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The control can handle 120V and 240V devices and can also be wired to control low-voltage devices. The electrical wiring used must also be rated for suitable temperature exposure (Honeywell advises wiring rated for 167degF). The sketch at left, courtesy of Carson Dunlop, shows an improper (too high) upper limit setting - this is an unsafe fan-limit switch setup which is likely to allow the furnace to overheat, risking heat exchanger damage and dangerous carbon monoxide leaks. |
At 120V the control can switch fan motor loads at 14 Amps full load (84 Amps locked rotor load), and the limit switch (presumably an oil or gas burner) can handle up to 8 Amps full load (48 Amps locked rotor load).
When controlling 240V devices the fan or blower control can handle up to 7 Amps full load (42 Amps LRA) and the limit switch can handle 4 Amps full load (24A LRA).
This furnace combination control is mounted on the furnace in a location where the bimetallic spring/probe (shown above in this article) will protrude into the warm air plenum to sense furnace air temperatures there. Each furnace manufacturer will provide instructions of where, on their system, the air temperature should be monitored for control purposes.
The reason the control manufacturer warns that the control tip should not touch any internal surfaces of the furnace is that doing so can cause improper reading of furnace air temperatures or could damage the control or prevent free movement of the bimetallic spring in response to temperature changes.
Of course if you are replacing an old control that has failed, just mount the control in the same location that held the prior unit. When replacing an old furnace limit control, make sure that the new control has a sensing tip of the same length as the unit being replaced. Otherwise the new control may not work safely.
The manufacturer provides details for surface mounting, rigid-bracket mounting, or swivel-mounting of the control. Which of these methods you choose depends on what mounting is needed to place the sensor probe in the proper location in the air plenum.
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Fan control wiring: As Honeywell's illustration shows, the two fan terminals are on the upper and lower left side of the control. Line voltage is wired at the bottom left push-in terminal. Load voltage (to the fan) is wired at the upper left push-in terminal. Limit control wiring: As the illustration shows, the two LIMIT terminals are on the upper and lower right side of the L4064B control. The Line (power in) wire is connected to the lower right push-in terminal, and the Load (wire to oil or gas burner) is wired to the upper right push-in terminal. |
The wiring diagrams shown in more detail below are typical for wiring the furnace combination control on heating systems. Remember that all electrical wiring of furnace controls (or any other electrical devices) must comply with national and local electrical codes as well as the specifications of the control manufacturer and the furnace manufacturer.
Wires are connected to the control using push-in terminals. A wire strip gauge is provided on the left side of many versions of this control. The control used for our photos came with additional push-in terminals (Part # 137813) that can be used to convert the push-in wiring connectors to screw-terminal connectors. This is a great idea if you expect to be changing wiring from time to time. (Dr. Jess Aronstein's research has demonstrated that repeated-use or re-use of push-in type terminals on electrical receptacles does not provide a very reliable connection. This may be true for this control as well.)
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In most applications in the U.S. and Canada the Honeywell L4064B combination control is used to switch on and off 115V-120V or 240V fans and heating burners, or on some gas equipment, the burner controls (the LIMIT switch function) operates at lowe voltage. The pull-out or break-off tab discussed below at "Low Voltage Control Installation Notes" is left in place for line voltage applications.
The left-illustration shows normal wiring for this set-up. The fan or furnace blower motor is controlled by the two left connections (green dots); the furnace limit switch which will turnoff the burner if the temperature limit is reached is controlled by the two right connections (red dots). Here the furnace limit switch is controlling a line-voltage device. The colored triangles refer to notes given below.
The right-hand illustration above shows the wiring for controlling line voltage when the jumper or pull out tab has been removed. You can see that in effect the installer in effect is replacing the missing jumper by installing a common wire to both terminals on either side of the contacts where the jumper was removed. This important detail permits this control to be used to control line voltage (120V) devices even if the jumper has been removed or the paper pull-out tab on older controls has been lost.
Additional installation details for this control in the latest form are available from Honeywell.
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To prepare the L4064B furnace control for use in a low-voltage application, simply remove the small red cardboard tab shown at left or break off the copper jumper between the two contacts in the same location on newer models of this control. . If you make a mistake and remove this tab and then realize that you need to use this control to handle line voltage (120V or 240V) you can simply install a jumper wire as we show in the right-hand illustration just above on this page.
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The diagram at left shows how to wire the Honeywell L4064B combination furnace control when it is used to control low-voltage equipment. This tab is found protruding from the control near the center of the bottom of its face. You'll see embossed on the control above this tab the words "Remove for Low Voltage". Newer versions of this control have a brass jumper in the same location. The brass jumper is broken off for low-voltage use and is not replaceable once it has been removed. |
The fan or furnace blower motor is controlled by the two left connections (green dots). The furnace limit switch which will turnoff the burner if the temperature limit is reached is controlled by the two right connections (red dots). Here the furnace limit switch is controlling a low-voltage device such as a heating furnace gas valve. The colored triangles refer to notes given below.
Notes:
Before wiring this or any control be sure to obtain the latest data from the manufacturer of the control and the furnace on which it is to be installed. Additional installation details for the Honeywell L4064B Combination Fan Limit control in the latest form are available from Honeywell.
After wiring this control make sure you've use the proper settings by reading over How to Set the Fan & Limit Control and then 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.
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 at page left show where you are in our document or website.
FURNACES, HEATING
DUCT SYSTEM DEFECTS
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.
Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.
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05/31/2008 - 09/05/06 http://www.inspect-ny.com/heat/Fan_Limit_Switch_Installation.htm © Copyright Dan Friedman 2008-1999 All Rights Reserved
