| The 24VAC is used primarily for two purposes. The first is control, such as in door bells, and thermostats. The other is to supply operating power, such as in irrigation valves, rotation encoders, low voltage lighting systems and CCTV cameras. In all cases the low voltage is used because it is safe and doesn't need an electrician to wire it, which is not the case for 110VAC or 220VAC circuits. Show
But why AC instead of DC? The advantages of AC are lower cost solenoids and motors, less relay contact erosion and better arc suppression. AC solenoids are cheaper than DC solenoids. This is because DC solenoids depend on the resistance of the coil wire to limit their current. AC solenoids can limit their current by their AC impedance. 24VAC solenoids and relays do not have the back-EMF problems that DC operated devices have, so there is less EMI noise. 24VAC can run longer distances than 12VDC. It isn't because of the alternating current, but because of the 24V. To supply 24 Watts will require 1 amp at 24VAC and 2 amps at 12V. The voltage drop in a wire is proportional the amps only. Losing 1 volt due to ohmic losses in the wire is also more significant for 12V than for 24V (8% versus 4%). AC/AC -- The most common source of 24VAC is the AC/AC wall mount power supply. This is a simple step-down low voltage transformer, with mains voltage AC in and 24V AC out. There are no other active or passive components (other than a fuse). The transformer is designed to give a certain voltage at a certain current, so often have to be designed specifically for the application. AC/AC adapters have no line regulation at all, so their output voltage is proportional to the input voltage. Wall transformers have a small amount of load regulation, since the transformer windings are a significant part of the load.  Since the mains voltage can vary, depending on the distance to the transformer, and the load the transformer sees from you and your neighbors, the output of a 24VAC transformer will also vary accordingly. Typical specifications for the mains are ±10%, so the output of the 24 VAC transformer will vary from 21.6VAC to 26.4VAC from this factor alone. 24VAC fundamentalsA 24V RMS sine wave has a peak voltage of 33.9 V. When rectified this results in the pattern shown in the diagram, with peaks of 33.9V and valleys of 0V. Filtering this pulsed waveform will even this out, but there will always be some ripple. The output of a 24VAC transformer is not actively regulated. The transformer is designed to source 24VAC at the nominal current listed on the label. If you draw more current the voltage will sag, if you draw less then the voltage will increase. Furthermore, the mains voltage can typically vary by ±10% depending how far you are from the mains substation, so even the nominal voltage of the 24VAC transformer can vary ±2.4V. Thus, equipment designed for 24VAC must be flexible in its acceptable voltage. Solenoids and relays actuated by the 24VAC power are very flexible in their input voltage, as are synchronous motors. A synchronous motor is designed to run at the AC frequency of the source power. The advantages are that the speed is independent of the load, can be used as timers, have less EMI than brushed DC motors and make efficient use of available power. The single diode, half wave rectification DC adapter is used for the cheapest of battery chargers, toy motors, battery adapters and other places that the 100% ripple and 50% duty cycle won't matter. Adding a capacitor can smooth out the ripple, but since it needs a larger capacitor than full wave bridge rectification, the cost trade off usually leans toward the full wave rectifier. An AC/AC transformer has the advantage over switchmode DC power supplies in that it can supply surge current. For example a sprinkler valve can have a holding power of 5 watts, but an inrush current of 8 watts. The transformer will sag in voltage during the inrush pulse, but will not have to be rated at the higher power. Capacitively coupled noiseRunning a twisted pair of 24VAC next to signal wires can cause trouble. The AC can capacitively couple to the signal wires to add 50Hz or 60Hz noise to the signal. Moving the signal wires away from the twisted pair, or shielding the twisted pair will solve the noise problem Thermostats and HVACThe 24VAC can be used in HVAC systems to send a signal from the thermostat to turn on the heat or cooling. It is also sometimes used to synchronize the thermostat's internal clock and supply power for electronics and pilot lights. The thermostat turns the unit on by shorting two wires together, known by the jargon "call." The wiring doesn't usually supply both lines of the 24VAC power up to the thermostat, but uses one line (Red) which connects to the correct "call" wire to turn on the heat or cool functions. For thermostats that require power to operate the other leg of the 24VAC supply can be run, which is often blue. This is sometimes called the "C" wire, short for "common," meaning "ground." Some thermostats also try to steal some power from the "call" wire, but if they steal too much the heater will turn on. A common multi-stage heat pump system has the following wiring & connections:
