Low voltage wiring
Low voltage or control power is used for 2 reasons. One, the voltage is low enough so as not to be dangerous. Two, the control components can be made to handle smaller loads and are more accurate that line load controls,
The control voltage most commonly used is 24 VAC. The source for this voltage is the 24 volt transformer which has a primary winding that uses the line voltage and a secondary winding that puts out 24 VAC.
The control voltage most commonly used is 24 VAC. The source for this voltage is the 24 volt transformer which has a primary winding that uses the line voltage and a secondary winding that puts out 24 VAC.
The 24 volt transformer
The transformer will only work with AC current. They are not generally designed to handle large loads. They are rated in volt-amperes or VA. VA is similar to wattage in that voltage multiplied by amperage equals VA. An example is a 120 volt transformer has a primary voltage, or input voltage of 120 volts. The secondary voltage or output voltage is 24. This is accomplished by varying the number of turns in each winding. If the primary had 100 windings, and the secondary had 10 windings, and 120 volts was applied to the primary, the secondary voltage would be 12 volts. Additionally, if 1 ampere was passing thru the primary, 10 amps would be available in the secondary. Transformers come in may shapes and sizes. They can have multiple input voltages and multiple output voltages. The video below shows some of the types available.
So, why does this transformer work? The transformer is an inductive load. Inductive loads are usually only used with alternating current. The reason for this, without going into long explanations is, the magnetic field that surrounds all energized wires is constantly rising and collapsing as the voltage rises then falls. When the field rises, it builds up in intensity, then collapses as the voltage drops to zero then rises again in opposite polarity. This continues at 60 complete cycles per second. When these fields rise and collapse, they cross any wire that is near them. The the field crosses the wire, it induces a current in that wire. There is no electrical connection between the 2 wires but power is transferred. The voltage of the power induced is dependent on the relation of the number of wires in the primary to the number in the secondary winding. When the wires are placed close to an iron core, the iron concentrates the magnetic field to make the transfer more efficient.
Two transformers in a circuit
This is about phasing of transformers. This is an operation that is seldom done anymore but was common when outdoor units and indoor units had separate transformers. If the transformer secondaries are hooked up incorrectly, there transformers actually fight each other. The result will be one transformer will fail. Usually the weaker transformer will fail. For this reason, only transformers of the same VA should be used. That is if one is 40 VA, then the other should be the same. The easiest way to do this is to hook one secondary wire to the other, then hook the other two together. If there is a spark when connecting the second set, the transformers are no phased. If there is no spark, you are ok. You can also use a voltmeter between the two. If the voltage is twice the secondary voltage, the transformers are out of phase. If the voltage is zero, its ok. The video below demonstrates this operation.
Terminal board
In most systems there are 2 terminal boards. One at the back of the thermostat and one at the air handler/furnace. So, what do the terminals mean? A little history first. When residential furnaces first used thermostats, 2 wires were required. The wire size was #18. The only wire available had a white wire and a red one. It was decided that the red would be power and white would be the switched line to power the furnace. So we have 2 of the terminals covered. So, Red is power and White is heat. Then there is Y. Y is used for cooling. Who knows why this was chosen over blue that seems to make sense we don't know but cooling is usually Y. Next is G. We chose to use G to operate the fan. C is usually used for the other side of the transformer or common. I won't say these colors make sense, especially to the electricians, but that's how they are usually done. The video below explains this concept.
Color code for heat pumps
The most complicated of the low voltage wiring is that for the heat pump. When there is only a furnace, you need 2 wires to operate the furnace. Add air conditioning, and you need another to operate the air conditioner and you need another to operate the fan with the air conditioner. That gives you 4. 5 if your thermostat needs a common to power itself. When we add the heat pump, we need one to operate the reversing valve for the change from heat to cool. Because the heat pump will not always be able to keep up with the heat load as the temperature drops, a second stage heat will be needed. So we will need at least 7. Looking at the colors below, you have the red for power, white for heat, yellow for cool, green for fan, orange for reversing valve if it is energized in cool, blue if the reversing valve is energized in heat, black for common. The brown can be used for second stage or supplemental heat. I don't mean to say this code is always used, but it seems to be the most common I have seen. The video below gives an explanation of this color code.
