Honeywell Furnace Temperature Fan Limit Switch Control – The fan limit control turns the fan on and off in furnace applications. It works off of temperature and also provides a high-temperature safety limit.This is a switch, actually two switches, which turns the furnace blower on and off based on the temperature of the heat exchanger. A helical bi-metal probe extends from the back of the fan limit control. This probe is located inside the heat exchanger and the bi-metal of the probe reacts to temperature.
Honeywell Furnace Temperature Fan Limit Switch Control - How It Works
How it Works – Honeywell Furnace Temperature Fan Limit Switch Control
When the furnace heat exchanger heats up the bimetal twists causing the round dial (shown in the picture) to turn. Based on where the manual selection switch is set, when it reaches the manual setting contacts inside the switch close. Then the furnace blower motor turns on. This provides a time delay for the fan to come on which allows the furnace heat exchanger to heat up. This eliminates (or limits) cold air from being blown out of the ducts when the furnace blower first starts.
Shut Down Cycle – Honeywell Furnace Temperature Fan Limit Switch
After the burners go out when the call for heat is over, the bi-metal in the fan limit control cools. The bi-metal begins to make the dial turn the other way. When the temperature in the furnace heat exchanger cools the bi-metal reaches the low set point on the dial. The contacts inside the fan limit control open and allows the blower to shut off. This allows the heat exchanger to cool off by dissipating the excess heat in the furnace heat exchanger. This makes better use of the heat produced by the furnace. Instead of allowing the heat to remain in the furnace this feature in the Honeywell fan limit control uses as much heat as possible. The heat still in the heat exchanger. Finally, this adds efficiency to the furnace.
Honeywell Furnace Temperature Fan Limit Switch Control - Safety and Control
The Honeywell fan limit control is also a temperature control limit switch. Another limit control switch which is integral to the Honeywell fan control is a high-temperature control limit switch. The circuit which directly runs through this switch controls the burners in the furnace. The temperature control limit switch inside the Honeywell fan control breaks the circuit to the burners if the temperature exceeds the high limit setpoint. This is a safety feature all furnaces are equipped with in case the temperature gets too hot inside the exchanger. If this happens the furnace blower contacts remain closed so the furnace blower dissipates the excess heat in the furnace exchanger.
Technical Specifics - Honeywell Furnace Temperature Fan Limit Switch
There are differences in these furnace control switches so if you replace the one in your furnace be sure to replace with an exact replacement and make sure you get the settings correct from the old fan limit switch control to the new fan limit switch control. Wiring is also an issue from some fan limit switch controls as there are little details to be aware of when wiring a new fan limit control switch from the old one. The first thing you will want to notice is the length of the insert for the control. Some are 11 inches, some are 8 inches and there are some that are 5 inches for the insert that goes into the heat exchanger. It is important to get this right otherwise the fan limit switch control may not function properly and cause nuisance problems.
Specifically, the list of Honeywell Fan Limit Switch Controls includes:
- L4064B2210/U - 11-1/2 inch insert
- L4064B2236/U - 8-inch insert
- L4064B2228/U - 5-inch insert
- L4064B2228/B - 5-inch insert
The list for Camstat Furnace Fan Limit Controls includes:
- F568 - 3-inch insert
- F569 - 5-inch insert
- There are other Camstats so the list is not inclusive of all the fan limit switches they offer. Always remember, the best replacement is an exact replacement.
The list for White-Rodgers Furnace Fan Limit Controls includes:
- 5D51-35 - 5-inch insert
- 5D51-90 - 8-inch insert
- 5D51-78 11-inch insert
Functionality of the Fan Limit Switch Control
The above-referenced fan limit switches offer features such as “Heat Assist” where the switch delays the blower turning on to allow the heat exchanger to heat up. This prevents or limits cold air on initial start-up of the furnace. Additionally, a time delay function to turn the blow off. This feature allows the furnace to cool down after the main burners extinguish and to dissipate the residual heat left in the exchanger after the burners shut down. These switches also have a dial function temperature setting to control the temperature at which the blower motor turns on and off. Lastly, they offer a high limit setting (adjustable) so if the heat exchanger is getting too hot it shuts the main burners down for safety. This is a safety function of the fan limit switch control.
All temperature ranges and adjustable temperature ranges vary for the fan on, fan off, and the high limit temperature safety. Additionally, make sure not to cross the line voltage with the control voltage as some use both line voltage and control voltage wired to the fan limit switch. Remember to run the wiring through the available slows and bushings of the device provides them. Make sure wiring is secure and away from the burners to prevent damage to the wire. Always follow the manufacturer’s instructions and never exceed the rated amps for the device. Lastly, always make sure the device is mounted properly and securely.
Wiring Diagrams - Furnace Temperature Fan Limit Switch
The following is a wiring diagram for a Honeywell Fan Limit Switch Control. Before beginning any wiring make sure you turn the power off. Turn the power off at the furnace and go a step further for safety and turn the power off at the breaker. You are working with high voltage (and low voltage) and it can cause you serious harm. Additionally, I recommend taking a photo of the old fan limit switch and how it is wired before you start ripping the old wiring out and removing the fan limit switch. You can use the photo as a reference for wiring the new EXACT REPLACEMENT - it is an exact replacement, right?
