This 3D video shows how low voltage wiring works in a typical heat pump system. We cover schematics, some electrical circuit basics, how the low-voltage circuit accomplishes its tasks, and some best practices to maximize the longevity of your low-voltage circuits.
There will be three diagrams: the condenser, heat strips, and air handler each having its own schematics.
Those schematics show various types of switches, including basic switches, pressure switches, thermal switches, float switches, and contacts. An open switch has no electrical path, meaning that the equipment can't turn on. On the other hand, a closed switch allows electricity to pass through, so the equipment can turn on. Switches will be either in the normally closed or normally open position, and the schematics will indicate the normal state of the switch. It's also worth noting that switches are power-passing devices and do not use or consume electricity. In the condenser, you will see high and low-pressure switches. The heat relay on the heat strips will have a prominent pair of normally open contacts. The air handler usually has a normally closed float switch, which opens when water fills it up and breaks the circuit.
Compared to switches, loads are power-consuming devices that transform electrical energy into some other form of energy. The contactor coil and reversing valve solenoids are examples of loads.
Within the thermostat, G (green) goes to the blower, Y1 (yellow) is for the contactor, O/B (usually orange, sometimes dark blue) is for the reversing valve, RH (red) is for constant 24v heat (RC is constant 24v cool), C (blue) is for common, and W2 (white) is for auxiliary heat. In this case, we are using ACC+ (black) for dehumidification.
The transformer is the source of the 24v power that starts everything. Transformers take high voltage (often 230v) from the power company on the primary and drop it to 24v on the secondary, which then goes to the integrated circuit board. The primary and secondary interact electromagnetically; they don't actually touch.
When the 24v power makes it to the integrated circuit board via SEC1 or SEC2, it passes through the 5-amp fuse and powers the R terminal on the terminal block. Due to that configuration, many techs wire the float switch to break R. Breaking R will stop power to the thermostat and the defrost board at the condenser. (However, some techs may break Y instead.)
Y goes from the thermostat to the terminal block and then to the contactor coil. The contactor coil needs 24v to pull the contactor in and close the contacts that allow power to reach the compressor and the condenser fan. Y goes in and out of the high and low-pressure switches before reaching the contactor coil.
W2 connects to the heat strips and W on the defrost board from the terminal block. So, either the thermostat or the defrost board has the capability to bring on electric heat.
The G terminal supplies constant 24v power to the blower fan inside the air handler.
Common feeds from the common side of the transformer and goes to the thermostat to complete the circuit. It then provides 24v common to the defrost board, which also completes the circuit on that side of the unit. We need common in all cases because it provides a path back to the transformer; otherwise, the circuits would all be open and would not work.
The O terminal's wire passes through the terminal block from the thermostat. Then, the 24v power goes to the condenser to energize the reversing valve solenoid in cool mode. (Note: Ruud and Rheem systems energize the reversing valve solenoid in heat mode, and these may have dark blue instead of orange wires.)
When we use ACC+ for dehumidification, the blower will only reach full speed when the DH terminal is energized on the terminal block.
When stripping back the wire jacket, try to minimize nicks by making a small vertical cut and pulling the sheath back. Cut the sheath you've pulled back. Cut the tips of the conductors, as you may have nicked them. Make careful cuts to expose the bare conductors; you want them to be able to reach the terminal block or touch under the wire nut, but they shouldn't be exposed.
When routing wires or cables through cabinets or other areas where they might get cut, be sure to use proper grommets. Don't run wires over metal objects like the capacitor, as shorts may occur. When routing the wires in a spot where they might chafe, rub out, or otherwise suffer damage, use a conduit whenever possible to protect the wires. Also, use zip-ties to secure wires with a bunch of slack.
Short circuits happen when there is an undesigned path, and open circuits happen when the path is interrupted completely. Try to minimize both as much as possible.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
Learn more about the 2022 HVACR Training Symposium at https://hvacrschool.com/symposium/.
There will be three diagrams: the condenser, heat strips, and air handler each having its own schematics.
Those schematics show various types of switches, including basic switches, pressure switches, thermal switches, float switches, and contacts. An open switch has no electrical path, meaning that the equipment can't turn on. On the other hand, a closed switch allows electricity to pass through, so the equipment can turn on. Switches will be either in the normally closed or normally open position, and the schematics will indicate the normal state of the switch. It's also worth noting that switches are power-passing devices and do not use or consume electricity. In the condenser, you will see high and low-pressure switches. The heat relay on the heat strips will have a prominent pair of normally open contacts. The air handler usually has a normally closed float switch, which opens when water fills it up and breaks the circuit.