ResolversAnother advantage of using 24VAC in your power distribution is to power "resolvers" which look like a motor, but use the phase to send back the angle of rotation of the rotor to a controller. Analog resolvers have high angular resolution, and are robust and dependable shaft encoders. The windings are designed to create a sine wave output and a cosine wave output. Comparing these two signals gives an absolute rotation reading. The zero reading is typically when the sine wave is maximum and the cosine is minimum. The position is found by taking the ratio of the sine and cosine winding voltages and taking the arctangent. The ratio makes the reading inherently immune to noise on the power line, and compensates for variations in temperature.Irrigation systemsMany sprinkler systems use 24VAC as a safe way to get AC power to timers, motors, valves, and solenoids. Lighting systemsA number of low voltage incandescent lighting systems have been installed in buildings. They typically run off of large torroidal transformers, which have multiple input taps to account for voltage drop at the bulb. A large building might have a closet full of these torroids.Solar systemsA solar panel and battery combination are intrinsically DC. But what if you need AC you have to find a way to invert it. PowerStream has a variety of 24VAC inverters with 12V, 24V and 48V inputs to allow security cameras, irrigation systems, etc. to operate off the solar power supply. Battery backupIt is often hard to find a good solution to give a low voltage AC system a battery backup. PowerStream also has developed a unique 24VAC battery backup. It can be inserted anywhere in a 24VAC system to charge a battery and supply an uninterruptible 24VAC to devices downstream.There are various converters available for converting 24VAC to useful DC power. A direction conversion to 24VDC to run PLCs is available, as are 24VAC to 12VDC converters, 24VAC to 5V converters, and even an adjustable DC output converter for special applications. In addition we have developed a fluorescent light ballast series that runs on 24VAC. Page 2
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If you plan on getting a smart thermostat and don’t have a c wire, please save yourself some headaches and get one. I promise it’s not expensive and just about anyone can do it. So, how do you do it? What is the C Wire on a Thermostat?The c wire is an extra wire that can be used to provide a return path for continuous 24V power for any application. It’s normally used to provide a return path to power the thermostat. The “c” stands for common. It is often labeled as “c” on thermostat backplates. Keep in mind that it is not necessarily labeled as c and the wire is not necessarily any particular color. Although there are some best practices, there are no strict standards that wire name and color adhere to. Although I’ve seen it stated in several places, it’s not entirely correct to say that the c-wire is what powers the thermostat. Typically, the wires that provide the power (often called the “hot” wires) are labeled Rc (cooling) and Rh (heating). They provide a source of 24V power coming from the HVAC control board. Sometimes they are separate wires and require separate connections. Other times they are the same wire. In that case it will typically be labeled Rh/c. In order to have a circuit that power can flow through, wires need to connect back to the HVAC control board to complete the loop. That’s what the other wires do. So, the c wire doesn’t actually provide the power. It provides a return path so that the thermostat can be powered without disrupting the other wires which are used as electrical on/off switches for your equipment. Why Do I Want a C Wire?Now that you know what the c wire is, you may be asking yourself, why do I want it? Today’s wireless thermostats require more power than the simple calculator-screen programmable thermostats of the recent past. Keeping a connection to a WiFi router or an automation hub is one power requirement that never existed. Trying to maintain a WiFi connection will drain a couple AA batteries within days. Many smart thermostats come equipped with a large, full-color touch-screen. This is another feature that contributes to the need for a constant power source. Using a c wire is the best way to provide the constant power that is required. Some smart thermostats claim to not need a c wire. One example is the Emerson Sensi WiFi thermostat. This quote from their website makes me think otherwise: “If you already have a c-wire connection, even though it is not required, we recommend connecting it to Sensi to help improve Wi-Fi connectivity and battery life.” If connecting a c wire improves WiFi connectivity and battery life, that means it’s less than optimal without it. I don’t know about you, but those are two things that I very much prefer to have at their optimal levels. If your thermostat loses power completely, your equipment can’t turn on. Do you really want to risk that happening? Smart thermostats are supposed to make our lives easier. Nobody wants to be replacing batteries once per week or even once every couple months. I want to be able to hook one of these things up to the wall and hardly even think about it for the next five years. I don’t want to be on vacation worrying about my thermostat batteries or WiFi connection. In fact, I don’t want to be anywhere and have to worry about anything related to my thermostat. How Do I Know If I Have A C Wire?