Hot Air Heating Furnace Basic Operating Steps
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Sequence of steps in furnace operation - how warm air heating systems work
Troubleshooting heating system boiler, furnace, burner, controls, or heat distribution problems
How to inspect & repair hot air heating systems - Furnaces
What are the basic components of warm air or furnace heating systems?
Duct system inspection, defects, repairs
Typical operating temperatures of warm air or hot air heating systems
Questions & answers about the operating sequence of warm air heating systems

Heating furnace operation: this article lists the sequence of steps in the operation of a modern warm air heating furnace as well as older heating furnaces such as the "octopus" furnace that may have little or no duct work. Understanding exactly what happens from the moment that the room thermostat calls for heat until the call for heat is satisfied can help troubleshoot a heating furnace that is not working properly.

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How does a Warm Air Heating Furnace work? Sequence of Operating Steps

This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects. Sketch at page topt courtesy of Carson Dunlop

1. The Building Thermostat senses temperature, turns on the furnace burner: As building temperatures drop a wall-mounted thermostat in the occupied space senses the temperature drop and in response, calls for heat at the furnace, causing the furnace oil or gas burner to turn "on". Details are at THERMOSTATS.

Sketch at left courtesy of Carson Dunlop.

2. The Gas or oil burner: The heat source such as a gas or oil-fired burner will turn on in response to the thermostat, and it will continue to run until either the high temperature or "FAN LIMIT" temperature is reached inside the warm air plenum or until the building thermostat inside the occupied space senses that the desired temperature there has been reached.

At that time the burner is turned off but the blower will usually continue to run for a bit longer for reasons we'll explain in more detail below. See OIL BURNERS or GAS BURNER Flame & Noise Defects

3. 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. Combustion gases leave the inside of the heat exchanger and flow through a flue vent connector to a building chimney where they are vented safely outside.

4. The furnace blower inside the furnace blower compartment draws returning cool air from the living area and blows it across the outside of a steel "box" called the heat exchanger, sending the now-warmed air onwards into the furnace's output side or "supply air" plenum where it is sent into the building warm air duct system for delivery to the occupied space.

5. Air ducts connect and permit movement of cool air from occupied space through furnace and deliver warm air back to occupied space: The building air duct system includes return or cool air ducts and warm air ducts.

Cool air ducts carry air from the occupied space into the furnace return air plenum, possibly through an air filter, and into the furnace blower compartment. Warm air supply ducts connect to the supply air plenum and carry warm air into the occupied space where it flows out of floor, wall, or ceiling warm air registers or diffusers. See DUCT SYSTEMS

The Octopus furnace shown at left delivered heat to the building with no ductwork whatsoever. Warm air rose into the first floor of the building through a floor grid mounted at the top of the furnace. Warm air rose to upper floors through stairwells or by ceiling-cut air registers.

6. Combination Fan & Limit Control: This control turns the furnace blower on and off at the proper times. The FAN ON setting on this control makes sure that the blower fan does not turn on (even though the building thermostat has asked for heat) so that the furnace wont' blow cool air into the occupied space.

Furnace FAN ON: When an adequate warm temperature has been reached inside of the furnace warm air plenum chamber the FAN ON switch turns on the furnace blower to deliver warm air to the occupied space.

Furnace FAN OFF: The furnace combination fan and limit control FAN OFF setting lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down and the heat it contained has been sent to the occupied space.

Furnace LIMIT: The LIMIT indicator setting is a safety control that will turn off the oil or gas burner if temperatures inside the warm air plenum exceed a safe level.

See FAN LIMIT SWITCH for details.

When does the furnace blower turn OFF in normal operation?

When the thermostat has been satisfied and turns off the oil or gas burner at the furnace, the fan limit switch will cause the blower or fan unit to continue to operate until the temperature at the supply plenum has reached or dropped below the "cut-in" or "fan-on" lower limit on the switch.

