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Hot Air Heating Furnaces, Controls & Switches
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• What are the controls and switches found on a hot air heating systems?
• What is a gravity heating system? How does a modern furnace work?
• Troubleshooting heating system furnace controls, ducts, filters, registers, dampers, controls
• How to inspect & repair hot air heating systems - Furnaces

This website answers most questions about central heating system controls and switches: 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 sketch at above left, courtesy of Carson Dunlop, explains the basics of how a modern warm air heating system or "furnace" works and shows how warm air flows in a building. The Carson Dunlop sketch at right shows how an older gravity heating system worked, such as the "octopus" france shown at the top of this page. This article provides details about the operation, controls, inspection, diagnosis, and repair of all types of warm air heating furnaces.

If you don't know what kind of heat your building uses, see our introduction at BOILERS, HEATING.

If your heating system is not working properly, see NO HEAT - BOILER / FURNACE DIAGNOSIS. 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.

Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks. 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 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

How to Identify, Reset, or Adjust Hot Air Heating System (Warm Air Furnace) Controls and Switches

If the heat in your building is provided by warm air that flows out of ceiling, wall, or floor air supply registers into the occupied space, or if your heating system uses a water-to-air heating system 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, and the air is being heated by a gas, oil, or electric furnace, or perhaps by a heat pump or a geo-thermal system.

The articles listed below and found at this website provide description of all of the major components of warm air heating systems, how to recognize or find each component, what it looks like, what goes wrong, and how to maintain, repair or adjust the component.

What is the Function of the Hot Air Furnace Fan Limit Switch?

The warm air furnace fan limit switch is a control which determines when the hot air furnace blower assembly turns on and off. The fan limit switch prevents the furnace blower from sending chilly air into the building if the oil or gas burner has not sufficiently heated up the furnace heat exchanger and supply air plenum. In the photographs above we show to illustrations of a hot air furnace fan limit switch as you're likely to find one at a typical furnace.

A fan limit switch on an oil fired furnace is shown the right hand photo above and a close up is shown at left.

The fan limit switch is about in the center of the above photo, and is partially hidden by a low voltage transformer and a metal electrical junction box which are mounted at the right side of the furnace cabinet.

The fan limit switch is also a safety control which protects the furnace heat exchanger from damage by turning the burner off on the furnace gas or oil burner if the temperature inside the warm air supply plenum (just above or just next to the heat exchanger) gets too high.

Details about the fan limit switch, how it works, and how it is set are provided at FAN LIMIT SWITCH

A Quick Guide to Oil Burner and Gas Burner Flame Sensors

Furnace Oil-burners use either a cad cell or stack relay to confirm that the furnace oil burner is operating properly and to avoid flooding the combustion chamber with un-burned oil.

Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: Aquastat Functions and Cad Cell Relay Switch Flame Sensors (hot water boilers and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.

A Guide to Oil Fired Furnace or Heating Boiler Cad Cell Relays

Flame sensing devices on oil-fired heating appliances: modern oil-fired heating furnaces, boilers, and water heaters use a Cadmium Cell sensor, usually located inside the oil burner tube, to "see" the presence of flame and thus to assure that the oil burner assembly stops pumping oil into the combustion chamber if flame ignition is unsuccessful. The cad cell itself, that is the little cell that "sees" the oil burner flame, is wired (often by a yellow wire) to the cad cell relay switch (see photo) which is usually a gray box with a red "reset" button located on top of or alongside the the oil burner assembly. The cad cell causes the relay to switch the oil burner off when a flame is not established or if flame is lost at the oil burner. Our photograph shows a modern Honeywell(R) R8184G 4009 cad cell relay for use on (typical) intermittent ignition oil burner equipment. Details about Cad Cell relay switches, how to inspect, test, and reset them can be read at Cad Cell Relays

A Guide to Stack Relay Control Switches on Boilers, Furnaces, or Water Heaters

Oil Burner Stack Relay Switches Explained Older oil burners used to heat a warm air furnace, a heating boiler, or a water heater may use a Stack Relay to prevent sending fuel into the heater if it has not been successfully ignited. Stack relays are an older type of flame sensor than the Cad Cell Relays which we discussed above, but they accomplish the same purpose (turn off the oil burner if the flame is not established). The "stack relay" is a bimetallic spring inserted into the flue vent connector located usually quite close to the heating boiler between the boiler top and the chimney. The bimetallic spring warms in response to hot oil burner exhaust, confirming that combustion is taking place. If combustion is not occurring a timer inside the stack relay turns off the oil burner to prevent flooding of the combustion chamber with un-burned oil. We provide details about stack relays, the function, inspection, testing, and setting of stack relay switches at STACK RELAY SWITCHES

