Updated: Oct 24, 2022
In the world of HVAC, low voltage shorts are a common occurrence on a service call. Low voltage shorts occur when an electrical circuit finds a shortcut to the ground or common that requires little or no resistance, rather than completing the designated path.
In an HVAC system, low voltage shorts are often indicated by a blown fuse on a control board or in the circuit wiring. Unfortunately, knowing that a short exists is only the first step. The fuse acts as a safety on the hot side of the transformer. It prevents a direct short between the hot and common from damaging the transformer. Remember that the common and ground are usually connected so if the hot shorts to ground, it will blow the fuse.
A circuit must have a load that gets powered between the hot to common and not a direct path without a load. On a fault, the fuse opens the electrical circuit so that a direct path cannot be completed between the hot and common side of the transformer. If a fuse is bypassed or is not installed, and there is short in the system wiring, this could damage components or potentially cause a fire within the unit or wiring.
Low voltage shorts can have several causes and if a technician does not know where to look, this can turn into a time consuming issue. A short may be obvious due to an exposed wire, rodents chewing on wires, wire insulation wearing down over time or some other complication, but do not expect this to always be the case. To tackle some of the harder to find shorts, we are going to look at techniques to isolate and nail down where a short has taken place. This will then allow us to begin the repair process.
We can break our search into the following three sections:
· Wiring to the Thermostat, Wiring to the Outdoor Unit, the Thermostat, and the Contactor
· Wiring to Safety Switches
· Control Board
Determining the Problem
When arriving to the service call and noticing a blown fuse, begin by making sure that the thermostat is not calling for fan, AC, or heat, then turn the power off to the unit. Replace the fuse and turn the furnace or air handler power back on. Noticing when a fuse blows is the first step before troubleshooting. A technician should not repeatedly allow a fuse to blow over and over until they have solved the problem. However, first noticing the problem is important. The problem could be intermittent or an obvious direct short.
Is the Problem in the Thermostat Wiring?
Start by making sure that the thermostat mode is set to off and change the fan to the on position. If the fuse blows at this point, you know that the short is in the green, fan wire. This is because on a call for “fan”, the thermostat connects the red power wire to the green fan wire, therefore making the green wire hot. If for some reason, the green wire is damaged and is contacting the ground frame of the unit or the common wire, this will blow the fuse. If the fuse blows, turn the power off and swap the damaged wire for a spare wire to fix the problem. If the fuse did not blow, when setting the thermostat to the fan on position, continue to the next step below.
Set the fan to auto and the system to cool. Then set the thermostat to call for air conditioning to turn on. At this point, the red wire will touch both the green and yellow wires in the thermostat. If the fuse blows at this point, the yellow wire is damaged and will need to be swapped. If the fuse did not blow, continue to the next step below.
Set the fan to auto and the system to heat. Then set the thermostat to call for heat by raising the temperature setting. At this point, the red wire will touch the white wire in the thermostat. If the fuse blows at this point, the white wire is shorted and will need to be swapped. If the fuse did not blow, the problem is not with the thermostat wiring or it is an intermittent issue.
(Installation Note: It is always a good practice to install thermostat wiring that contains more wires than is needed for the system to operate, I.e., run 6 wire for applications requiring 5 wires. This way, if there is a shorting problem down the road, just one wire can be switched out without having to replace the entire wiring.)
Finding the Thermostat Wire Short with the Multimeter
First, turn the power off to the furnace or air handler. Before beginning to test with the multimeter, it is important to realize that the common thermostat terminal on the control board, is connected to ground. The multimeter should be set to measure electrical resistance and testing is easiest if an alligator attachment is used on at least one probe.
When using a multimeter for testing, you will notice that when touching common with one probe and the ground frame with the other probe, your meter will read 0.00 ohms of resistance. This indicates that a direct, non-resistant connection exists. This is the same reading that is present when holding the two multimeter probes together.
For the following procedures, we will be testing for a connection between any potential power wires and the ground/common terminal, which would indicate a short. If no connection exists and the wires are good, the meter will read OL. If there is a short between a potential power wire to common/ground, the meter will display 0.0 ohms to 0.5 ohms.