When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:

On most heating systems the burner or heat source will continue to run all of the time that the building thermostat is asking for heat, and will stop running as soon as the thermostat is satisfied. If the furnace oil or gas burner is very high capacity, or if the furnace fan/limit controls have been set to cause this effect, the burner may on some systems cycle on and off periodically while the warm air blower continues to run.

How is heating furnace efficiency or economy measured? What does Furnace AFUE mean?

Each furnace model is assigned an AFUE number. AFUE is an abbreviation for Annual Fuel Utilization Efficiency. In short, the AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat. Higher AFUE is better. If your furnace has an AFUE rating of 90, that means that for every dollar you spend on fuel, 90 cents worth of heat is delivered into your building. The remaining 10 cents is lost in inefficiency such as heat that escapes up the chimney along with the products of combustion.

AFUE is not the whole story of heating cost efficiency. A high-efficiency heating system that has not been cleaned and serviced may be running poorly and wasting money. In fact, an 85% AFUE heating furnace that has not been cleaned might be running at an efficiency much lower, perhaps 65%.

Furthermore, if your building is drafty or poorly insulated, you may be delivering heat at high efficiency but losing it from the building much faster than necessary. These articles can help with a more complete approach to saving money on heat: First see HEATING COST SAVINGS METHODS then for more detail check out our articles at HEAT LOSS RATE CALCULATIONS and HEAT LOSS INDICATORS and INSULATION IDENTIFICATION GUIDE

What are the Typical Air Temperatures Supplied by Warm Air Heating Furnaces?

A conventional gas fired warm air furnace will provide heated air at 140 to 170 degF at the supply plenum. Naturally the temperature of this air drops as it moves through the ductwork system and will be cooler exiting the air supply registers.

A high efficiency gas fired furnace will provide heated air at lower temperatures, typically at 110 F to 140F at the supply plenum.

Watch out: as Carson Dunlop Associates warn (sketch at left), under-sized or restricted ductwork will reduce airflow over the furnace heat exchanger and can thus overheat and even damage the furnace heat exchanger.

So if air temperatures at the supply plenum (or even at supply ducts) are abnormally high don't think that's a great heating system - instead you may need the system checked by an HVAC expert technician.

See FAN LIMIT SWITCH for details on the temperature operating limits and controls for warm air heating systems.

Signs of Improper Warm Air Heating Furnace Operation

Furnace Oil or Gas Burner Cycling On and Off Frequently

If a furnace oil or gas burner cycles on and off frequently while the furnace blower continues to run, the heating system probably needs service, inspecting, testing, or adjustment.

An over-sized or gas burner that is putting heat energy into the furnace too fast for the blower to extract it can cause this symptom.
A dirty or blocked air filter or a dirty or blocked blower fan can cause this symptom because it is reducing the rate of air flow across the heat exchanger. If you want to save money on heating costs be sure the warm air flow is not reduced or obstructed by dirty or improper air filters.
Improper furnace control settings can cause furnace oil or gas burners to cycle on and off too often; check with your heating service technician.

Reduced or no Air Flow at Heating Supply Registers

If you are getting some but not much air flow out of heat supply registers there is probably a problem with:

the air filter(s) - dirty or damaged - see AIR FILTERS for HVAC SYSTEMS
the blower fan is dirty or has a loose fan belt - see DIRTY A/C BLOWERS
the duct system itself, such as collapsed, blocked, or disconnected duct work - see DUCT SYSTEMS
if the system is a combined heating and air conditioning system, the cooling coil may be blocked by dirt - see DIRTY COOLING COIL
if the system has always delivered only poor air flow at the supply registers, the blower may be dirty or defective, or the return air supply may be inadequate - see INCREASING RETURN AIR

If you are getting no air flow whatsoever but the furnace burner seems to be running there is probably a problem with:

the blower fan motor or fan itself has failed
the duct system has become totally blocked or disconnected - see DUCT SYSTEMS
the furnace is not running at all - see NO HEAT - BOILER / FURNACE DIAGNOSIS

A Guide to Hot Air Heating Furnace Inspections - Troubleshooting

Furnaces follow a simpler operating sequence than hot water boilers and steam boilers which we discussed above.
Hot air heating furnaces use different controls than hot water boilers since the furnace distributes heat by warm air. For example, a fan/limit switch is normally used to turn the blower on and off.
A Working Definition of a hot air furnace can aid explaining the system to clients and owners:

A furnace is a steel or cast iron "box" connected to a series of duct work which runs through the house bringing hot air to grilles (registers).

