Quck answer
To bypass a thermostat and test it, follow these steps:
1. Turn off the power to the HVAC system at the circuit breaker or fuse box to ensure safety.
2. Locate the thermostat and remove the cover to access the wiring.
3. Identify the wires connected to the thermostat terminals (R, C, W, Y, G) and make note of their colors.
4. Disconnect the wires from the thermostat terminals.
5. Connect the R (power) and G (fan) wires together using a jumper wire or alligator clips.
6. Turn on the power to the HVAC system.
7. If the fan starts running, the thermostat is likely functioning properly. If not, it may need to be replaced.
8. Reconnect the wires to their respective terminals and replace the thermostat cover.
9. Restore power to the HVAC system.
By following these steps, you can bypass a thermostat to test its functionality before deciding if it needs to be replaced.
If any part of your HVAC system is not functioning, it is possible that the thermostat is faulty. While the most convenient way to determine if an HVAC thermostat is faulty is by testing it with a multimeter, you can also bypass the thermostat by twisting two of the thermostat wires together if you do not have a multimeter on hand. This is a simple and safe method, as the wires carry low voltage from the transformer, although precautions should still be taken.
Knowing how to bypass a thermostat can be useful for making your furnace or AC compressor work even when the thermostat is not functioning. It is also handy knowledge to have when remodeling and there are exposed thermostat wires hanging from the wall, but you have not yet purchased a new thermostat.
Step-by-Step Instructions for Bypassing a Thermostat
1. Power Off
Some HVAC systems have a furnace, an air conditioner, and a blower, while others only have a furnace and blower, and some have a reversible heat pump refrigeration unit. Regardless of the number of components, most systems are controlled by a single 240-volt double-gang breaker in the main panel. Locate this breaker in the electrical panel and turn it off.
2. Access the Thermostat Wires
Use a flathead screwdriver to remove the cover plate of the thermostat. There will be a slot where you can insert the blade of the screwdriver between the faceplate and the main body of the thermostat. Insert the screwdriver and twist slightly to pop off the faceplate. Use a Phillips screwdriver to remove the mounting screws that secure the temperature sensor inside the thermostat to the base plate. Remove the sensor to expose the thermostat wires, which are connected to the thermostat terminals on the base plate.
3. Identify the Wires
Refer to the standard color code. If your thermostat is the typical five-wire type used in many HVAC systems, the terminals will be labeled with the letters R, W, Y, G, and C. R represents the red wire, which is the power supply wire for the thermostat. W represents the white wire, which controls the heating system. Y represents the yellow wire, which controls the cooling system. G represents the green wire, which controls the blower motor. C represents the common wire, which is usually blue or black in color.
4. Choose the Function to Bypass
To bypass the thermostat, you will use the red wire to directly supply power to one of the other wires (except for the common wire). Choose the function you want to test. Starting with the blower controlled by the green wire is often the most useful, as you can hear it when it starts. If you have a two- or three-wire thermostat, use the white wire. In a two-wire system, you do not have a choice, and in a three-wire system, the blower usually comes on automatically when the thermostat calls for heat.
Warning
Never connect the red wire and the blue or black common wire together. Doing so may cause one of the fuses in the control board to blow.
5. Connect the Wires Together
Combine the R wire with the wire you have chosen and then switch the breaker back on. If you have chosen the green wire, you will hear the blower in the air handler turn on immediately when you flip the breaker. If you have chosen the white wire, you will hear the furnace and blower start at the same time, or if you have a two-wire thermostat, you will only hear the furnace start. If you have chosen the yellow wire, the air conditioning system should start.
If you are conducting a comprehensive test of your thermostat, you should repeat this procedure for each wire in succession (excluding the common wire). If any component – furnace, AC unit, or blower – fails to start, that component may be defective, or the issue may be with the control board, but it is not the thermostat. At this point, you will need an HVAC technician to further troubleshoot the problem.
6. Finish the Test
Turn the breaker back off, untwist the wires, and reattach them to their respective terminals. If the wires are simply hanging out of the wall because you do not yet have a thermostat, just separate them. These wires only carry 24-volt power from the transformer, so they will not deliver a dangerous shock if someone accidentally turns the breaker back on.
Need to see the process visually? Watch Benjamin Sahlstrom’s video below for a step-by-step demonstration:
Why Does a Thermostat Stop Working?
Every thermostat, including the one that controls the coolant flow in a car radiator, has a temperature sensor. The temperature sensor in a thermostat that controls a central air system uses either a thermistor or a bimetallic strip. It sends electronic signals to a switching device that relays the signals to the appropriate wire terminals. Low-voltage thermostats also have circuitry that controls the system’s operation sequence.
There are many reasons why a thermostat may not send a signal to the control board. The terminals may be dirty or corroded, the wires may not be properly connected, or the thermostat may not be mounted level. These problems can be easily fixed by oneself, but internal issues are best left to a technician, who may recommend replacing the thermostat.