The first step is to remove the thermostat cover or face plate (the switching section that can easily be removed from the thermostat back plate). The next step is to disconnect the thermostat wiring from the control board, except for the common wires.
Once this is done, the multimeter is used to test each wire along with the common terminal. This is done by clipping one probe onto the common terminal and using the other probe to connect to each of the other wires, one at a time. If no connection or short exists between each wire and the common, the multimeter should read OL. An exception to this is for the Y wire that leads from the furnace or air handler to the outdoor unit. This because the Y wire is connected to the contactor coil along with the common on the other side of the coil.
Wiring to the Outdoor Unit and Contactor
Between the Y wire leading to the outdoor unit contactor and the common terminal, where the common wire is coming back from the outdoor contactor, there will be a reading in ohms. This is a measurement of the resistance value of the coil on the contactor. This value can be confirmed by reading the resistance value across the contactor coil at the outdoor unit and comparing. However, if the reading between the common terminal and the outdoor unit Y wire is 0.5 Ohms or less, this may be an indication that a short exists. A contactor coil will measure a resistance value greater than 0.5 Ohms.
If the measurement between each wire to common is OL, then the thermostat wiring from the control board to the thermostat is intact and not the problem. Additionally, if the Y wire from the furnace or air handler to the outdoor unit does not have a resistance reading lower than 0.5 Ohms to the common, then that is also not the problem.
At this point, the thermostat face cover can be put back on and the process of testing each wire to common can be performed again with the multimeter measuring for electrical resistance. The power to the furnace or air handler remains off. The thermostat can be set for fan on. This will allow the technician to test the thermostat for an internal short to common.
Be aware that a reading in megaohms may appear when testing the red wire, especially if the thermostat has a battery. This is very close to OL so it does not indicate a short. Remove the batteries for the test to be sure and measure again. If each wire passes the testing process, then the thermostat is good and is not causing the short.
Testing the Safety Switches and Wiring
The power remains off. At this point, we can disconnect the low voltage wiring harness on the control board that leads to the safety switches. We can then test the safety switches and wiring for a short to ground.
The first thing to notice in a gas furnace is that there is a designated common wire that is part of the wiring harness that is connected to ground (as indicated in the picture above by the green/yellow wire). Be sure to reference the manufactures’ wiring diagram to determine which wire is the common/ground or look for a wire connecting directly to the ground frame or the base of the gas valve.
After locating the common wire, a multimeter can be used to test each wire against the common wire, at the harness. This is a connection to both common and to the ground. To test, press one probe into the side of the common wire and use the other probe to press into each of the other wire slots on the harness. (remember to not deform the front of the wire connector with the probe. If no short exists between the wires and the common, the multimeter should read OL.
If one of the wires measures 0.00 Ohms to common, then a short exists to ground in either the wiring or the switch. Remember, some of these wire sections may lead to multiple safety switches wired in series such as the thermal limit and roll out switches. If a short is indicated in the wiring leading to the safety switches, disconnect the safety switches from each other. Continue the process of isolating and testing wires, switches, or components against ground/common as necessary as well as visual inspect until the short is identified. Make any necessary repairs, as needed.
If all the previous tests have been completed and there is no indication of a short in the wiring, thermostat, contactor, safety switches, or any other low voltage components, the control board can be tested. This should only be done as a last resort as utilizing the electrical resistance measurement of the multimeter to test the control board can possibly result in damage to the control board. This is because the multimeter sends low levels of current through the board to test for resistance. The power must remain off for this test.
To test for an internal short in the control board, remove the fuse and the thermostat wiring from the thermostat terminals on the control board. At this point, take a resistance reading between the C (common) terminal and the R (power supply) terminal on the board. If the multimeter reads less than 0.5 Ohms of resistance, the board has been shorted and will need to be replaced.
For additional examples and information, check out our video on Finding Low Voltage Shorts Fast! HVAC Troubleshooting!
Check out our section on Thermostat Wiring Diagrams!
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Published: 04/08/2021 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/