Basic Electrical Checks

 Happy New Year to everyone!  Let’s start this first blog of the new year with some basic electrical troubleshooting.

Since most HVAC equipment today has solid state electronic controls (computer boards), POWER to the unit being worked on is very critical.  Polarity, grounding, dedicated circuits, etc, all affect how a unit is going to function or possibly malfunction.

Let’s start with having the furnace on a dedicated circuit. If the furnace “shares” power with another furnace, appliance, or even a flourescent light, there could be back-feeding from that other device that will cause nuisance issues.  Something as little as 2 to 3 volts present on the neutral wire can cause controls to respond with a reversed polarity fault code because the board sees power on both legs and doesn’t know what to do.

Of course, true reversed polarity (hot and neutral switched) of the incoming power will render the unit inoperable. 

One of the most important wires to any unit is the ground wire. This provides a number of things for the unit to work properly.  FIRST and FOREMOST, the ground wire is a safety.  If a component develops a short, but does not have a good unit  ground,  the unit can become a potential killer. Example — a motor develops a short but there is a poor ground to the furnace, Without the ground, the circuit breaker or fuse may not “pop” and shut the unit off.  Along comes Mr. homeowner or Mr. Service Tech, and he grabs the unit and is leaning on a pipe — he now becomes the ground and the “fuse”.  He is now the recipient of the short and could be electrocuted or badly hurt!

Too often, a dedicated ground is not run to the unit and the conduit is used as the ground. This is acceptable under some codes, but what happens if corrosion from basement dampness gets into the connectors? The ground to that unit is now compromised and can cause problems and safety issues with the unit.

Why is correct polarity critical?  Why does the power wire (or hot) have to go to the correct place?  I’ll give you another example — How does flame rectification work? We pass line voltage through the sensor through the flame to ground (another reason for a good ground :>)) to create a DC microamp signal back to the control. Any time the power switch is turned on at the unit, there is power at the flame sensor (you can measure it to ground from the tip of the sensor and is usually anywhere from 40 VAC to 110 VAC depending on the control and manufacturer) because the control is always looking for flame.  If you reverse the hot and neutral to the control, now you have no voltage present at the sensor to look for flame and it has no way to detect it when it is present. Now the unit locks out due to ‘loss of flame” because there is no way to detect it.

When I teach classes, I always tell service techs that they need to perform 3 checks on the electrical power to the furnace to assure proper grounding, clean neutral, and proper polarity. Those 3 checks are:

  1. check the hot to neutral and the hot to ground. BOTH should read the same. 
    1.  If hot to ground is less than hot to neutral (by even 2 volts) then you have a poor ground to the unit.
    2. If hot to neutral is less (by even 2 volts), you have a poor connection in the neutral.
  2. Check the neutral to ground
    1. they should read “0” volts.  If voltage is read between neutral and ground, the furnace is not on a dedicated circuit and you are getting a back feed from something else on that circuit.
  3. and of course, make sure the hot is connected to the LINE terminal on the unit.

Of course, you must have the proper meter to perform these checks but by doing these as part of a start-up or any service call, you can eliminate a lot of problems with the equipment.

One last thing to check is the polarity and grounding of the low voltage transformer.  With your meter, read from the “R” terminal to ground — it should read around 24 VAC.  Then read from common  to ground — it should read “0” VAC.  Most units today use a grounded common.  If you read say, 18 VAC from R and 10 VAC on the common, you lost the ground to the transformer and the unit will not work.  Even LOW VOLTAGE polarity and grounding is critical with today’s equipment.

A lot of service issues can be eliminated if the time is taken to do these simple, yet important checks of the power to a unit.


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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