Guide to Draft Hoods on Gas Fired Furnaces, Boilers, Water Heaters - Purpose, Inspection, Repair

What is a Gas Appliance, Furnace, Water Heater, or Boiler Draft Hood

The photo at left is of a York gas fired furnace which displays a conventional draft hood opening - the large horizontal opening space shown in the middle of the furnace. The purpose of this opening is to permit additional air to flow into the flue vent connector (stack pipe) and chimney when the gas burner is operating. This additional air flow avoids excessive draft at the gas burner. Too much draft at the gas burner could result in improper gas combustion. (The gas burner will be below this opening and behind the cover with the louvered openings. The louvers provide combustion air to the gas burner.)

The draft hood or draft regulating device we are discussing here is normally used only on gas-fired heating equipment, not on oil-fired equipment.

Details about draft control for gas fired heating systems, including furnaces (like the furnace shown in our photo above) or boilers, are discussed at Furnace Draft Hood on gas fired equipment.

(Details about draft control on oil fired heating systems, including furnaces or boilers, are discussed at Draft Regulators barometric dampers on oil fired equipment.)

Guide to Furnace or Boiler Flue Gas Spill Switches on gas fired equipment - Purpose, Inspection, Repair

What is a furnace, boiler, or water heater flue gas spill switch? A spill switch may be found at the draft hood on any modern gas fired appliance, such as a heating furnace (hot air heat), a heating boiler (hot water heat or steam heat), or a water heater. This little sensor, or two or more of them, form an important safety device that feels the heat of escaping combustion gases that ought to be going up the flue or chimney. Since escaping combustion gases in a building are dangerous (forming a potentially fatal carbon monoxide hazard), if the sensor gets hot from flue gases flowing past its surface, it is designed to turn off the fuel supply to the gas burner. Details about flue gas spill switches used on gas-fired appliances such as furnaces and water heaters can be read at Spill Switches

Guide to Inspecting Barometric Dampers or Draft Regulators on Oil Fired Furnaces, Boilers,

Barometric dampers are devices used to regulate the draft on oil-fired heating equipment such as furnaces, boilers, or water heaters. On oil fired equipment the barometric damper, or draft regulator is typically a round Tee inserted in the flue vent connector between the heating appliance and the chimney. The face of the tee contains a round "door" with an adjustable weight. The service technician adjusts the weight to control the swing or opening of this door which in turn controls the amount of excess air that can enter the flue and chimney when the oil burner is operating. The barometric damper or draft regulating device we are discussing here is normally used only on oil-fired heating equipment, not on gas-fired equipment. Details about draft control on oil fired heating systems (such as the oil fired heater shown in the photo at left), including furnaces or boilers, are discussed at Draft Regulators barometric dampers on oil fired equipment. (Details about draft control for gas fired heating systems, including furnaces or boilers, are discussed at Furnace Draft Hood on gas fired equipment.)

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

FURNACE CONTROLS & SWITCHES
  CAD CELL RELAYS
  DRAFT HOODS
  DRAFT REGULATORS
  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
  SPILL SWITCHES
  STACK RELAYS
  THERMOSTATS
  ZONE DAMPER CONTROLS

• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Thanks to reader Eric Haver for technical edits 3/24/2009

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
• Guide to Warm Air Heating System Components (Furnaces, ducts, registers, blowers, duct dampers, air filters, gas or oil hot air heat):
• Draft Hoods on gas fired heating equipment, function and safety of heating flue vents
• Draft Regulators & barometric dampers on oil fired heating equipment and safe venting of flue gases
• 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): how furnace combination fan and limit control switches work, how to set the fan limit switch, how to test the limit switch
• 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
• 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 automatic or electrical zone dampers in heating or air conditioning ductwork

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

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• ...

More Information on Building Diagnostic Inspections and Repairs

• 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
• 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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