Guide to Warm Air Furnace Heating Systems
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A Guide to Warm Air Heating Systems - how forced hot air and gravity hot air furnaces work
How to determine what kind of heat you have: photos
Furnace filter guide
Duct system defects, inspection, repairs
Guide to furnace controls, stack relays, fan limit switches, installation, setting, testing, replacement
Furnace heat exchanger leak guide
How to manually turn on a furnace or air conditioning blower fan
Guide to troubleshooting heating system furnace controls, limit controls, and fan controls
Questions & answers about how to buy, install, maintain, and fix warm air heating furnaces, ductwork, and controls

Warm air furnace installation, troubleshooting, repair guide: this article series answers just about any question about forced air or warm air furnace central heating system troubleshooting, inspection, diagnosis, and repairs in residential buildings and homes. We explain how furnaces work, what controls and settings are used, what goes wrong, and how to fix it. This page is the starting point for our series of heating furnace diagnosis and repair articles. The key heating furnace components are introduced here at FURNACES, HEATING.

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Warm Air Furnace Inspection, Diagnosis & Repair - where to start?

If you have no heat, see HEATING LOSS DIAGNOSIS-FURNACES. You can also continue your heating furnace diagnostic procedures with the articles at page left headed under FURNACE CONTROLS & SWITCHES. If your furnace is fired by an oil burner, you can go directly to those diagnostics at OIL BURNER INSPECTION & REPAIR. Gas burners are discussed at GAS BURNER Flame & Noise Defects.The basics of how furnaces work can be read at FURNACE OPERATION DETAILS.The Sketch at page top courtesy of Carson Dunlop

At this website we describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards.

First make sure you understand what type of heating system is installed

If you don't know what kind of heat your building uses, we explain how to figure out the answer in more detail at HEATING SYSTEM TYPES.

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These photos will help you determine what kind of home heating system you have. Here we're showing heating equipment just as you'll see it in your home, with all of the access covers and panels in place. Articles at this website offer lots more detail including photos of individual heating system controls and components along with explanatory text.

Warm Air Heating Systems - Furnaces: If the heat in your building is provided by warm air that flows out of floor registers (above left), a wall air supply register (above right) or a ceiling air supply register (photo at left) and on into the occupied space, then the air which warms the living space is probably being delivered through large or small diameter ducts, registers, air filters, and a furnace blower.

The heated air is being heated by a gas, oil, or electric furnace, or perhaps by a heat pump or a geo-thermal system then your heat is provided by a warm air furnace (sketch at page top, for example)

Cooler air (hopefully also from the same occupied space) flows back to the furnace through one or more air returns and ducts into the furnace return air plenum from which it enters the furnace itself to be re-heated. Some older warm air systems (illustrated below) are less sophisticated and may have no ductwork at all, and worse, may heat cold air from the basement and send it one-way into the occupied spaces of the home.

What's the Difference Between Forced Hot Air Heat and Gravity Hot Air Heat?

Your heating furnace may located in a basement, in a crawl space, in an attic, or even in an outdoor utility closet or an attached garage. In all cases, some heating equipment (oil, gas, coal, wood, geothermal, electric, solar) is used to transfer heat to air that is then delivered to the occupied space of the building.

Illustration of Simple Gravity Furnaces using a Floor Register (Ductless Warm Air)

Older hot air heating systems were comprised of a furnace that heated air, sometimes just air from the basement. The warm air rose into the upper areas of the building by convection (warm air, which is less heavy than cold air) rises, displacing colder, more dense air in the building). You can see one of these old under-floor convection furnaces in our photos at left.

Popularly called a "gravity furnace" (cold air falls by gravity, and warm air defies gravity by rising), you will see only two "pipes" or ducts on the unit.

A flue gas exhaust flue (the smaller diameter steel "pipe" that exits near the bottom left of the gravity furnace and connects to a brick chimney in our photo - and that larger diameter round duct at the top center of the gravity furnace.

That large round warm air supply plenum or duct delivered warm air into the building through a large floor grate in the first floor above. Our arrows show the direction of air flow through this gravity furnace.

In this photo we can't see the cool air intake but almost certainly it's at the bottom of the unit and is in this horribly inefficient unit, is taking cold wet basement air and heating it up before sending it upstairs. We do see a little of this furnace's repair history - that abandoned motor on the floor in the bottom center tells us that an oil burner was installed and had to have a motor replacement.

Warm air rose from this gravity furnace upwards from the first floor grate into the rest of the building also by convection (or "gravity") flowing up a stairwell, or upstairs through registers cut in the first floor ceilings.

