This limit control replacement guide will help you to diagnose and replace the furnace limit control. This would cost over $100 to have a repairman do it for you. We will help and you can keep the $100.
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The limit control is an electrical switch that opens when the temperature around it gets above the setpoint. Some are a round button-like switch and are referred to as snap disc limit controls. Others have a long metal sensor or a long mounting arm with a snap disc on it. On older furnaces, the control also turns the blower motor on and off at a preset temperature. These are referred to as fan/limit controls.
Before replacing, we will verify the switch is defective. First, turn the power off to the furnace. If you have an ohmmeter, you can measure the resistance across the switch. If the temperature is below the setpoint, the resistance across the switch should be zero.
** HOT TIP: A simple electric meter is a must have for any serious do it yourselfer.**
If you do not have an electrical meter, you can temporarily bypass the switch and see if the furnace works.
**WARNING** THIS IS FOR DIAGNOSING THE PROBLEM ONLY! THE FURNACE SHOULD NOT BE RAN WITH THE CONTROL BYPASSED. IT COULD RESULT IN A FIRE OR OTHER SERIOUS DAMAGE.
To replace the control, begin by ensuring the power to the furnace is off. Then, on fan/limit type controls, mark the wires and write down there connecting points. (If the control is a fan/limit control, there will be four wires connected to it. Two of the wires are low voltage (24vac) and go to the limit switch. The other two wires are line voltage (120vac) and go to the fan switch.)
**NOTE** A simple limit switch will normally only have two wires and you cannot really hook it up wrong.
Then, remove the wires by pulling them off the connectors. Remove the two screws holding the control in place and remove the control from the furnace.
When performing a limit control replacement, it is very important to get the proper replacement. There are many different types and setpoints. The new control should match the old one in the following ways:
1. The control should be the same style (snap disc, helical bimetal, fan/limit, etc.)
2. The opening setpoint should be the same.
3. The length of the sensing element should be the same.
After choosing the right replacement, mount the new control in the furnace. Then, reconnect the wires ensuring they are in the proper location.
Complete your limit control replacement by turning the power back on to the furnace. Run the furnace through a complete heating cycle to verify proper operation. You should check to make sure the furnace has proper air flow.
This can be performed by measuring the outlet temperature and the return air temperature. Compare the difference between the two readings with the recommended temperature rise.
The recommended temperature rise is usually on the label plate inside the furnace. Also you should ensure that the supply temperature is below the maximum listed on the label.
If the readings are outside the recommended temperatures, you should check the blower motor operation, filter, and duct system for obstructions.