Gas Furnace Definitions and Terms

This page contains a list of terms, words and meanings associated with a gas furnace and/or heating system. We have chosen to eliminate many of the words associated with other forms of heating or cooling a home UNLESS they relate directly to the gas furnace system.

If you don’t see a word listed and want to know what it means, scroll to the bottom of the page and ask! We will try answer as quickly as possible and probably include the term in this page for future reference!

Terms and Definitions Related to Gas Furnace

Air Flow –
The direction in which the distribution or movement of air occurs.

Annual Fuel Utilization Efficiency (AFUE)
An industry rating used to measure the overall efficiency of heating equipment fueled by natural gas. It’s easily related to as the amount of heat you will get from your heating equipment for each dollar you spend on fuel. Higher AFUE ratings, means more efficient heating equipment. learn more at our Gas furnace ratings page.

Blower –
An air handling assembly for moving air throughout a ducted system. Usually consists of a outer case, a blower motor and a wheel-like fan system.

Blower Motor –
A small electric motor that attaches to, and turns the blower wheel.

BTU (British Thermal Unit)
A standardized unit of measurement for a specific amount of heat that’s required to increase the temperature of one pound of ice or water, by a single degree (Fahrenheit). BTUH – The number of BTUs in an hour.

Carbon Monoxide
CO, commonly known as Carbon Monoxide, is a deadly and colorless, odorless gas, produced by the burning of fuels. High CO amounts in the home can be caused by a leaking exhaust or flue system, or a cracked heat exchanger.

CFM (Cubic Feet per Minute)
Cubic feet per Minute, or CFM, is a measurement of the rate of air flow a blower system delivers.

Condensing Furnace
A condensing furnace as it relates to home use, is a term used to describe a gas furnace that extracts all the heat from the exhaust gases of a furnace. When approximately 90% or more of the heat from exhaust is recaptured, water vapors within the exhaust actually condense into liquid, thus the name.

Contractor License
A contractors license or state certification, is required in almost all 50 states to assure the safety and well being of unknowing homeowners. When you hire a heating contractor, you should make sure they are licensed and insured to work in your area, for the job you are hiring them to do.

Crawl Space
The crawl space is more in line with your home, but worth mentioning. In homes without basements, it’s very common to build a small storage section under the home, to raise the home 1-4 feet off the ground. These spaces, very much like a short basement, are called the crawl space. (Crawl, because that’s generally the only way to get under the home)

Direct Vent
Draws in fresh outside air for combustion and then vents the combustion gases directly outside as well.

Downflow Furnace
A downflow gas furnace gets return air into the top of the furnace and pushes warm air out of the bottom.

Duct or Ductwork
A system of closed sheet metal, fiberglass board, large insulated flexible host or other  material used for distributing air through a home or building.

Thermocouple & Flame Sensor –
The Thermocouple or flame sensor is a small metallic pin-like device that sits directly in the path of the flame of the pilot light on standing pilot systems. When the flame is lit, the thermocouple or flame sensor sends an all-good signal to the control board or gas valve, letting the furnace know there is an ignition flame present, and that its OK to open the gas valve and ignite.

Furnace Filter
A filter is a pleated or other type of device used to remove dust and other particles from air to protect the HVAC equipment. Filters vary how well they work and it is measured as MERV rating.

Flue Pipe
In a gas furnace heating system, the flue pipe is the exhaust system for your gas furnace. Flue systems can be made of PVC, sheet metal piping or even chimney flue systems.

Gas Furnace
Part of a heating system that converts natural gas or propane into heat.

Heat Exchanger
Built inside your gas furnace, the heat exchanger is a large aluminum or stainless steel chamber that facilitates the transfer of heat energy from the source to the air that is forced through the duct system.

Horizontal Furnace
A furnace that lies on its side, pulling in fresh return air from one side and expelling warm air from the other. A Horizontal Left or Horizontal right furnace are common terms related to the poise of a gas furnace.

Hot Surface Ignitor
A small ignition device that allows the gas furnace to start up. These ignitors have all but replaced the older standing pilot light systems and are most commonly made from ceramic or silicon nitride.

“Heating Seasonal Performance Factor”, is the term used to measure the efficiency value of heat pump systems.

Heating, Ventilation, and Air Conditioning.

Load Calculation
A mathematical set of calculations to determine the heat gain and heat loss in a building for proper heating system size design.

Natural-Draft Furnace
A furnace in which the natural flow of air from around the furnace provides the air to support combustion.

Gas Package System (Gas Pack) –
A single packages system of air conditioning and a gas furnace where all components are located in one cabinet, usually situated on the side or roof of a home.

A temperature control device, typically found on a wall inside the home. The thermostat monitors the inside temps (and sometimes the outside temps also) and tells the gas furnace when to start and heat the home or space.

Ton –
A ton is a measurement heat absorption of air conditioning or cooling. It is based on the amount of cooling you would get if you placed a big old chunk of ice in the corner and melted it in a 24 hour period. For example, having 1-ton of cooling means your system has the same capacity as melt one ton of ice in a 24 hour period.

Upflow Furnace
A furnace that pulls air in from the bottom and expels warm air from the top.