Bob and Tim are discussing the job that they are about to start. They are at a residence and the system has a blown low-voltage fuse. Tim said, “Let’s just change the fuse and try it and see if the system will start up. The only thing it can do is blow another fuse."

Bob said, “I think there is a better way. Fuses do not blow for no reason, it means there must be some problem in the low-voltage control circuit. I think we should shut the control voltage off and see what the Ohm readings are in the low-voltage circuit. The fuse is blowing during the cooling cycle so it stands to reason that we probably have a problem in the cooling circuit."

Tim asked, “How many devices do we have in the cooling low-voltage circuitry?"

Bob explained, “This is an older system so there is no circuit board. The low-voltage power supply is in the furnace in the garage. Looking at the diagram in Figure 1, the low-voltage transformer (power supply) is in the indoor furnace control panel and it passes power to the thermostat through the “R” wire. The thermostat can be called a distributor, it distributes power to the power consuming devices through the other three wires at the thermostat. The “R” wire is the hotwire and the other three wires are “Y” cooling, “G” indoor fan wire, “W” the heating wire. For example when the thermostat calls for cooling voltage passes back down the “Y” and “G” wires which will start the indoor fan and the outdoor condensing unit. When there is a call for heating, the thermostat sends a signal down the “W” wire to the furnace and the furnace starts. So the thermostat controls what is operating, the voltage to the various circuits just pass through the thermostat, the thermostat is not considered a power consuming device."

Tim said, “I think I have this down and I can look at the diagram and follow what we’re doing. What do you suggest we do next”?

Bob explained, “Notice that the “R” wire went to the thermostat directly, when the thermostat contact closes it would route the “R” wire back to the appropriate device. There are two more wires leaving the indoor section or furnace and those two wires are the “Y” and the “O” and they both go to the outdoor condensing unit. The “O” wire is the common wire from the transformer. Remember that the potential 24 V voltage at the transformer would be checked between the “R” and the “O” wires. There was no need to take the common wire to the thermostat because it doesn’t do anything at the thermostat, it has no job at the thermostat unless there is a battery to charge or a light bulb to light, so it goes merely to the device where it’s needed. In this case it would be the cooling relay in the condensing unit. In the heating cycle the “O” side of the transformer would be furnished directly to the power consuming devices in the furnace. Now let’s get down to business and see if we can find a short circuit or something else causing the problem”.

Tim then asked, “What would be the first step?"

Bob explained, “turn off the power to the furnace and disconnect the two wires on the transformer and replace the fuse. There is no power on circuit so we will not blow the fuse. Now set the Ohm meter on R X 1 and take a reading across the two wires.”

Tim removed the wires and checked the Ohm reading and said, “The Ohm reading is 24 Ohm’s, what does that tell us?"

Bob then explained, “The transformer on a typical system like this would be rated at 40 V/A. If you divide the voltage in to the volt amp rating 40/24 = 1.67 amperes, (amps), this is the maximum amps that the transformer can deliver. The fuse will be rated somewhere close to 1.67 amps."

Tim said, “Sure enough the fuse was a 2 amp fuse. So that means that the current flow through the fuse from the transformer was greater than the transformer could deliver over a long period of time.”

Bob said, “That is a good deduction. Now if we carry this a little further what we can find out using a formula is we can determine what the resistance should be in the circuit, R=E/I, R=24/1.67 =14.45 Ohm’s. If the resistance is less than 14.45 Ohms, the current draw in the circuit will go up, and blow the fuse. Measure the resistance in the circuit and see what it reads.”

Tim measured the resistance and said, “The resistance is 10 ohms”.

Bob said, “Now we have to go and find what that low resistance is because 10 ohms will cause 2.4 amp current flow (24/10=2.4). Since the unit was running and cooling let’s measure the “Y” wire to the “O” wire and see what we get.”

Tim took the wires loose at the terminal board in the indoor unit and checked the resistance and sure enough it was 10 ohms. They moved out to the condensing unit and disconnected the two wires on the compressor contactor and checked the coil in the contactor and it read 10 ohms. They had found the problem. This lower than normal ohm reading through the contactor coil would be described as a “shunt”, not a “short”. A short is normally described as a 0 ohm reading, just like touching the two leads of your ohm meter together would give you a zero reading.

Tim then said, “Wow, that was an exercise in electricity. It’s interesting that you can run down a bad circuit using a VOM (Volt OHM Miliamp meter). I guess you have to use one long enough to become comfortable with it."

Bob then suggested, “When I first started I took my VOM meter in the house to the kitchen table and got every device that I could find and checked the resistance through it. I was able to determine an awful lot about power consuming devices. You have to remember that the more resistance equals less amp draw. If a device starts dropping in resistance, it could very well mean that some of the coils are rubbing together and reducing the resistance, a coil such as the one in the contactor is just a long piece of wire. The wire is coded with a very thin coat of shellac are far to keep it from touching copper coil to copper coil. I un wound several coils like that to see approximately how much wire there was in a coil. A high resistance coil is a very long wire”.

Publication date: 3/19/2018