Air Conditioning Denver : Troubleshooting a low voltage short : Weatherwise Heating and Air Conditioning
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Air Conditioning Denver : Troubleshooting a low voltage short

by Weatherwise on 06/19/11

Low voltage shorts in your air conditioning system are some of the most difficult failures to diagnose.  Some shorts are direct shorts, which commonly causes a fuse in the furnace to open.  If the control system in the furnace is not protected with a low voltage fuse, additional failures may occur to include damaging the transformer in the furnace.  

One of our Denver air conditioning technicians came across two different low voltage shorts this week.  Both shorts had similar readings caused by different reasons.  The first short in Denver gave the technician a reading of 24 volts at the condensing unit contactor, but the contactor would not engage.  The most logical first reaction is that the contactor coil is defective.  When a new contactor would not resolve the issue, further troubleshooting was needed.  When an experienced tech comes across this type of instance he or she learns not to trust what his meter is reading.  A backfeed or improper common through the control system can give false readings.  To resolve the backfeed problem, the ac tech isolated the control wiring at the furnace by disconnecting the thermostat wiring at the control board in the furnace.  Testing the resistance of the wires determined that none of the wires were shorted to ground or to each other.  
When all of the control wiring is removed from the diagnostic, then the ac system can be tested by jumping the controls directly at the control board.  The Denver air conditioning repair technician needs to make sure the thermostat is disconnected as well.  In this instance, the air conditioner tested ok with the thermostat removed.
The 24V reading from the thermostat to the contactor was a false reading.  When the thermostat was replaced and the control wiring was connected the entire system ran properly.
The second low voltage short failure that week was traced to improper wiring. The homeowner had their furnace replaced over the winter and the control wiring for the air conditioner in the furnace was improperly connected.  The technician began by reading 24V at the contactor in the condenser, but the contactor would not engage again.  When the common leg of the control wiring was bypassed by connecting the common side of the contactor to ground the contactor engaged. This lead the technician to trace the missing  common control wire.  The failure was in the furnace.  Whoever had installed the furnace had not wired in the common control wire.  Instead the low voltage wiring to the condenser contactor had been looped between the R lead and Y lead at the furnace terminal board. No common leg of low voltage wire was connected at all.  Rewiring the air conditioner control wiring in the furnace easily fixed the problem.
Low voltage shorts come in a variety of disguises.  It takes time and experience to properly troubleshoot shorts.  Weatherwise Heating and Air Conditioning strives for 100% diagnostic service.  Weatherwise guarantees to properly diagnose your heating and air conditioning equipment right the first time, or the service call  is free.  For the best air conditioning service Denver has to offer call Weatherwise today.

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