Thermal Units is the term for determining heat volume.
It is defined as: the amount of heat required to raise the temperature of one pound of water 1 degree Fahrenheit.
Carbon Monoxide: A poisonous gas produced by incomplete combustion of fuel gas. Concentrations of as low as 35 parts per million or sometimes less can be harmful. See also Safety
Circulating fan: Fan assembly that moves air through the furnace throughout the structure.
Chassis ground: An attachment of a circuit to the metal casing of an appliance.
Closed: The position of an electrical switch that allows electricity to pass through.
Combination gas valve: A gas control valve that incorporates a pressure regulator, an electric solenoid, a pilot safety valve and a manual shut off knob.
Combustion air: Air used to supply the burners with enough oxygen to allow complete combustion.
Continuity: An electrical term indicating that the wire or component tested is broken or more accurately has an open circuit.
Dedicated breaker: A circuit breaker in the service panel that has only one appliance wired to it.
Diagnostic code: These are lights on the IFC that indicate by numbers of flashes the conditions that are present in the furnace.
Direct current: An electric current that keeps the same polarity all the time. Common source would be a battery.
Downflow: A furnace design that moves air from the top of the furnace to the bottom. Used in buildings with no basement.
Draft hood: A device used on older gas furnaces to bring ambient air into the vent gasses to cool the vent gasses and provide a way for downdrafts to exit without affecting the burner flame.
Pilot dropout: The amount of time it takes for the pilot safety to shut off all gas supply to the burners when the pilot light is extinguished.
Ductwork: These are usually galvanized metal piping that distributes heated air throughout the structure.
Flame rod: A small diameter stainless steel rod on a gas furnace used to sense flame rectification by being placed in the flame.
Flame safety control: A control, sometimes incorporated into an IFC, designed to insure that gas is not moved through the burners or flame is established under dangerous conditions. In the oil furnace it is usually called the cad cell relay. In this application, it will shut down the oil supply if light is not sensed within a preset time after the oil is sprayed
Fan and limit switch: A combination switch that provides for energizing the circulating fan and limit function to shut off the burners if the temperature in the heat exchanger gets too high. This video explains this switch: https://youtu.be/QpHfFdk04u0
Forced air: An appliance that uses a fan to force the air throughout the structure.
Gas chain: A series of components organized in series with each other used to control the flow of gas to the burner(s). Usually found on appliances built before 1970. Replaced by the combination gas valve
Gas cock: A manually operated valve in the gas supply line used to cut off gas supply to a gas applianc
Gas regulator: A device that lowers line gas pressure to the proper pressure for the appliance.
Gas valve: An electrically operated valve that includes a manual shut off, gas pressure regulation and sometimes a flame safety control.
Ghost voltage: This is a voltage sometimes read that has no amperage with it and is usually a feed back from the controller and indicates either a failed control, or the controller is not allowing power thru.
Hard lockout: The term used to describe the condition that will not allow restart. Usually unit will attempt restart in one hour if the control is and integrated furnace control.
Heat exchanger: A sheet metal barrier between the burned gasses and the air passed through the furnace.
HSI: Hot surface ignitor. A device that when electricity is passed through, warms to a high temperature to ignite gas burners.
Hot stick: See above HSI
In wc: Inches of water column. A pressure equal the rise of water in a u tube manometer.
Inducer: A fan assembly used to draw burnt gasses from a gas furnace and create negative pressure in the heat exchanger
Induced draft: A gas furnace that uses a small fan to pull the burned gasses through the furnace to the vent. Usually are newer high efficiency types.
Inshot type burner: A burner that usually has a horizontal flame that is used in gas appliances that use an inducer to pull the flame into the burner tubes.
IFC: Integrated furnace control Electronic control used to control most operations for newer gas furnaces including flame safety control, circulating fan control, inducer control and safety sensors.
Ignition control: An electronic control used on gas furnaces to light the burners and prevent the burners from operating if there is an unsafe condition.
Interrmittent pilot: A pilot burner that only operates when there is a call for heat.
Gas line pressure: The supply pressure that the utility sets for the pressure from the meter to the appliance.
Latent heat: The heat released or absorbed by a change of state, usually liquid to gas or gas to liquid.
Line voltage: Electrical term for the voltage used to operate the larger loads of the furnace (such as circulating fan motor or inducer). For most gas furnaces 120v.
Load: An electrical term for a device that consumes electricity and does work. (example: a light, a motor)
Lockout mode: A term to indicate that the furnace has seen an unsafe condition and will not continue the sequence. A blinking light sequence will indicate where the failure is. Rebooting the unit will clear the code, but if the condition is still there, it will reoccur.
Limit switch: A heat actuated switch that usually opens on a rise in temperature. Usually used to turn off burners or other energy source in unsafe conditions.
