Lets start by explaining what the C wire is, and why it's needed. In the olden days thermostats were simple switch devices, that used Mercury Switches to complete the circuit and turn on the heat/AC. Mercury switches were commonly used in bimetal thermostats. of the movable mercury drop provided some hysteresis by moving the bimetal spring slightly beyond the point it would normally assume, thereby holding the thermostat off slightly longer before flipping to the on state and then holding the thermostat on slightly longer before flipping back to the off state. The mercury also provided a very positive on/off switching action and could withstand millions of cycles without degradation of the contacts. Because of this, there was no reason to run a return wire to the thermostat. For example a thermostat that only controlled heat, would only require 2 wires. Newer thermostats that offer clocks, backlit displays, WiFi, etc. are controlled using circuit boards and integrated circuits.

These new circuits require a path for electricity to return to the source, and so require an extra wire. This new wire is known as the C wire, or Common wire. If you're lucky when you upgrade to a newer thermostat that requires this connection, there will be an extra (unsused) wire in the cable at the thermostat. portable air conditioning unit partsIf you're not, you'll have to run new cable to the thermostat.how much is a coil for an ac unit If you have heat and AC, you'll have to pull an 18/5 cable. installation of air conditioning unitsIf you have just heat you can get away with pulling 18/3 cable, but you may want to pull 18/5 anyway to make adding AC in the future easier. There are no standards for wire color, so any wire could be used for any purpose.

The most common color code would be (note: this is for forced air furnaces, heat pumps and other systems may be different). This solution is illustrated in this video from Honeywell. With this solution you lose the ability to manually turn on the blower fan, but the fan will still run properly in the Auto position. WARNING: This procedure involves modifying the wiring in the furnace, and may not be approved by every manufacturer. Check with the furnace manufacturer, and all local codes before attempting this procedure. Make sure the breaker for the furnace is OFF before you begin. A transformer uses coils of wire, magnetism, and a bit of magic to transfer energy from the primary side of the transformer to the secondary side of the transformer. Usually during the transfer, the voltage is either increased or decreased. In the case of our furnace we're likely talking about taking 120VAC, and transforming it into 24VAC. Once the voltage has been reduced, we can use the lower voltage and a thermostat to control the furnace.

Now that you know even less about transformers than you did before, lets look at a diagram. This is an actual wiring diagram from a furnace, but you'll notice I've highlighted a few things. First, in red I've highlighted the 120V primary side of the transformer. I've also highlighted the secondary side of the transformer in a couple shades of blue. This was done to illustrate that one side of the transformers secondary winding (light blue), is attached to the R or power terminal. While the other side of the secondary winding (dark blue), is attached to the C or "neutral" terminal. On a schematic or wiring diagram, a transformer will look something like this. Often you'll see a number written on each side, which denotes the expected voltages on each side of the transformer. Notice in the schematic above that the top side lists 120V (120 volts), while the bottom lists 24V (24 volts). When actually digging through HVAC equipment, a transformer will look something like this.

Notice the rectangular middle section, flanked by a bulge on each side. These are the typical physical characteristics of a transformer. Transformers typically have a volt-ampere (VA) rating, which can be used to determine the amount of current that can safely flow through the transformers coil wires. To determine the maximum current, simply divide the VA value by the voltage. For example, a 120V/24V 40 VA transformer would be capable of 1.66667 amperes on the secondary. 40VA / 24V = 1.66667A and .3333 amperes on the primary 40VA / 120V = .3333A Normally this is not a problem, since the only things supplied by the transformer are switches and relays. If you install a thermostat that draws more current than the transformer can carry, you're going run into problems. So in this case, you'll have to upgrade the transformer and any fuses that protect it (since the fuses are sized based on the VA rating).To explain the components of a forced air system: a fan draws air from the house into the system through return air vents (Image 1).

During cold weather, gas burners create heat in the heat exchanger (Image 2), which in turn heats the air. The hot air goes up into the plenum and from there flows to the supply trunk line (Image 3) where it is distributed through vents throughout the rest of the home. The conditioned air then re-circulates back into the system as needed. During hot weather, you turn on the air conditioner and cooling coils inside the plenum (Image 4) then cool the air, which is then distributed throughout the house. You need a load calculation to make sure the furnace is large enough to supply the new room. You can get it online from companies like DuctWork or go to local HVAC pros to get a load calculation worked out. Generally speaking, if the room existed when the house was completed, the furnace is probably powerful enough. If it is an addition that was added after the house was built, you may need a new furnace. A couple other components to a gas forced air system is a source of fresh air from outside the house to aid in combustion, and you'll notice chimney pipes that carry flue gases from the hot water heater and the furnace to the outside of the house.

Mark the center point for the takeoff and drill a pilot hole using the cutting bit in the hole cutter. Preset the cutting tool to the correct diameter. Place the center guide in the pilot hole and start the cutting bit along the circumference of the hole. Use a right-angle drill because you have to work in a tight space, but you can attach the cutting bit to any drill. The cutting bit then swivels around the center guide (Image 1), forming a perfect circle. Note: Sheet metal hole cutters and right-angle drills are available at most rental centers or specialty stores. After the takeoff is in place, reach in and bend the tabs over (Image 2) to secure it to the duct. When doing any heating or ventilating project, you may need to pull a permit, so check with your local municipality. In some cases, you may even need to hire a pro to help with this kind of project, so check on that as well. Install the ceiling register boot (Image 1) so you will know how long to cut the supply duct.

Center it between the joist and screw into place with self-tapping screws. Note: Use a power nut driver to drive self-tapping sheet metal screws. Measure the length you need for the supply duct. Take the measurement from about 1" inside the takeoff to about 1" into the boot collar. Cut the duct to length with a round duct cutter. You need to use a round duct cutter because the cutter actually removes a thin strip of metal along the cut line (Image 2), which makes the cut a lot smoother. It is almost impossible to cut the metal with any other tool. Snap the duct together. It has a special snap lock built in at the factory. Install the damper into the duct by drilling a hole into the side of the duct and then screw the damper into place. The damper control indicator will show you whether the damper is open or closed. Put the crimped end in first, then put the other end of the duct in. The takeoff rotates to make it easier to fit the duct into place. Secure both ends with a few 1/2" sheet metal screws through the collar.

Dampers are critical for the efficiency of a forced air system. They allow you to adjust the amount of air flowing into each room balancing the system. Make sure the damper control is on the bottom of the duct. Add a couple of support brackets to help support the weight of the duct. Finally, attach the register to make it look a little nicer. For the system to work, you have to re-circulate the air back to the furnace. That is why you need a cold air return (Image 1). Expert Note: If you had no return air in your room, you would create a positive pressure. That would then reduce the flow of conditioned air to that space. Nail a piece of sheet metal onto the back of a stud cavity. Cut two holes -- one in the stud cavity and the other one into the existing return (Image 2). Secure start collars (Image 3) into both holes. Use non-insulated, flexible ducting to connect the two returns (Image 4). Use plastic cable ties to secure the ducting.