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Furnace blower fan limit switch temperature settings: This article describes in detail the setting of furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems. The photo at the top of this page, courtesy of Honeywell Tradeline Controls, labels all of the controls and wiring terminals in a Honeywell combination fan and limit control type L4064B.
This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.
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Here are the functions and and normal settings for the warm air heating furnace control switch such as the Honeywell Tradeline L4046 and 4046B combination fan and limit control:
As we explained above, the furnace fan limit control turns the furnace blower on and off at the proper times. Below we detail the functions and normal settings for five controls found inside this device.
The basics of how furnaces work can be read at FURNACE OPERATION DETAILS and the key heating furnace components are introduced at FURNACES, HEATING. This website discusses these systems and heating components in detail in articles listed at the left of these pages. If your heating system is not working properly, see NO HEAT - BOILER or NO HEAT - FURNACE.
1. Furnace FAN OFF Setting
The furnace combination fan and limit control FAN OFF setting (blue arrow in our photo at left) lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down and the heat it contained has been sent to the occupied space.
The FAN OFF setting is the left-most metal finger protruding through the round silver plate of the fan limit control. Move the FAN OFF setting lever to the temperature at which the fan is to stop in order to prevent sending cool air into the occupied space.
From the factory the FAN OFF finger is usually set to about 90 °F.
At the end of a furnace-on heating cycle, after the gas or oil burner shuts down, the furnace blower will continue to operate for a time.
This continued fan operation achieves the following:
2. Furnace FAN ON Setting
When an adequate warm temperature has been reached inside of the furnace warm air plenum chamber the FAN ON switch turns (green arrow in our photo at above left) on the furnace blower to deliver warm air to the occupied space. The FAN ON setting on this control makes sure that the blower fan does not turn on too soon (even though the building thermostat has asked for heat) so that the furnace will not blow cool air into the occupied space. This setting also prevents the blower fan from cycling on and off too frequently during a heating cycle.
The FAN ON setting is the metal finger second from left-most, protruding through the round silver plate of the fan limit control. This finger is set to a temperature range from a minimum of 20 °F above the FAN OFF set point, to a maximum o f 30 °F below the LIMIT OFF set point.
From the factory the FAN ON is usually set to about 130 °F.
3. Furnace LIMIT OFF Setting
The LIMIT OFF indicator setting (red arrow in our fan limit switch photo at above left) is a safety control that will turn off the oil or gas burner if temperatures inside the warm air plenum exceed a safe level. This is the highest temperature setting on a furnace combination control. It is set to the furnace warm air temperature at which this safety switch is to turn off the oil or gas (or other) burner or heat source. On the Honeywell L4064B the LIMIT OFF is set to a temperature between 100 °F and 250 °F.
If the temperature inside the supply plenum reaches the "high" limit set at the LIMIT OFF finger, the switch will
turn off the oil or gas burner. This condition may not ever happen under normal conditions with most hot air furnace systems - on those systems the burner continues to run all of the time the thermostat
is asking for heat. The LIMIT OFF or "HIGH" or "MAX" on the furnace fan limit switch is a safety device.
From the factory this setting out of the box is 200 °F.
4. Furnace LIMIT STOP Setting
The LIMIT STOP control is an extra safety device to make it difficult for an amateur to set the LIMIT OFF to an unsafe or too-high temperature. This control setting could be changed by inserting a pen tip or similar object into the round hole visible in the LIMIT STOP plate near the right end of the temperature dial and slog on the silver plate of the fan limit control.
The LIMIT STOP is set at the factory to 200 °F. You should not change this setting as to do so maybe dangerous.
5. Furnace FAN ON Manual or "override" Switch
Fan override switch, also called a "manual fan switch" or "fan on switch" if present, is usually a white button that can be set to cause the furnace blower fan to run continuously or to run automatically driven by the temperatures sensed by the combination fan and limit control switch.
The fan override switch is indicated by the white arrow in our photograph.
Manual-On furnace fan: Push the white fan override button "IN" to set the fan to its MANUAL position to force the furnace blower to run continuously. This setting can be used to move (and presumably to filter) building air through the heating duct system at any time of year.
After completing these control settings be sure you test the combination fan and limit switch for safe and proper operation. See FAN LIMIT SWITCH TROUBLESHOOTING for details.
At FAN LIMIT SWITCH TROUBLESHOOTING we discuss troubleshooting furnace blower fan operating problems such as a blower that runs continuously or one that turns on and off to frequently.
Reader Question: which way do we push or pull the fan switch to go between AUTOMATIC and MANUAL-ON
"(Pull out the button to force the fan to "always on". Push the white button back in to return the fan to automatic operation" - It seems opposite on the diagram you have.. have a look. It says "pull automatic". I'm pretty sure mine is the same unit. I'm having trouble with blower motor always staying on after heater runs... any ideas? Thanks! Rob - 10/17/12
Rob, thanks for the suggesitons; I have revised, corrected and also clarified the article above, adding photos, and tuning up the text. We welcome critique, corrections, or suggestions from our readers.
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