Compared to switches, loads are power-consuming devices that transform electrical energy into some other form of energy. The contactor coil and reversing valve solenoids are examples of loads.
Within the thermostat, G (green) goes to the blower, Y1 (yellow) is for the contactor, O/B (usually orange, sometimes dark blue) is for the reversing valve, RH (red) is for constant 24v heat (RC is constant 24v cool), C (blue) is for common, and W2 (white) is for auxiliary heat. In this case, we are using ACC+ (black) for dehumidification.
The transformer is the source of the 24v power that starts everything. Transformers take high voltage (often 230v) from the power company on the primary and drop it to 24v on the secondary, which then goes to the integrated circuit board. The primary and secondary interact electromagnetically; they don't actually touch.
When the 24v power makes it to the integrated circuit board via SEC1 or SEC2, it passes through the 5-amp fuse and powers the R terminal on the terminal block. Due to that configuration, many techs wire the float switch to break R. Breaking R will stop power to the thermostat and the defrost board at the condenser. (However, some techs may break Y instead.)
Y goes from the thermostat to the terminal block and then to the contactor coil. The contactor coil needs 24v to pull the contactor in and close the contacts that allow power to reach the compressor and the condenser fan. Y goes in and out of the high and low-pressure switches before reaching the contactor coil.
W2 connects to the heat strips and W on the defrost board from the terminal block. So, either the thermostat or the defrost board has the capability to bring on electric heat.
The G terminal supplies constant 24v power to the blower fan inside the air handler.
Common feeds from the common side of the transformer and goes to the thermostat to complete the circuit. It then provides 24v common to the defrost board, which also completes the circuit on that side of the unit. We need common in all cases because it provides a path back to the transformer; otherwise, the circuits would all be open and would not work.
The O terminal's wire passes through the terminal block from the thermostat. Then, the 24v power goes to the condenser to energize the reversing valve solenoid in cool mode. (Note: Ruud and Rheem systems energize the reversing valve solenoid in heat mode, and these may have dark blue instead of orange wires.)
When we use ACC+ for dehumidification, the blower will only reach full speed when the DH terminal is energized on the terminal block.
When stripping back the wire jacket, try to minimize nicks by making a small vertical cut and pulling the sheath back. Cut the sheath you've pulled back. Cut the tips of the conductors, as you may have nicked them. Make careful cuts to expose the bare conductors; you want them to be able to reach the terminal block or touch under the wire nut, but they shouldn't be exposed.
When routing wires or cables through cabinets or other areas where they might get cut, be sure to use proper grommets. Don't run wires over metal objects like the capacitor, as shorts may occur. When routing the wires in a spot where they might chafe, rub out, or otherwise suffer damage, use a conduit whenever possible to protect the wires. Also, use zip-ties to secure wires with a bunch of slack.
Short circuits happen when there is an undesigned path, and open circuits happen when the path is interrupted completely. Try to minimize both as much as possible.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
Learn more about the 2022 HVACR Training Symposium at https://hvacrschool.com/symposium/.
In this video we're going to talk through the low voltage electrical circuit on a typical heat pump system note, this is a carrier system and we're going to be talking through the condenser. The heat strips and the air handler components may vary slightly across manufacturers, but in this video we're going to focus on reading a schematic showing the different electrical circuits and highlighting the components as well as some common problems that occur. That can result in issues in the low voltage circuitry. So let's get started first, we have a condenser and our condenser diagram have heat strips or electric key with our heat strip diagram, and we have our air handler with our air handler diagram.
You may note that this is an ecm configuration, meaning it has a variable speed, blower motor. First, let's talk about switches. This is a symbol for a simple electrical switch shown in the normally open position. You can see the difference between normally open and normally closed.
Switches are shown in the powered off position in the equipment. Here we show some pressure, switches, thermal switches or heat activated switches and float switches. We also show a type of switch called contacts that are commonly used in contactors and relays here, they're in the normally open and the normally closed position. Open means that there is no path for electricity to move through closed means.
