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 Mobile View HEATING SYSTEMS AIR FILTERS for HVAC SYSTEMS AIRBOUND HEAT SYSTEM REPAIRS ANODES & DIP TUBES on WATER HEATERS ANTI SCALD VALVES APPLIANCE EFFICIENCY RATINGS ASBESTOS in BUILDINGS BACKFLOW PREVENTERS BAROMETRIC DAMPERS BLOWER DOORS & AIR INFILTRATION BLUERAY Recall BOILERS, HEATING BOILER CONTROLS & SWITCHES BOILER LEAKS CORROSION STAINS BOILER LEAKS, HOW TO LOCATE BOILER NOISE SMOKE ODORS BOILER OPERATING PROBLEMS BOILER OPERATION DETAILS BOILER PRESSURE & TEMPERATURE SETTINGS CARBON MONOXIDE/DIOXIDE CARBON MONOXIDE WARNING CHIMNEY INSPECTION DIAGNOSIS REPAIR CHIMNEYS & Flues - Asbestos Transite Pipe COOL OFF HEAT Thermostat Switch DRAFT REGULATORS - barometric dampers DUCT SYSTEMS DUCT SYSTEM DEFECTS DUCT & AIR HANDLER ODORS DUST FROM HVAC? ELECTRIC HEAT ELECTRICAL POWER SWITCH FOR HEAT ENERGY SAVINGS in BUILDINGS FAN CONVECTOR HEATERS - HYDRONIC COILS FLUE VENT CONNECTORS FREEZE-PROOF A BUILDING FURNACES, HEATING 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 FURNACE OPERATION DETAILS GAS PIPING, VALVES, CONTROLS HEAT EXCHANGER LEAKS HEAT LOSS in BUILDINGS HEAT LOSS DETECTION TOOLS HEAT LOSS INDICATORS HEAT LOSS PREVENTION PRIORITIES HEAT LOSS R U & K VALUE CALCULATION HOUSEWRAP AIR & VAPOR BARRIERS HEATING COST SAVINGS METHODS HEATING LOSS DIAGNOSIS-BOILERS HEATING LOSS DIAGNOSIS-FURNACES HEATING OIL CLOUD WAX GEL POINT HEATING OIL EXPOSURE HAZARDS, LIMITS HEATING OIL SLUDGE HEATING SMALL LOADS HEAT PUMPS HEATING SYSTEM INSPECTION PROCEDURE HIGH EFFICIENCY BOILERS/FURNACES INSULATION INSPECTION & IMPROVEMENT MOTOR OVERLOAD RESET SWITCH NO HEAT - BOILER / FURNACE DIAGNOSIS ODORS FROM HEATING SYSTEMS OIL BURNERS OIL BURNER INSPECTION GUIDE OIL BURNER NOISE SMOKE ODORS OIL BURNER SOOT & PUFFBACKS OIL FUEL TYPES & CHARACTERISTICS Oil Odors: Leaky Oil Tank Piping OIL SAFETY VALVES OIL TANKS OIL TANK LEAKS & SMELLS OIL TANK PIPING DEFECTS OIL TANK PRESSURE OIL TANK SLUDGE OIL TANK TESTING OIL TANKS, BURIED PLASTIC HEATER VENT PULSE COMBUSTION HEATERS RADIANT HEAT RADIANT HEAT Floor Mistakes to Avoid RADIANT SLAB FLOORING CHOICES RADIANT SLAB TUBING & FLUID CHOICES RADIATORS RELIEF VALVES - TP Valves on Boilers Relief Valves - Water Heaters SAFETY DURING HEATING INSPECTION Safety Recalls BLUERAY Recall CHIMNEYS & Flues - Asbestos Transite Goodman HTPV RECALL Heat Recovery Ventilator RECALL Lennox Furnace Manuals Lennox WARNING Weil McLain RECALL SPILL SWITCHES STACK RELAY SWITCHES STEAM HEATING SYSTEMS TANKLESS COILS THERMAL EXPANSION of MATERIALS THERMAL MASS in BUILDINGS THERMAL MASS FLOOR SLABS THERMAL MASS in UPSTAIRS THERMAL MASS WALL DESIGN THERMAL TRACKING & HEAT LOSS THERMOSTATS Transite Pipes, Chimneys & Flues WINTERIZE A BUILDING More Information  InspectAPedia ® Home & Site Map InspectAPedia Blog - News Updates Air Conditioning & Heat Pumps Bookstore Electrical Environment Exteriors Heating Home Inspection Insulate Ventilate Interiors Mold Inspect/Test Plumbing Water Septic Roofing Structure Accuracy & Bias Pledge Contact Us |
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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. 
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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.
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.
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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
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.
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 |
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 |
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.)
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 |
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
For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:
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03/24/2009 - 09/05/06 - InspectAPedia com/heat/FurnaceControls.htm © 2009 - 1997 Copyright Daniel Friedman All Rights Reserved - InspectAPedia® is a Registered U.S. Trademark