A fan, usually at the furnace, circulates air from cold-air return grilles through duct work to the furnace where the air is heated and then returned through "supply ducts" to the living area.

Operating sequence of a hot air heating furnace:

The building or building room cools,
as temperature drops the thermostat switch closes (calls for heat),
oil (or gas or electric) burner is turned on, oil is pumped from tank to burner, ignited, burned,
Hot gases pass through a fire chamber (steel can)
Hot gases pass through a heat exchanger (steel separating building air from combustion chamber gases)
and out an exhaust flue,
past a barometric damper to a chimney to outdoors.

ASHI 9.2.C. The inspector shall open readily open-able access panels provided by the manufacturer or installer for routine homeowner maintenance
Understanding functions=understanding implications: E.g. : wet basement, steel furnace, high CO risks from rusted heat exchanger.

Heating System Operating Sequence Inspection Tips

The rust on the exterior around the gas burners of this gas fired furnace could be due to improper and unsafe operation.

Expert inspection should be ordered to assure that this system is safe and functional.

If the furnace turns on immediately when the thermostat is turned-on or up from the living area what does this mean? Normally a hot air furnace oil or gas burner will turn on immediately when the thermostat calls for heat.

However the blower or fan unit that delivers warm air to the living area will not turn on until the fan limit switch in the hot air supply plenum or furnace top senses that the air in that area is warm. This prevents blowing cold air on building occupants.

Similarly, when the thermostat is satisfied (the room has reached the thermostat set temperature), the thermostat will "turn off" the furnace. What actually happens is that the oil or gas burner will turn off immediately, but on most hot air furnace systems, the blower will continue to run, perhaps even for several minutes, as it extracts the remaining heat from the furnace heat exchanger and hot air supply plenum.

Two reasons for this continued operation of the furnace blower fan past burner shutdown include:

Cooling down the heat exchanger avoids cracks or damage that could occur if the furnace heat exchanger gets too hot.
The cool down cycle of the blower avoids wasting heat by failing to distribute it to the living area from the furnace.

Make a thorough visual inspection for signs of damage or improper operation, missing parts, water damage, rust, mechanical damage, soot, smoke, improper venting, missing or damaged safety controls, etc. We detail these topics in articles listed below. Thanks to reader Cyrus for editing suggestions.

Details of Heating System Controls and Switches

For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:

Hot Air Heating Furnace Basic Operating Steps
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
CAD CELL RELAY SWITCH on oil fired furnaces or boilers as flame sensors & safety devices
Stack Relay Switches Explained 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
Gas Piping, Valves, Controls including gas regulator assembly and other controls

Types & Combinations of Water (hydronic heating boilers) and Air (warm air furnaces) Heating Systems

Definition of Water to Air Heat Exchanger Heating Systems

A technical note is necessary about determining what kind of heat or what type of heating system is installed: some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator).

The heat exchanger in a water to air heating system is then placed inside of an air handler or blower compartment where a blower fan circulates building air from return ducts to a plenum where air is blown across the heat exchanger and then the warmed air is delivered to the occupied space through additional warm air ducts or radiators.

More about Water to Air Heat Exchanger Heating Systems

Some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator). The heat exchanger in a water to air heating system is then placed inside of an air handler or blower compartment where a blower fan circulates building air from return ducts to a plenum where air is blown across the heat exchanger and then the warmed air is delivered to the occupied space through additional warm air ducts or radiators.

