Diagnosing the failure of a low voltage transformer in a furnace or commercial building roof top unit
It is very unusual for a low voltage transformer in a residential furnace to fail and when they do fail, it is normally a symptom of some other component failing.
The photo above is of a low voltage transformer from a furnace.
WARNING: These repair activities involve working on high voltage equipment that poses serious electrical shock risk. Follow all safe electrical work practices and ensure that only qualified, trained service technicians perform these repair activities. Hazardous Voltage! Failure to disconnect power before servicing could result in death or serious injury. Disconnect all electric power, including remote disconnects, before servicing. Follow proper lockout/tagout procedures to ensure the power cannot be inadvertently energized.
A recent service call on an air conditioner involved a blown low voltage transformer. There was no low voltage power (24V) out to the air conditioning unit or inside the furnace. We measured the voltage across the two primary side terminals of the low voltage transformer and found 120V. Then we measured the voltage across the two low voltage terminals of the low voltage transformer and found 0V. As a check, we disconnected both leads on the low voltage side of the low voltage transformer and re-checked voltage. We still found this voltage to be 0V. This indicates a failed low voltage transformer.
As a low voltage transformer rarely is the source of the failure, we proceeded with troubleshooting. We first looked for any additional loads that were connected to the low voltage circuit that may have been pulling too much current.
To calculate how much current a low voltage transformer will supply, identify the VA rating of the low voltage transformer. This VA value is usually indicated on the transformer sticker. A typical low voltage transformer inside a residential furnace is rated at 40 VA. Then divide the 40 VA by the secondary side voltage rating which is 24 volts. So 40 VA / 24 V = 1.67 A. So the maximum circuit ampacity of the low voltage circuit is 1.67 A.
Add up all of the individual loads on the low voltage circuit and the sum of all these current loads must be lower than the 1.67 A calculated current from the transformer. If the sum of the current loads in the low voltage circuit exceeds the 1.67 calculated amperage, then replace the 40 VA transformer with a larger transformer, or remove some low voltage loads and place these on their own transformer. For example, if a humidifier solenoid valve is on the low voltage circuit and the sum of all the low voltage current loads exceeds the calculated current from the transformer, then install a dedicated low voltage transformer for the humidifier.
In the case of our service call, there were not any added loads in the low voltage circuit. We began to check each load on the low voltage circuit looking for a damaged component, a failed component, or signs of a short to ground in the circuit.
Small animals such as rodents or snakes can cause damage to components. Usually this type of damage is obvious and there may be nesting materials from the animal present that must be removed.
Components do fail and this case can be harder to notice visually. One of the more likely components to fail is the contactor in the outdoor unit (the air conditioning unit). The contactor changes position (from open to closed and from closed to open) a large number of times over a given season. The mechanical stress on the contactor from the repeated cycling can lead over time to failure of the contactor in such a manner that current is allowed to follow an unintentional path through the failed contactor to the metal structure of the air conditioner (or furnace, whatever the equipment is) and to a ground wire attached to the appliance. This will result in a short in the low voltage circuit that may raise the current load on the transformer to the 1.67 A maximum which will cause the transformer to fail.
The above photo is of a contactor that is similar to the one that failed. The electrical leads supplying the contactor from the power source connect to L1 and L2. The various loads in the unit connect to T1 and T2.
Shorts can happen at other places in the equipment such as if low voltage wires are pinched under a metal cover on the component or if vibration causes the low voltage wiring to rub on parts of the machine and over time the outer insulation is wore off the wire resulting in a short to ground. Rodents as well are known to chew insulation off of wiring runs causing shorts.
Note that in the drawing of a low voltage circuit above, a short to ground can happen anywhere along the circuit and in any component.
It might seem that the low voltage fuse that is found on the circuit board of the furnace would blow before the low voltage transformer would fail on high current draw, and of course this can happen. However, the fuse in the low voltage circuit (usually an automotive style fuse) is normally rated for 3 or 5 amps and a short that involves a lower current draw than this such that current draw on the transformer is below the 3 amp or 5 amp rating of the fuse would result in the transformer failing before the fuse would open.
The photo above shows a pink automotive style 3 amp fuse on a furnace circuit board. The amperage rating is normally stenciled on the end of the fuse and is normally fairly easy to read, even though this particular fuse amp rating is harder to read.
The photo above shows a violet colored 3 amp automotive style fuse on a residential furnace control board.
If it is not clear what amp rating fuse is installed, then check the installation and service documents that should be with the equipment. A call to the manufacturer may be necessary.
In the case of our recent service call, this is exactly what happened. The contactor had failed in a way that resulted in a short that overloaded the transformer. However, the low voltage fuse never blew.
Troubleshooting the contactor did not reveal any fault. Measuring resistance from each of the low voltage terminals (with wires disconnected) to ground indicated very high resistance (OL on the meter – for open loop). However, when we replaced the contactor, the short went away and the new transformer was drawing less than 1.67 A current draw.
There is a handy diagnostic tool called a Lil Popper that is a resettable circuit breaker rated usually at 5 A that has two electrical leads that can be plugged into the terminals where the automotive style fuse normally goes. During circuit testing, if a high current condition exists, the Lil Popper circuit breaker will open and can then be reset by pushing the reset button. This saves one from burning through a bunch of fuses during circuit troubleshooting.
The photo above is of the resettable breaker called a Lil Popper.
If the equipment in question is a roof top unit instead of a residential furnace, it is good practice to ensure that any low voltage transformer high side is wired for the correct line voltage. It is common for the low voltage transformer that is shipped with the roof top unit to have options for either 230v or 208v primary voltage. Based on results of a voltage test using your voltage meter, ensure that the correct primary side terminals are connected.
For troubleshooting a failed gas valve in a residential furnace or commercial building roof top unit, review the blog on troubleshooting a failed gas valve.
There is a handy blog for helping to decode the meaning behind Trane furnace model numbers for residential furnaces. Or, for decoding the meaning behind Trane roof top unit model numbers, there is a separate blog.