Make: An electrical term meaning to allow electrical power to pass through.
Manifold: A pipe with several fittings used to distribute gas to the different burners
Manometer: A pressure measuring tool that can measure low pressures in
inches of water column.
Media: A type of filter that uses a type of paper or fiberglass to catch particles moving thru the ductwork.
Natural draft: A gas furnace that uses the heat of the burning gasses to move the products of combustion
Nozzle: A part of an oil burner that takes high pressure oil and sprays it into a fine mist to prepare it to burn.
Normally open: The position of contacts of a switch when power is not applied. Open means no power can pass through.
OEM: A part designed for a specific appliance model as opposed to general replacement part
Ohmmeter: An electric meter used for measuring the resistance of a circuit
Open: The position of an electric switch that will not allow electricity to pass through.
Overfired: The condition of a furnace having too much gas inputted into the burners, resulting in damage to the heat exchanger. See setting the firing rate
Pilot safety: A control system that uses a pilot light to light the main burner and warm a thermocouple that creates a small voltage to hold open a safety shut down valve open. When the pilot goes out, the thermocouple cools and the safety valve shuts down the gas supply.
Primary: Electrical term for the winding of a transformer that is connected to the power source. (Household outlet etc).
Primary air: Air drawn into the gas burner to mix with air. As the air/fuel mixture comes out of the burner, secondary air is added from the surrounding air and is ignited.
Primary control: A term used for oil furnace flame safety controls such as the cad cell relay or the stack switch. They are used to shut off the burner if there is no flame detected.
PSC: An abréviation for permanent split capacitor. Theses motors are low to moderate starting torque motors used in fan applications with higher efficiency than shaded pole motors. See also motors
Prepurge: Operation of inducer prior to flame initiation to clear any gasses from the heat exchanger.
Regulator: A device that reduces pressure to a lower pressure.
Return air: Ductwork designed to bring air that is circulated through the structure back to the furnace to be heated.
Ribbon type burner: A burner that is like a long pipe with holes in the top where the flame comes out.
RLA: Running load amps. This is the maximum amperage that the air circulating fan motor can draw. The RLA is listed on the model plate of the motor. A much lower reading may indicate blockages in the ductwork. A high reading may indicate failing bearings or a motor that is too small for the application.
Rollout switch: A high temperature limit switch that must be manually reset placed near the burners to detect flames that have "rolled out" from the front of the furnace.
Secondary: Electrical term for the winding on a transformer that is used as a power source. In furnaces, it is usually a low voltage source for control power.
Secondary heat exchanger: This is a part on high efficiency furnaces. It is a finned coil mounted directly above the blower assembly designed to remove more heat from the exhaust gasses.
Sensible heat: Heat that can be measured by a thermometer.
Series: An electric term to indicate a switch that if opened will shut off the power to some control or load resulting in interruption of the circuit.
Short: An electrical term indicating that the circuit has bypassed the load and passed through to the other side of the circuit with little or no resistance causing excessive current and consequent opening of circuit protection (fuse or breaker)
Single pole double throw switch: A switch that will allow power to go through one way when in the rest position, and another in the energized position, but not both at the same time.
Split phase motor: A single phase motor with 2 winding in the field. One winding is the run winding that is powered all the time. The other winding is the start winding. It is used to boost the rotor to start the motor. It may or may not be in the circuit all the time.
Stack temperature: Temperature of exhaust gasses as they leave the heat exchanger
Static pressure: The air pressure that is created by the fan blowing air into the ductwork system. The level of static pressure is dependent ability of the fan to increase the pressure and amount of air the outlets are able to remove from the system.
Standing pilot: A gas ignition system that uses a standing pilot to ignite the main gas burners.
Rollout switch: A high temperature switch mounted near the burner assembly to detect flame rollout and shut off the burners
Terminal board: Connection screws used for connecting thermostat wires to the furnaces
Transformer: A small coil that transforms line voltage to control voltage. Usually 24 v a/c
Thermocouple: A junction of 2 differing metal wires that when heated will create a small voltage in the wires to hold open a gas valve.
Unconditioned space: This could be an attic space, crawl space, or outside
Upflow: A furnace design that moves air from the bottom of the furnace to the top of the furnace. Used in buildings with a basement.
Venting: Piping used by furnaces to remove burned gasses from the structure to the outside.
Venturi: A reduction in the size of a passage that gasses are passing through causing the gasses to speed up.
Water column: The measuring scale for determining fuel gas pressure. 28 inches water column equals 1PSI.
Well: As used in hot water boilers it is a fitting mounted on the side of the boiler that penetrates into the water jacket into which a temperature sensor is installed.
2 stage furnace: This furnace uses 2 regulators to control the amount of gas burned. The first stage usually runs at 1/2 the pressure of the second stage.