Water-to-air heating systems will use both a separate water heating boiler and a blower or air handler system.

Watch out for corrosion on water to air heat exchanger heating coils, as we explain just below.

Corrosion on Heating & Air Conditioning Heat Exchanger Coils & Condenser Coils

Corrosion on heat exchanging coils in heat pumps, air conditioners, or water-to-air heating coils eventually leads to leaks and costly repairs that require replacement of the coil and re-charging of the HVAC system refrigerant. Corrosion may occur as a function of age and use and failure to keep the coil clean, but there are some special sources of corrosion in HVAC systems that you should watch for:

Corrosion of HVAC equipment coils due to their use at indoor swimming pools: chlorine used in pools is very volatile and rapidly leaves the pool water as chlorine gas. While the chlorine levels may be below a health hazard to humans at indoor pools, the operation of indoor HVAC systems that condition swimming pool room air exposes the coil to high volumes of possibly corrosive chlorine, reducing the equipment life.
Corrosion of HVAC equipment coils due to presence of Chinese Drywall in the building. See CHINESE DRYWALL HAZARDS for details.

Definition of Dual Water and Air Heating Systems

Some buildings are heated by a combination of separate hot water systems (circulating hot water through radiating devices like baseboards or radiators in some areas) and hot air systems (circulating warm air through ductwork into the occupied space in other areas).

These buildings will have both a hot air furnace and a completely separate hot water heating boiler installed. In this case these are completely separate heating systems and usually each serves different building areas.

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Questions & answers about the operating sequence of warm air heating systems

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FURNACES, HEATING
FURNACE CONTROLS & SWITCHES
  BACKDRAFTING HEATING EQUIPMENT
  CAD CELL RELAY SWITCH
  DIAGNOSE & FIX FURNACE
  DRAFT HOODS
  DRAFT REGULATORS
  ELECTRIC MOTOR OVERLOAD RESET SWITCH
  ELECTRICAL POWER SWITCH FOR HEAT
  FAN AUTO ON CONTROLS
  FAN LIMIT SWITCH
    What is a Fan & Limit Switch?
    Sensing Furnace Temperatures
    How to Set the Fan & Limit Control
    How to Test the Fan & Limit Control
    How to Install the Fan & Limit Control
    How to Wire the Fan & Limit Control
  FAN LIMIT SWITCH TROUBLESHOOTING
  FAN MOTOR START CAPACITORS
  FIRE SAFETY CONTROLS
  HEAT EXCHANGER CLEANING
  HEAT EXCHANGER LEAKS
  Reset Switch - Primary Control
  Reset Switch - electric motors
  SPILL SWITCHES
  STACK RELAYS
  THERMOSTATS, HEATING / COOLING
  ZONE DAMPER CONTROLS
FURNACE HEAT EXCHANGER LEAKS
FURNACE OPERATION DETAILS
FURNACE OPERATING TEMPERATURES

Thanks to our reader Cyrus for editing help and critique 2/1/2009
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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How does a Warm Air Heating Furnace work? Sequence of Operating Steps

When does the furnace blower turn OFF in normal operation?

When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:

How is heating furnace efficiency or economy measured? What does Furnace AFUE mean?

What are the Typical Air Temperatures Supplied by Warm Air Heating Furnaces?

Signs of Improper Warm Air Heating Furnace Operation

Furnace Oil or Gas Burner Cycling On and Off Frequently

Reduced or no Air Flow at Heating Supply Registers

A Guide to Hot Air Heating Furnace Inspections - Troubleshooting

Heating System Operating Sequence Inspection Tips

Details of Heating System Controls and Switches

Types & Combinations of Water (hydronic heating boilers) and Air (warm air furnaces) Heating Systems

Definition of Water to Air Heat Exchanger Heating Systems

More about Water to Air Heat Exchanger Heating Systems

Corrosion on Heating & Air Conditioning Heat Exchanger Coils & Condenser Coils

Definition of Dual Water and Air Heating Systems

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

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