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Hot Air Heating Furnace Basic Operating Steps
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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. The articles at this website 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. The limitations of visual inspection of heating systems are described. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

A Guide to Hot Air Heating Furnace Inspections

• 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 burner is turned on, oil is pumped from tank to burner, ignited, burned,
• Hot gases pass through a fire chamber (steel can) 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: eg: wet basement, steel furnace, high CO risks from rusted heat exchanger.

Heating System Operating Sequence Inspection Tips

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 fam 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 satisified (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 blower fan include:

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.

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

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 Relays 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 techincal 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.

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.

Definition of 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.

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - principal author/editor of the InspectAPedia ^© Website
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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More expert information on this topic

More Information on Inspecting and Repairing Heating Systems

• Carbon Dioxide Gas Toxicity
• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• Dust from HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• 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.)

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