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Guide to Combination Fan & Limit Switches on Warm Air Furnace Heating Systems
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The warm air furnace fan limit switch (shown at above left on a gas fired warm air furnace) is a control which determines when the hot air furnace blower assembly turns on and off. The heating furnace 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 (shown at above right on an oil-fired warm air furnace and in our sketch at left) where we can see about 3/4 of the silver colored dial where the fan limit switch settings are made.
The switch, whose cover has been removed, is about in the center of the 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 soot and foil tape above the oil burner assembly also tell us that this system has been operating improperly with back pressure in the combustion chamber. The photo at above left shows the same type of combination furnace control in use on a gas-fired heating furnace.
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This would be an unusual condition but might occur if air ducts were blocked or if someone has fouled up the system controls. Details: In normal warm air furnace operation, by moving building air across the heat exchanger, the blower is warming air that will be delivered into the occupied space, but at the same time this process is keeping the heat exchanger from reaching too-high a temperature. If a furnace blower fan fails to start but the furnace heat source (gas or oil fired burner) is running, the heat exchanger would be come overheated and may warp and crack. |
The fan limit switch is designed to prevent this damage by shutting off the burner if plenum temperatures reach the high limit.
A fan limit control switch is found on both oil and gas fired heating furnaces of all types.
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The dial turns the blower fan on, off, and provides an upper limit temperature setting for safety. The white button you can see at the lower right corner of the furnace fan limit switch is called the manual fan switch - a manual override which can cause the furnace blower fan to turn on and run continuously regardless of furnace temperature. On systems where we have installed high quality air filtration to address an indoor air quality issue, and where the fan is rated for continuous duty, we may pull this switch out to keep the fan on continuously. |
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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. |
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As the air in the furnace plenum warms up the bimetallic spring expands, turning a gear which turns the fan limit control dial (shown in the photo above). As the fan limit switch control dial rotates, mechanical "fingers" on the back of the dial operate electrical contacts inside the switch to turn the fan on or off and at the upper limit to turn off the furnace oil or gas burner as well. When you move one of the little sliding temperature set points on the face of the dial you're moving the position of the mechanical fingers on the back of the dial. |
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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.
FURNACES, HEATING
DUCT SYSTEM DEFECTS
DUCT & AIR HANDLER ODORS
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
FURNACE CONTROLS & SWITCHES
CAD CELL RELAYS
STACK RELAYS
DRAFT HOODS
SPILL SWITCHES
DRAFT REGULATORS
FURNACE OPERATION DETAILS
HEAT EXCHANGER LEAKS
STACK RELAY SWITCHES
For details about the controls, components and switches commonly found on hot hot air heating systems see the articles listed below in which we explain how to identify, set, re-set, repair, replace, or avoid problems with the components of a furnace or warm air heating system.
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02/25/2010 - 09/05/2006 - InspectAPedia.com/heat/FanLimitSwitch.htm - © 2010 - 1988 Copyright Daniel Friedman All Rights Reserved - InspectAPedia® is a Registered U.S. Trademark