There is an electrical path. This means, when a switch is closed, things will turn on and when it's open, they'll turn off switches are power, passing devices they don't use or consume electricity. Now, let's show some common components on this system. Here we show our low pressure switch and how it's shown on our two different diagrams, the connection diagram and the standard schematic diagram that come with a carrier condensing unit.
Here we show our high pressure switch with our connection diagram and our schematic ladder diagram. Now, let's take a look at our heat relay on the inside here we're showing the contacts for our heat relay. You can see that they're in the normally open position. These power on are electric heat strips, which act as auxiliary heat or backup heat for the heat pump.
Next, we show our integrated circuit board for our air handler. You can see our 24 volt power coming into the top at sec-1 and sec-2 from the transformer. Our 5 amp fuse and our terminal block for our low voltage connections, as well as the plug that goes to the heater and motor a common component in a residential air conditioning system is the float switch or the condensate overflow switch. These switches are normally closed when they fill with water.
The switch opens, which breaks the circuit and shuts off the system. Now, let's go over, our thermostat g is for the blower call or 24 volts to the blower. Motor y is the contactor which turns the condenser on in both heating and cooling mode. In a heat pump, o or b is the reversing valve, with o being the most common orange, which is powering the reversing valve to operate in cooling mode, meaning that when the system operates in cooling mode, it energizes the o terminal and the orange wire rh is Constant 24 volt power for heating mode and rc is constant: 24 volt power for cooling mode. In most cases, these are the same c is for common, which is the path back to our transformer w2 or ox auxiliary heat is our electric heat strips and, in this case, we're using the acc setup for dehumidification. This is set up within the menu of the thermostat loads. Are power consuming devices loads, convert power into other forms of energy? In most cases in low voltage circuitry the loads are the coils within the low voltage circuit, such as the contactor coil, the reversing valve solenoid and other relay coils. Let's take a look at our contactor coil.
This is inside our contactor. A reversing valve solenoid is also an electromagnet when it energizes it allows the valve to shift into cooling mode. In this case, this is a dc relay for heat mode. The transformer is the source of all the 24 volt power for the control circuitry, the high voltage electricity comes into the primary of the transformer and goes out the secondary, dropping the voltage from 230 volts to 24 volts.
In this case, these primary and secondary coils within the transformer do not actually touch electrically. Instead, they interact electromagnetically. The 24 volts from the transformer now connects to the integrated circuit board, travels through the 5 amp fuse and then powers the r terminal on the terminal block. Here we show wiring the float switch so that it breaks r or 24 volt power, some choose to break y.
Instead. This means that when the float switch opens it shuts off power to both the thermostat and the condenser defrost control. Here we trace each circuit. You can see we're tracing one side of r after it comes back out of the float switch to where it connects at rh on the thermostat.
The other side goes outside to the defrost board. It powers the defrost board with constant 24 volts, because the defrost board needs to have constant power in order to power its timer logic and defrost circuitry. The y circuit goes into the terminal block, which acts as a junction point and then goes out to bring on the contactor y or yellow, is used to energize the contactor coil, which pulls the contactor in and allows the condenser fan and the compressor to run. Here.
We show the y circuit going in and out of our high pressure switch and low pressure switches before it goes to our contactor coil, as shown in the diagram. If either of these switches open, it won't allow the system to run next. We show our auxiliary heat, which is white or w2. Once again, this wire connects to our terminal block on the heat strips and then goes outside and connects to w on our defrost board. This allows the defrost board to back feed and bring on electric heat, as well as the thermostat, so either the thermostat or the defrost board on the outside has the capability of bringing on the electric heat. Next we have the g terminal. The g terminal is for a constant 24 volt power applied to our blower. Keep in mind that many variable speed, blower motors will not come up to speed unless 24 volts is applied on g during cooling mode.
As you can see, this feeds down to the blower motor and brings the blower motor on this is used, especially during the constant fan on setting on the thermostat. Next, we show common, which actually feeds from the common side of the transformer secondary and then goes inside to the thermostat, providing 24 volt common to the thermostat, then outside to the defrost board. Providing 24 volt common to the defrost board common is necessary in all cases because it provides the path back to the transformer in order to complete the full electrical circuit out of the loads. As you can see, it also runs to the loads here.