How to Spy on your Parents Through the Gravity Heat Registers

We know that the air register at left is a warm air supply register because it has those moveable louvers that would be absent on a cool air return register cover.

But now let's be honest - we don't know for sure if the air register at left is connected to ductwork or if it's just letting warm air rise by "gravity" (we say "convection") from a floor below.

It's easy to figure out however. Just open the louvers and look through the grating. If you find yourself looking into a duct, typically full of trash and debris in an older home, it's connected to ductwork.

If you find that your can see right into a room below, this is a simple gravity or convection register. When the author (DF) was a boy, we used to spy on our parents and their friends by peering down at them through a register like this after we had been sent upstairs to bed. They were not up to much besides drinking, talking about stuff we couldn't understand, and playing canasta.

Illustration of Octopus Furnaces that Added Warm Air Ducts

Octopus furnaces (illustrated below) added warm air ducts that conducted air directly to different areas of the building. You can see at below left why the heater was called an "octopus" furnace. It may not have always had the eight arms of an octopus but it sure looks like one, with those ducts waving all around. At below left you can also see the stack relay (black rectangular control) on the exhaust flue of this octopus furnace.

Octopus warm air furnace (C) Daniel Friedman

Octopus furnace with asbestos duct wrap (C) Daniel Friedman

At in both of the above photos and enlarged at above right we illustrate that white paper-like duct wrap that was just about always an asbestos paper material. On the octopus furnace at above right you can also see a rectangular heating control switch near the top of the furnace. Those black Tee-shaped controls visible on the ducts themselves are manual duct dampers that allowed the occupants to balance warm air flow among different areas of the building.

Examples of Modern Forced Warm Air Furnaces

Modern warm air furnaces classed as central heating include a heat source (oil burner, gas burner, electricity, solar,etc.), a blower assembly, a cool return air plenum, a warm air supply plenum, and connections to supply and return ducts that bring cool air from the occupied space, pass it through the air handler, and move it as warmed air back into the building.

Depending on their physical position a furnace may be an upright unit or a horizontal unit suited for low basements or crawl space installation.

Heating system air flow direction determines whether we call the vertical air handler an upflow unit (cool air enters at the bottom) or a downflow unit (cool air enters at the top of the unit).

Our upflow/downflow furnace illustration at left was provided courtesy of Carson Dunlop Associates.

Below at left is an up-flow high efficiency gas fired furnace that we installed.

Air enters that silver plenum at the base of the unit and flows upwards through an air filter, blower assembly, heat exchanger and supply plenum into supply ductwork. Our arrows indicate the direction of air movement through the upflow furnace.

Horizontal warm air furnace in a crawl space (C) Daniel Friedman

At below right is a horizontal furnace. We guess that return air is entering at the right end of the unit because those flex ducts are larger than the smaller flex duct headed up from a (not visible) connection to the supply plenum at the left end of the unit. A look inside through the blower compartment access door on the other side of this unit would answer the question.

See FURNACE CONTROLS & SWITCHES and for details about how furnaces work and their controls, see FURNACE OPERATION DETAILS.

Are you sure it's a furnace and not a boiler like the one at left?

A furnace is not a boiler, and vice versa. A boiler should not be called a furnace, or your heating technician will know she or he can charge you extra because you have no idea what is going on, and what they do will seem more mysterious (and expensive) than ever.

Except for one trouble making company who messed everyone up by making a hot water heating boiler and calling it an "Iron Furnace". Oh well. Boilers heat water and furnaces heat air. Usually.

Do you see ductwork, or water pipes?

If the heat in your building is provided by warm or hot metal radiators, heating baseboards containing finned copper tubing, or wall convectors that look like a radiator but contain finned copper tubing, or if heat is provided by flexible rubber, plastic, or metal tubing run in building floors or ceilings, then the warm or hot water circulating in those devices is probably being delivered by piping circulating water heated by a heating boiler, or possibly by a steam boiler or a heat pump or geo-thermal system.

See BOILERS, HEATING and RADIANT HEAT Floor Mistakes to Avoid. If your heating radiators have valves which hiss and let air escape as heat is coming on your heat is probably being delivered in pipes which circulate steam from the steam boiler up through radiators in the occupied space. See STEAM HEATING SYSTEMS .

How does a Warm Air Heating Furnace work?

The Building Thermostat senses temperature, turns on the furnace burner. The oil or gas burner will continue to run (usually) until the call for heat is satisfied at the thermostat.

Heat exchanger: Hot combustion gases produced by the oil or gas burner circulate inside of the furnace's metal heat exchanger causing it to get hot.

