Upflow and Downflow furnace schematics (C) Carson Dunlop AssociatesGuide to Warm Air Furnace Heating Systems
Furnace inspection, troubleshooting, & repairs

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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. Sketch at page top courtesy of Carson Dunlop Associates.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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

Warm air register in a floor (C) Daniel FriedmanIf you have no heat,


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


[Click to enlarge any image]

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.

Wall warm air supply register in a NYC apartment © D Friedman at

Articles at this website offer lots more detail including photos of individual heating system controls and components along with explanatory text.

Ceiling air supply register for heating or air conditioning © D Friedman at

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)

Gravity warm air furnace (C) Daniel Friedman

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

Floor register for heating © D Friedman at

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.

Octopus warm air furnace (C) Daniel Friedman

Below you can also see the stack relay (black rectangular control) on the exhaust flue of this octopus furnace.

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.

How to Spot an Invisible Octopus Furnace

Footprint of an octopus furnace, probablhy coal fired, now removed from this Poughkeepsie Home (C) Daniel Friedman at

The round footprint of lighter-coloured concrete on the basement floor of this Poughkeepsie New York home tells us that most-likely there was previously a round octopus furnace installed in the home.

At the time of our inspection the home had been heated for decades by a steam boiler and the octopus furnace was long gone. The combination of the estimated age of the home (about 100 years old), its location, and history of heating in the North Eastern U.S. support this opinion - now principally of just historic interest.

Examples of Modern Forced Warm Air Furnaces

Upflow and Downflow furnace schematics (C) Carson Dunlop Associates

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.

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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

Below 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.

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

A look inside through the blower compartment access door on the other side of this unit would answer the question.


for details about how furnaces work and their controls,


oil fired hydronic heating boiler (C) Daniel Friedman

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.


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.


How does a Warm Air Heating Furnace work?

How a furnace works - schematic (C) Carson Dunlop Associates

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.

LARGER VIEW of heating furnace fan limit switch

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


Continue with Warm Air Heating Furnace System Diagnosis & Repair

If you have no heat,


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

Gas burner off © D Friedman at 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.


Continue reading at FURNACE CONTROLS & SWITCHES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see FURNACE HEATING FAQs - questions & answers about furnace inspection & repair




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