Hot Air Heating Furnaces, Controls & Switches InspectAPedia® -
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
Questions & answers about warm air heating system controls & switches
Furnace operating controls & switches: this article explains how to identify & set or adjust the controls and switches found on warm air heating furnace systems.
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This article series 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.
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.
How to Identify, Reset, or Adjust Hot Air Heating System (Warm Air Furnace) Controls and Switches
The articles listed below and found in this article series 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.
Comparing Forced Hot Air Heat with Gravity Hot Air Heating Systems
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.
What is the Function of the Hot Air Furnace Fan Limit Switch?
Details about the fan limit switch, how it works, and how it is set are provided at FAN LIMIT SWITCH. Excerpts are below.
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 our gas furnace control 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 the all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: AQUASTAT CONTROL Functions and CAD CELL RELAY SWITCH (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 RELAY SWITCH
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.)
Questions & Answers regarding this article
Questions & answers about warm air heating system controls & switches
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Additional technical contributors & reference sources for this article are listed below.
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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 & 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.
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 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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