Honeywell Furnace Fan Switch Wiring for Control and Limit - Line Voltage and Control Voltage Examples
Honeywell Furnace Temperature Fan Limit Switch Wiring Advisory
It is important to note in the above diagram that there are several different ways to wire this control. Additionally, since there are different voltages for different purposes and uses, great care should be taken to properly wire the control. Mixing the 24-Volt circuit with the higher voltage circuit will cause further problems such as a blown transformer and other issues with the low voltage circuit. This is also considered a fan limit switch and it is a safety switch. Proper wiring of the control is important for safety reasons. Many manufacturers are different in their engineering and control wiring. Additionally, models produced by a single manufacturer can be wired differently from one model to the next. If you do not understand basic control circuits or basic electrical circuits then please call a professional to handle this problem for you. Even if you think calling a professional is expensive. If you make a mistake it could cost you a lot more than calling in a professional especially for control and wiring issues.
Honeywell Furnace Temperature Fan Limit Switch Troubleshooting
So you are having problems with your furnace and you think its the fan limit switch control? How do you know this? You should follow a logical step of furnace fan limit switch control troubleshooting to eliminate anything else. Here is a list of basic steps to follow and things to look for when troubleshooting a fan limit switch control. Some fan limit switches require a special tool to make adjustments to the temperature settings. Always handle the control with care as it is specifically calibrated from the factory for controlling and maintaining accurate temperatures. Rough handling or not using proper tools to set the settings may cause the control to lose calibration and be inaccurate.
- Before you get started, check the air filters in the system. Dirty clogged air filters can cause high limit switches to open. This opens the safety circuit and prevents the furnace from heating.
- Check the line voltage and the control voltage with a voltmeter. You should have proper line voltage and control voltage. The control voltage is usually around 24-volts give or take 10%. Don’t forget to kill the power before working with the wiring.
- Is the furnace blower running all the time but you do not have heat? If so, have you checked the safety circuit for an open limit switch. Of course, the fan limit switch control has a limit or does it? The could possibly be other limit switches in the circuit and normally when any limit switch opens. especially a high limit caused by excessive heat, the safety circuit automatically triggers the blower fan to run at full speed while shutting off the main burners. This action helps cool down the problem. If the blower continues to run without any heat then you likely have a problem with a high limit fan switch or another limit switch in the system. To trace out the safety circuit you need to be able to read the furnace wiring diagram and follow the circuit out checking each switch. Helpful Hint: Safeties in a furnace are typically wired in series with each other and usually comprise of a single circuit.
- Does the blower keep running too long after the main burners shut down? If so, have you check the temperature settings on the fan limit switch control. These settings are normally adjustable on some fan limit switch controls. Following the instructions for the “fan limit switch control” make the adjustment to shut the fan down sooner. This will require turning the temperature setting up on the control. Don’t confuse this setting with the high limit setting.
- Do the main burners light but then shut down again before the fan turns on? If this is the case, the temperature setting for the “fan on” needs to be turned down to bring the fan on sooner. This could also indicate other problems such as bad fan limit switch. If you have adjusted this temperature all the way to its lowest setting and you continue to have this problem, remove the fan limit switch control and look at the insert portion of the switch. See if you observe anything abnormal such as a broken metal helix or something else broken. If so then replace. Additionally, even if you don’t see any problems or anything broken and it is set at the lowest setting then you are likely looking at replacing the fan limit control.
- If the furnace fan limit switch is sticking and either not allowing the fan to turn off or turn on then it’s time to replace the switch. Follow the basic rules and replace it with an exact replacement.
Furnace Temperature Fan Limit Switch Wiring Diagram Sample Illustration for Safety Circuit
The safety circuit in the above diagram is for illustration purposes only. The entire circuit is not shown. This diagram simply illustrates what was written above. That safety circuits typically have more than one safety in the circuit and they are normally wired in series. There could also be other types of switches in the circuit such as a pressure switch or centrifugal switch for a forced or induced draft motor.
Furnace Temperature Fan Limit Switch Settings
The settings, especially for the high limit (if its adjustable on your control), is completely up to the manufacturer of the furnace. At no time should you ever exceed the setting as recommended by the manufacturer of the furnace? Most switches are made to not allow you to exceed this rating to keep you within safety limitations of the temperature of the furnace. Many fan limit switch controls, including Honeywell, have a limit stop which prevents the limit indicator lever from being adjusted beyond the equipment manufacturer’s specifications. Never exceed this setting or limitation for the furnace fan limit switch control.
Conclusion | Honeywell Furnace Temperature Fan Limit Switch
Hopefully, we have provided you with enough information here to help you resolve any issues with your furnace. Remember, safety is always paramount so if you have any doubt call a professional to fix the problem. Never re-engineer the system and use the same wiring and same parts as what is in the furnace following the rule above: An exact replacement is also the best replacement. Turn the power off before working with any wiring and take a photo of it to help you rewire the exact replacement as the old device. Follow the manufacturer’s instructions to a “T” and you should be okay.
Honeywell Furnace Temperature Fan Limit Switch Control
Honeywell Furnace Temperature Fan Limit Switch Control
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