Guide to Warm Air Furnace Heating Systems

Warm Air Furnace Heating Systems: inspection, diagnosis, repair, how forced hot air and gravity hot air furnaces work
- What's the Difference Between Forced Hot Air Heat and Gravity Hot Air Heat?
- How does a Warm Air Heating Furnace work?
Questions & Answers about how to buy, install, maintain, and fix warm air heating furnaces, ductwork, and controls
References

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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. Sketch at page top courtesy of Carson Dunlop Associates.

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 Related Topics 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.

Warm air register in a floor (C) Daniel Friedman

Wall warm air supply register in a NYC apartment (C) D Friedman

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 Associates.

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 Related Topics 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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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
  Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
  Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
  Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
  Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
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.
"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,
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.)
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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, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
Links to our list of additional information on heating system inspection, repair, maintenance
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