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Guide to Temperature Settings on Combination Fan & Limit Switches for Furnaces - Honeywell L4064B
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Here we describe 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.

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 / FURNACE DIAGNOSIS. This website answers most questions about central heating system 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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How to Choose the Proper Settings for a Warm Air Furnace Combination Fan & Limit Control Switch

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.

1. Furnace FAN OFF Setting

The furnace combination fan and limit control FAN OFF setting 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 degF.

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:

• The remaining heat from the hot furnace heat exchanger is salvaged and sent into the occupied space
• The furnace heat exchanger is cooled down more uniformly by the air flow over its surfaces in order to help reduce the chances of heat exchanger cracking
• The temperatures in the occupied space remain more even

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 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 degF above the FAN OFF set point, to a maximum o f 30 degF below the LIMIT OFF set point.

From the factory the FAN ON is usually set to about 130 degF.

3. Furnace LIMIT OFF Setting

The LIMIT OFF indicator setting 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 degF and 250 degF.

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.

In the unusual event that temperatures inside the supply plenum become too high, when temperatures reach the LIMIT OFF setting the fan limit switch will turn off the burner to prevent damage to the furnace heat exchanger (from warping or cracking from excessive temperatures)

From the factory this setting out of the box is 200 degF.

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 degF. You should not change this setting as to do so maybe dangerous.

5. Furnace FAN ON 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..

FAN ON SWITCH Settings:

• 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.
  
  Some experts opine that running the fan continuously will make heat in the building more uniform and more comfortable. (This will not be true in every building and for every heating system and in some cases can cause blowing of uncomfortably cool air into some rooms during the heating season.)
  
  
• Automatic furnace fan: Pull the white fan button "OUT" to the AUTO to cause the fan to cycle on and off automatically in response to temperatures sensed by and set on the fan limit control dial.

After completing these control settings be sure you test the combination fan and limit switch for safe and proper operation. See How to Test the Fan & Limit Control for details.

...

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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.

  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.

• Hot Air Heating Furnace Basic Operating Steps
• Cad Cell Relays on oil fired furnaces or boilers as flame sensors & safety devices
• Draft Hoods on gas fired heating equipment, function and safety
• Draft Regulators & barometric dampers on oil fired heating equipment
• 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
• 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
• Power Switches to turn on or off heaters for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
• 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 zone dampers for heating zone control on hot air heating system ducts

• Honeywell Corporation, Minneapolis, MN 55408. Honeywell has sales offices in all principal cities in the world and has manufacturing facilities in Australia, Canada, Finland, France, Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A. Honeywell Form Number 60-0450 7-75, residential division. Honeywell's latest product data for this type of control can be found in English at http://customer.honeywell.com/Techlit/Pdf/69-0000s/69-0117.pdf

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