One of the most frustrating things for a technician is servicing a malfunctioning piece of equipment and then suddenly it’s working. You don’t know what you did, you can’t get it to fail again and you have to explain to your customer what you did to fix it.
This scenario is usually caused by a ground or phase issue disguised as a flame sensor, ignitor, twinning, or circuit board problem.
Follow these steps to troubleshoot and find the issue:
The first thing to do is carefully remove the outer door, check for fault codes, cycle the power and let the equipment run without touching anything.
Look for obvious ignitor problems such as cracks, hot spots or the ignitor not heating up at all. Look at the flame sensor - does it look dirty, is there metal scale grounding it to the burner, is the porcelain insulator cracked, is the wire going to the sensor melted or broken?
If everything looks ok, carefully move all the ground wires to see if anything is loose. Relocate ground wires on painted surfaces to shiny chassis surface. You may need to sandwich the burner ground wire between the manifold and the burner housing. When you are sure all the internal grounds are tight cycle the furnace again, if it works normally your problem is solved.
Check your main ground wiring: 1. Check for 120V from the door switch to chassis ground. 2. Check for 0V from L2 (neutral) to chassis ground. 3. If they are backward flip them around.
If everything looks good but you are still having a problem, test the unit ground by first finding an outlet near the equipment that is properly grounded (use an inexpensive outlet tester as shown in the above image).
Field fabricate a ground wire jumper cord by using an old three prong extension cord. Remove the female end of the cord and using electrical tape, individually isolate the hot and common wires leaving the ground wire exposed (see image 2).
Plug the male end of your jumper cord into the outlet you previously tested. Check for voltage at the furnace between the green jumper cord wire and the equipment chassis; if voltage is < 3 volts attach the green wire to the furnace chassis, cycle the power and see if the furnace works normally. If it does, repair the ground to the furnace. If the voltage between the green jumper cord wire and the furnace chassis is > 3 volts, check for knicks, staples or sharp duct edges cutting into the high voltage wire. The circuit should be one continuous wire between the furnace and breaker panel.
If the ground is in tact, you may have a Phase problem causing your issue.
The power supply in the U.S. is rated at 60 Hertz, which means that it takes 1/60th of a second for the current to complete a cycle or 60 cycles/second. Phase is used to characterize two or more signals whose relationship is rising together (in Phase) or rising opposite of each other (out of Phase).
To check the phasing between your primary voltage and secondary transformer without an oscilloscope, check the voltage between your door switch hot lead and your transformer secondary hot lead. If you have around 96 volts your furnace power and transformer are In Phase.
If you have around 144 volts your furnace power and transformer are Out of Phase and you need to swap the high voltage leads to your transformer (see diagram below).
It is common to find new equipment that has been twinned together on two separate circuits, make sure that both circuits go to the same main leg on the electrical panel to avoid phasing issues.