*Before messing with any sort of wiring, it’s always a good idea to shut the power off... To check if you already have a c wire hooked up, start by taking your current thermostat off the wall to expose the wiring. Check the labelsYou will probably see a few wires connected to some labeled terminals. If you happen to see one connected to the terminal labeled “c”, then you are in luck. You have a c wire! If there is a terminal labeled “c” and it’s empty, you probably don’t have a c wire. Hang on, though! It might still be there… Are there any extra loose wires?Sometimes installers run the c wire from the control board to the thermostat location but don’t hook it up. You may want to take off the thermostat backplate and pull out the wires to see if the extra wire is there. Check The Control BoardIf you still aren’t sure if you have a c wire, go to your HVAC control board. Check to see if there is a wire attached to the “c” terminal. If there is no wire, you don’t have a c wire and will probably want to get one. If there is a wire connected, note its color and go back to your thermostat. Whichever wire matches the color, should be your c wire. Four Ways To Fix the C Wire ProblemDon’t get stuck pulling your hair out trying to run a smart thermostat without a c wire. If you’re anything like me, you need to keep all the hair you have! Don’t worry, it’s not that big of a deal to add a c wire. If you can handle programming a smart thermostat, you can figure out how to add a c wire. Here’s four ways to fix the problem: 1. Purchase An “Add-A-Wire” KitIn most cases, this solution is the easiest and best solution to the problem. An “add-a-wire” kit allows your thermostat to use your existing wires and still get constant power without using power stealing. The kits are installed at the HVAC control board. Installation requires a screwdriver and the ability to read a wiring diagram. You’ll have to remove the access panel to your furnace to locate the panel. Then, shut off the power before doing any work. Although the 24V power doesn’t pose much of a safety risk to you, shutting off the power reduces the risk of damaging the board. Keep in mind that your control board is probably a fairly expensive piece of equipment. So, if you have any doubt about what you may be doing, it’s probably best to contact a professional. There’s a few different manufacturers that make the add-a-wire kits. The Venstar kit has a proven track record and is the lowest cost kit I have found. The technology behind these kits is pretty simple, so there’s no reason to spend more if you don’t have to. Also, if you purchase an ecobee thermostat, it comes with an add-a-wire kit (they call it a PEK – power extender kit). Here’s a link to the Venstar kit: Venstar ACC0410 Add-A-Wire Accessory | Check Price 2. Purchase A 24V C Wire AdapterThe 24V adapter (transformer) has two wires that plug into the thermostat to power it. The other end just plugs into a regular outlet. This is the easiest solution, but probably my least desirable. It’ll work just fine, but you’ll have an unsightly wire running from the thermostat to the outlet. You could buy any 24V transformer to do the job, but this adapter is specifically made for smart thermostats. It has the two wire ends already stripped and ready to install in the thermostat. It also has extra long wires in case you don’t have an outlet near your thermostat. 3. Install Another WireThis is the best solution in my opinion. However, it’s not the simplest solution. It requires running a wire from the HVAC control board to your thermostat. If your home is already finished, this means trying to fish the new wire through the walls without destroying them. Depending on how far from the control board and how finished your walls are, this can range in difficulty from easy to hard. 4. Use A Power Stealing thermostatWhat is a Power Stealing Thermostat?Some thermostats, such as the popular Nest thermostat, have tried to get around the no c wire problem by using a technique commonly referred to as power stealing. Power stealing uses the existing circuits to “steal” a small amount of power. The “stolen” power charges a battery, and the battery powers the thermostat. The existing circuits are designed to act as on/off switches. Their function is not to actually power anything. There is a certain “threshold” current running through the wire that represents the switch point. Any current above the threshold is “on” and any current below the threshold is “off”. So, there can be a small amount of current running through the circuit without triggering the “on” condition. That is the current which can be used to power the thermostat via power “stealing”. How Can Power Stealing Cause Problems?The problem with power stealing is that the HVAC circuits were not designed for this. It provides a very small amount of power so doing it well is difficult. If you steal too much, you turn your equipment on when you don’t want to. If you steal too little, your battery gets drained and eventually the thermostat will go dead. More power can be stolen when the HVAC system is on than when it is off. The thermostat can charge its backup battery when the equipment is running but tends to slowly discharge when the equipment is not running. This means that during times of minimal usage, the chance of running your backup batteries really low or even completely dead is a possibility. To make matters worse, some thermostats like the Nest have a permanent battery. Rechargeable batteries like this will tend to lose their charge capacity over time (as anyone with a smartphone has ever found out). The power stealing with the Nest may work fine at first, but as the battery gets older, you may end up with problems. For more information about the problems with power stealing, here is a great post on the Ecobee site that explains why they don’t use it. Final ThoughtsSmart thermostats use too much power to be run by batteries. Power stealing can work, but it can be unreliable and is using your wiring in a way it wasn’t designed for. A c-wire is the best option and makes sure that your smart thermostat will have power as long as your house does. My recommendations for adding a c-wire: Best option: Add-a-wire kit Any comments or questions? Please leave a comment below and I’ll be happy to start a conversation! |
A thermostat that draws a small amount of power from the low-voltage wiring to power its electronics
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