Low voltage troubleshoot
When there is a failure of the low voltage transformer, the transformer itself is seldom the problem. Generally, there is an overload in the circuit. When there is an overload, the amperage draw goes up and the weakest part of the circuit fails. This part is usually the source of the power, the transformer. When the technician finds the transformer failed, then replaces it and starts the system, usually the result is another failed transformer. In many cases, the transformer is protected by a fuse. Sometimes the fuse is installed inside the transformer which means the transformer must be replaced. Sometimes the fuse is located in the circuit, sometimes it is mounted on the control board. In this case, the transformer will not fail but the fuse will blow. In order to not burn up the new one or keep blowing fuses, there is a method to this operation. Replace the transformer as usual. Before power is applied, a clamp meter is placed on one of the secondary wires. If the amp draw is over the rated load of the transformer (a 40 VA transformer should not draw over 1.6 amps), there is probably a short. If there is a fuse inside the transformer, this method cannot be used. I recommend that the fused transformer not be used. A fuse can be added the the secondary line after the troubleshoot is complete. If there is a fuse on the control board, it can be jumpered and the clamp meter placed around the jumper. If the draw is excessive, immediately open the circuit. If the amp draw is normal, you can start turning on different loads until the draw increases, Look below to "how to find the low voltage short" for further information. The video below covers how this is done.
For an idea of what happens when a transformer is overloaded, check the video below.
How to find the low voltage short
Low voltage or control voltage shorts (usually 24 volts) troubleshoot is somewhat similar to any electrical troubleshoot process.
The most common symptom is the low voltage fuse (usually mounted on the control board) has blown. On older units, the fuse may be mounted in the transformer. If it blows, the transformer must be replaced. The fuse is usually an automotive type. You will probably replace the fuse and it will blow immediately or blow when the unit starts. To eliminate constant blowing of fuses, the fuse terminals can be jumped. Be careful here. There is a way to do this without damaging components. With the power off, connect the jumper. Then clamp the jumper wire with a clamp ammeter. In this way, you can detect and overdraw of power. Any draw of more than 1.6 amps (residential) indicates a short. If the short is found, immediately cut off the power. At this point, all wires should be removed from the terminal board. The video below gives a demonstration of this part of the process.
The most common symptom is the low voltage fuse (usually mounted on the control board) has blown. On older units, the fuse may be mounted in the transformer. If it blows, the transformer must be replaced. The fuse is usually an automotive type. You will probably replace the fuse and it will blow immediately or blow when the unit starts. To eliminate constant blowing of fuses, the fuse terminals can be jumped. Be careful here. There is a way to do this without damaging components. With the power off, connect the jumper. Then clamp the jumper wire with a clamp ammeter. In this way, you can detect and overdraw of power. Any draw of more than 1.6 amps (residential) indicates a short. If the short is found, immediately cut off the power. At this point, all wires should be removed from the terminal board. The video below gives a demonstration of this part of the process.
With all the wires removed, except the common, we can turn on the power. The amperage draw on the meter should be zero. At this point, we can install one wire to the terminal board to its position. The first wire to connect should be the R. The common is already connected. If there is a short when power is applied, you know the problem is between R and common. The problem must be in the R wire somewhere. The common is connected to the chassis of the unit, so somewhere the R wire is touching the chassis. Probably the first action would be to remove the R wire from the thermostat. Is the short still there? If it is, the problem is between the terminal board and the thermostat. Look for removed insulation near penetrations of the unit.
If the amp draw is 0, then we must connect each wire in turn. The white is usually for heat. Connect the white to W. Then turn on the thermostat to a call for heat. If the amp draw is excessive, the problem is in the W circuit. The problem is in the W wire or load it connects to. If there is no excessive draw, connect another wire and call for that component to be energized. When the amp draw increases, you know the circuit that is causing the problem. This is a slow, methodical process. Do not skip any steps.
If the amp draw is 0, then we must connect each wire in turn. The white is usually for heat. Connect the white to W. Then turn on the thermostat to a call for heat. If the amp draw is excessive, the problem is in the W circuit. The problem is in the W wire or load it connects to. If there is no excessive draw, connect another wire and call for that component to be energized. When the amp draw increases, you know the circuit that is causing the problem. This is a slow, methodical process. Do not skip any steps.