The furnace blower inside the furnace blower compartment draws returning cool air from the living area and blows it across the outside of the heat exchanger, sending the now-warmed air onwards into the occupied space.

Air ducts connect and permit movement of cool air from occupied space through furnace and deliver warm air back to occupied space.

Sketch courtesy of Carson Dunlop

Combination Fan & Limit Control: This control turns the furnace blower on and off at the proper times.

Details of just how a warm air heating system works and how its controls function as well as how these components are inspected, tested, set, or re-set are provided at FURNACE OPERATION DETAILS

Continue with Warm Air Heating Furnace System Diagnosis & Repair

If you have no heat, see HEATING LOSS DIAGNOSIS-FURNACES.

You can also continue your heating furnace diagnostic procedures with the articles at page left headed under FURNACE CONTROLS & SWITCHES.

If your furnace is fired by an oil burner, you can go directly to those diagnostics at OIL BURNER INSPECTION & REPAIR.

Gas burners are discussed at GAS BURNER Flame & Noise Defects.

Frequently Asked Question about warm air heating furnace system diagnosis & repair

Question: My furnace gas burners go on but the blower does not start and then the flame goes off. What's wrong?

I have a american standard furnace Freedom 80 the burners go on but the blower fan does not and then the flame shuts off what's wrong - Robert

Reply: as a safety feature on furnaces, the fan limit switch turns off the burner if the blower does not start

Robert, if the furnace blower fan will not start, the limit switch on your furnace will turn off the burner in order to avoid overheating and damaging the heat exchanger.

You'll need to inspect and fix the blower fan problem. If your furnace blower uses a drive belt to connect the fan to the driving motor check that the belt is in place and not slipping. Other blower fans use a direct drive motor whose shaft spins the fan assembly. IN either case check that the motor is starting;

See BLOWER FAN OPERATION & TESTING for our article on diagnosing and fixing these problems.

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Questions & answers about how to buy, install, maintain, and fix warm air heating furnaces, ductwork, and controls

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FURNACES, HEATING - see detailed links at page left

or see these detailed articles:

Hot Air Heating Furnace Basic Operating Steps
BLOWER FAN OPERATION & TESTING how to test a heating or cooling fan that is not working
CAD CELL RELAY SWITCH on oil fired furnaces or boilers as flame sensors & safety devices
Draft Hoods on gas fired heating equipment, function and safety
Draft Regulators & barometric dampers on oil fired heating equipment
Electrical Power Switches: Where to Turn On or Off Heaters for Furnaces & Boilers, Heat Pumps or Electric Furnaces or Air Conditioners
Fan Limit Switch on hot air furnace heating systems gas or oil fired
Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
Gas Piping, Valves, Controls: Guide to LP and natural gas meters, valves, tanks, piping, gas leaks, gas regulator assembly & other gas controls
Oil Burners: Guide to Oil Burners for heating systems, boilers & Furnaces: basic parts, operation, maintenance, performance & money-saving tips
Oil Tanks: Guide to Heating Oil Underground & Above ground Oil Storage Tank Leaks, Testing, Problems & Solutions, Home Buyer's / Home Owner's Guide
Oil Piping: A guide to heating oil piping, valves, controls, leaks, repairs for heating systems
Power Switches to turn on or off heaters for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
Stack Relay Switches: Guide to finding, resetting, maintaining stack relays on oil fired furnaces or boilers as flame sensors & safety devices
Thermostats & Heat Controls for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
Zone Dampers: A guide to zone dampers for heating zone control on hot air heating system ducts

Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Links to our list of additional information on heating system inspection, repair, maintenance
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Ask a Question or Search InspectAPedia

Warm Air Furnace Inspection, Diagnosis & Repair - where to start?

First make sure you understand what type of heating system is installed

What's the Difference Between Forced Hot Air Heat and Gravity Hot Air Heat?

Illustration of Simple Gravity Furnaces using a Floor Register (Ductless Warm Air)

How to Spy on your Parents Through the Gravity Heat Registers

Illustration of Octopus Furnaces that Added Warm Air Ducts

Examples of Modern Forced Warm Air Furnaces

How does a Warm Air Heating Furnace work?

Continue with Warm Air Heating Furnace System Diagnosis & Repair

Frequently Asked Question about warm air heating furnace system diagnosis & repair

Question: My furnace gas burners go on but the blower does not start and then the flame goes off. What's wrong?

Reply: as a safety feature on furnaces, the fan limit switch turns off the burner if the blower does not start

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair