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The Dreaded Intermittent Short

If you’re a HVAC service technician, you might have categorized your previous service calls into areas of thought – first are the service calls that were easy to fix because you had seen it before.  Second are calls that you will eventually consider a learning experience – oftentimes these are the service calls you will never forget.

Inevitably, you will see a lot of unusual HVAC problems the longer you’re in the field.  The following is a story of one of my most memorable learning experiences that started out as a simple “no cooling” complaint.

A homeowner called my dispatcher with a typical no cooling complaint.  It was their first summer at a new house and the air conditioner seemed to work fine for them all spring.  But as the summer days get longer and hotter, air conditioners are forced to work harder.

My dispatcher scheduled this call for the middle of the next day; I arrived after lunch to diagnose the system.  The customer explained that the air conditioning unit had been working fine until a few days ago, but now it wouldn’t turn on and the thermostat display was blank.

They pointed me in the direction of the thermostat and I checked it for 24-volts.  There was no voltage at the thermostat so I explained that it needed voltage from the furnace to have a display.  I then proceeded to the furnace and checked for 24-volts.

When I pulled the door off the furnace, I found another clue – the transformer was discolored and looked overheated; at our shop we called this a “melted transformer.”  I explained that when there is 120-volts going into the transformer and no low voltage coming out, the transformer cannot be fixed, only changed, and that it’s usually a symptom of having a short somewhere in the low voltage system.  I assured the homeowner I could solve this problem, all I had to do was find the problem in the low voltage wires, fix the problem wires, and replace the transformer.

A service call that was supposed to be easy turned out to be very challenging.

I checked all the wires with my OHM meter, from the air conditioner to the furnace, and from the furnace to the thermostat.  I checked them to ground then checked them against each other.  They all checked out “good.”  In the past, I have found low voltage wires that were cut or smashed during construction, or dogs and rodents had chewed the wires and caused the short that would kill the transformer.  But I could not find a short in this system.

I installed a new transformer and checked the amperage draws, still not knowing what had happened to the first transformer.  I generally don’t like changing parts without knowing what caused the old part to fail so I also installed a 3-amp inline fuse in the system.  If the low voltage system ever drew higher than 3-amps, the fuse should “blow” before the transformer would melt.  When I left, the homeowner was happy to have air conditioning again, but I knew the melted transformer was a symptom, not the problem. At my shop we had a saying, “The difference between a service technician and a ‘parts-changer’ is that the service technician fixes the problem, not just the symptom.”  But in this case, I left this house not knowing if the problem was really fixed.

I was on my way home later that day when I was contacted by my dispatcher.  They said the air conditioning unit from my earlier service call had stopped working again and the thermostat was blank.  I then realized this was an intermittent problem and had my service dispatch schedule me to go to this residence the following morning.

The first thing I did when I arrived back at the residence was to check the 3-amp low voltage fuse.  I found it was “blown” (had electrically opened).  This meant the short in the wires had come back.  I explained the situation to the homeowner – how the low voltage fuse had done its job of protecting the equipment and that there was a short somewhere in the system – I just had to find it for them.

Once again I checked all the wires, following them as far as I could in the walls and the attic, feverishly searching for any hints of a problem, but they checked out good for a second time.  I knew I could not go any larger on my fuse size as the average amperage draw on residential HVAC systems is 1.5 amps.  The 3-amp fuse I installed was the largest fuse that should be used.

Starting to lose confidence in my ability to fix a simple short, I reviewed the service call in my head… I had to be missing something.  The system seemed to have an intermittent short in the low voltage wires as there was no obvious evidence of damage to the wires,   but I must have missed a clue.

I started from the beginning by asking the customer for details on the problem, reiterating how any additional information could help.  The homeowner said the system worked fine after I left, then as the 5:00 o’clock news was coming on, it quit and the thermostat went blank.

This was the clue I was missing – the system had been failing at a certain time and I wasn’t at the house at that time when I had originally troubleshot it.  I told them I was going to put another 3-amp low voltage fuse in for a temporarily fix and I would return at the end of the day.

I returned at 4:30PM and they let me know that they had run the A/C all day without any problems.  So I waited – paced back and forth between the indoor furnace and the outdoor condensing unit.  I wasn’t exactly sure what I was looking for but I knew something was happening in the late afternoon that was causing a short.  It was now almost 5:00 o’clock and nothing had happened yet.  I kept asking myself, “What could be happening at 5:00PM that only occurred when the condensing unit was running, but didn’t occur during the winter or spring?”

I knew something was about to happen and I was determined not to miss it.  I kneeled down next to the condenser, observing a unit that was running fine.  Then it happened.  The lawn sprinklers came on.  One of the sprinklers was aimed directly in the location of the condensing unit.  Before I could make it out of the area, I was soaked from head to toe.  By the time I had regrouped myself, the condensing unit had stopped due to the 3-amp fuse electrically opening back at the furnace.

I was drenched, but thrilled – I had found the missing piece to the mystery!  Now soaking wet and not wanting to walk back into the house this way, I knock on the customer’s front door and asked them to shut off the sprinklers to the backyard.  Ah-Ha!  The homeowners had recently renovated their backyard and landscaped the previous spring season.

Upon closer investigation, I found the condenser wires were slightly cut with bare copper showing.  This bare wire was the short I had been looking for, but it was only a short circuit when the combination of the condensing unit was running and water from the new sprinklers came on at 5:00PM.  I patched the wires, turned the sprinkler head in the other direction and the problem was fixed.

There are two categories your service calls can fall into – the easy to fix and the ones that will be a learning experience, the kind you will never forget.  Try to remember, no tech has seen it all, you may see some difficult service calls as you get more heating and cooling seasons under your belt, but you can’t always have all the answers.

About the Author

Tony Albers is a highly successful trainer who has taught heating and air conditioning classes designed specifically to meet the needs of today’s busy technicians and engineers.  For the past 33 years he has worked to advance the field of heating and air conditioning by teaching classes for IHACI, RSES, Southern California Gas Company, and San Diego Gas & Electric.  In the last 23 years he has traveled extensively throughout North America for US Air Conditioning Distributors and Venstar, giving workshops and seminars for large HVAC distributors and manufacturers.  Albers has been on the Continuing Education Committee for the Institute of Heating and Air Conditioning Industries and has written articles for Indoor Comfort News, HVAC Insider, and the Southern California Chapter of ASHRAE.