Both the contactor coil and the reversing valve coil, as shown in the wiring diagram, the o terminal, is for the orange conductor, which again passes through the terminal block on the air handler and then outside to energize the reversing valve solenoid coil during cooling mode on this Particular system, in this case we're using acc plus to energize the dehumidification terminal on the terminal block. The blower will only come to full speed when the dh terminal is energized by the thermostat. Here we show the proper practice for stripping back the outer jacket on thermostat wires when connecting thermostat wires together, make sure to connect the proper colors as shown strip them back to the appropriate length. Not too long and not too short, do not leave excess bare conductors and do not strip back too little, as shown here under the wire nut.
The same thing is true: when going in the terminal block, you want to make sure you have full coverage without any excess common o for reversing valve g for blower y, for contactor, w for electric heat and d h for dehumidification make sure to use appropriate grommets Or connectors when routing wires or cables through the cabinet, this is a common source of chafing and abrasions, which can lead to short circuits if done improperly, don't run wires loosely through the inside of the cabinet, where they can rub out against metal parts. This is also true of high voltage wires, ensure that they're routed properly and not run over the top of metal parts where short circuits can be caused. The same thing is true of our electrical conductors inside our condenser ensure that they're not rubbing on top of metal parts or copper tubing where they can become damaged when needed, use, zip ties or other standoffs to secure conductors, so they don't chafe. Next, let's show a simplified version of the ac electrical circuit, rather than being a heat pump, we're only showing the low voltage side of what energizes our compressor contactor for simplicity, our 24 volt power comes out of our transformer travels through our fuse, then up to power. The thermostat, when calling for cooling the thermostat energizes the y terminal, which then can bring on our contactor by energizing the electromagnetic contactor coil. It then needs to travel back to the other side of the transformer common to complete the circuit. Let's see inside the contactor to better understand how switches and loads work here, the switch is on top shown as the contacts the electromagnet on the bottom is the load which allows the contacts to pull in and out or open and close when energized. The circuit is closed, allowing the compressor and condenser fan to run here.
The normal state is open when the contactor coil energizes, then the contacts can close, allowing a path to bring on the equipment. Here we show the normal state of operation when everything's working properly. The thermostat is on the contactor coil energizes, the contacts close and the fuse is not blown. We have neither a short or an open when the r circuit is open, the thermostat is off.
The fuse. Cannot be blown by an open circuit, but the contactor cannot energize and the contacts remain open. Therefore, the system does not run in this case. The open circuit is shown as a broken conductor or wire.
A short circuit or a dead short between hot and common will result in a blown fuse. This is because the electricity is allowed to travel, bypassing the load, making this circuit shorter than it should be. This short causes a high current, which forces the 5 amp fuse to blow protecting the circuit, because the fuse is blown or open. Thermostat will now be off, the contacts will be open and the contactor coil will be de-energized.
Again short circuits can happen in many places. Often, where conductors run through cabinets in many cases, it happens because technicians strip back the wire jacket, as shown here, resulting in nicks and damage at the base of the wires. This can result in either an open or a short if conductors are stripped back too far. This can also result in bare conductors that can result in a short where it touches other wires or ground.
Here we show another type of dead short from hot to common, resulting in a blown fuse and a blank thermostat. Here we show a connection between two energized circuits red and switch leg, yellow going to the contactor. In this case the red will power, the yellow all the time and the system will run constantly. Here we show another common cause of electrical short circuits, especially when metal stud work is used.
The penetration up at the top of the metal stud work can be sharp, causing damage to the conductor and resulting in a short circuit. Another common cause is lawn implements damaging the control wire on the outside. This is why it's best to run control wires through a conduit. This video has been a summary of some of the most common causes of problems within the low voltage electrical circuit, and we've touched on the purpose of the circuits, some of the most common components, how these components are represented on diagrams and some basic electrical terms, such As loads switches opens and shorts, this is not a comprehensive video on the topic and it is not a diagnostic video. It simply shows you some of the most common visual inspection points and most common things to avoid in order to prevent issues in an upcoming video. We're going to show in more in-depth diagnostic process for low voltage circuitry, including how to use your electrical meters in order to diagnose problems fully thanks for watching our video. If you enjoyed it and got something out of it, if you wouldn't mind hitting the thumbs up button to like the video subscribe to the channel and click, the notifications bell to be notified when new videos come out, hvac school is far more than a youtube channel. You can find out more by going to hvacrschool.com, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more.
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