Skip to main menu Refrigerators and air conditioners both contain heat pumps. In a refrigerator, heat is moved from the food storage sections and discharged to the kitchen air. Air conditioners work the same way — they move heat from the inside of a building and discharge it to the outside air. Conventional or air source heat pumps differ from those in a refrigerator or air conditioner because they are reversible — they can concentrate heat from the outside air and move it inside to provide warmth, as well as move heat out of the building to provide cooling. To do this, air-source heat pumps (and central air conditioners) need a large outside unit to exchange heat with the outdoor air. Geothermal Heating and Cooling Systems In a geothermal heating and cooling system, the heat pump is connected to the building by a distribution system — most commonly air ducts. And the heat pump is connected to the earth through a series of pipes called a “loop”  The system exchanges heat with the earth, meaning that no noisy or unsightly outdoor unit is needed.

All heat pumps use a vapor compression cycle to transport heat from one location to another. In heating mode, the cycle starts as the cold liquid refrigerant within the heat pump passes through a heat exchanger (evaporator) and absorbs heat from the low-temperature source (fluid circulated through the earth connection). ac unit fan blades not spinningThe refrigerant evaporates into a gas as heat is absorbed. best central heat air conditioning unitsThe gaseous refrigerant then passes through a compressor where it is pressurized, raising its temperature to over 180 degrees F. air conditioning unit running costsThe hot gas then circulates through a refrigerant-to-air heat exchanger where the heat is removed and sent through the air ducts.

When the refrigerant loses the heat, it changes back to a liquid. The liquid refrigerant cools as it passes through an expansion valve, and the process begins again. Although heat pumps are complex internally, they are marvels of compact design for reliability. Some include features such as additional heat exchangers for water heating, and microprocessor-based automatic controls and protection devices.Refrigerant lines must be vacuumed after installing or reinstalling an A/C unit. Moisture inside refrigerant lines can contaminate the lines, causing your air conditioning unit to become sluggish. Anytime an A/C unit is installed, or reinstalled due to repair, the refrigerant lines must be vacuumed. Refrigerant already in the lines combines with the moisture that enters the lines. As soon as the A/C unit is turned on, the moisture begins to freeze, clogging the lines. This forces the A/C unit to work harder to cool the air in your home. Removing the moisture -- through use of a vacuum pump -- will allow your A/C unit to function effectively.

Turn your A/C unit into the "Off" position. Remove the service valve caps, using a wrench. The two gold-colored valves are located on the refrigerant lines, where the lines enter the condensing unit. The valves are located toward the bottom of the back side of the A/C unit, or the side A/C unit that faces your home. Install the refrigerant manifold gauge hoses onto the service valve lines of the A/C unit. Thread each valve onto its respective hose by turning the valve in a clockwise motion, just as you would thread a watering hose onto an outside water spigot. The left hose of the gauge attaches to the larger, low-pressure line's service valve, while the right hose attaches to the smaller, high-pressure line's service valve. Check the sight glass on the vacuum pump to ensure it has enough oil. The oil level should be at the fill line located on the sight glass; some pumps will have an "Oil Level" line. If it is not, add more oil. The oil inside the vacuum pump does double duty, trapping contaminants from the A/C unit and keeping the pump from over heating while it is in use.

Place the vacuum pump onto a flat surface next to the A/C unit. Thread the last -- and center -- hose from the refrigerant manifold gauge onto the intake valve of the vacuum pump. Thread it in a clockwise motion similar to Step 3. Switch the pump into the "On" position. Place both manifold gauge valves, located on either side of the manifold, into the "Open" position. Air and moisture trapped in the A/C lines will now begin to exit through the vacuum pumps exhaust valve. Track the refrigerant manifold's left gauge readout, measured in inches of mercury, until the gauge reaches 29 inHg. Close each manifold valve and quickly shut off the vacuum pump. Wait 20 minutes, and then check to be sure the reading remains at 29 inHg. A reading other than 29 inHg, may mean a leak in the refrigerant system. Disconnect the refrigerant manifold gauge from the A/C unit and the vacuum pump from the refrigerant manifold gauge. Turn on your A/C unit to be sure it is working properly. Things You Will Need Wrench Vacuum pump and vacuum pump oil Refrigerant manifold gauge Tip References Robinair: AC Solutions/AC Vacuum Photo Credits Jupiterimages/liquidlibrary/Getty Images Suggest a Correction

In this article, I am going to explain the function and wiring of the most common home climate control thermostats. This information is designed to help you understand the function of the thermostat to assist you when installing a new one, or replacing or up-grading an old one. We will focus mainly on the basics of home heating / cooling thermostats, and first of all, I will explain the function of them. The thermostat is the control device that provides a simple user interface with the internal workings of your homes climate control system. By the use of an adjustable set-point, the job of the thermostat is to turn on either the heating or cooling system to maintain the desired room temperature in the home, and to turn off the system when the desired temperature is achieved. The most basic of systems (such as an older ‘heat only’ forced air / gas furnace with a standing pilot light) only need two wires for control. They connect to a two-wire thermostat (generally a mechanical thermostat with a mercury filled ball connected to a coiled bi-metal strip).

A basic two-wire thermostat can be compared to a simple single-pole switch that you will find throughout your home, only instead of you turning the switch on and off as required, a mechanical or electronic temperature controlled mechanism is the operator of the switch. The terminals are usually marked ‘R’ and ‘W’. They usually operate at 24V ac power, and the source of this control power comes from a control transformer mounted either just outside, or inside the furnace body. The line voltage feeding the furnace (to operate the fan blower motor) is transformed down to a safer level of 24 volts (the gas control valve needs 24 volts to open), and after making a series loop through at least one safety device (the most basic and mandatory one is an over-temperature shut down), the power goes up to the thermostat, and when the room temperature falls below the set-point, the contacts close completing the circuit to the gas valve allowing it to open, the main burner to ignite, and begin the heating cycle.

In this the most basic of heating systems, as the temperature of the heat exchanger rises, another contact is closed on the line voltage side of the equation, and the fan blower motor starts moving air through the heat exchanger and out through the ducting in the home. If the fan fails to run for any reason, the heat exchanger will get too hot and the hi-limit temperature device will open the circuit to the gas valve, closing the valve and stopping the heat cycle. If your home’s system of this vintage has provisions for air-conditioning (cooling), then the thermostat wiring will have at least three wires (some will require a separate ‘R’ terminal for heating and cooling and will be labeled ‘Rh’ and ‘Rc’ now needing a minimum of 4 wires), ‘R’, ‘W’, and a ‘Y’ terminal. When the mode is selected for cooling (basic heat / cool thermostats have a mode selector for either ‘heat’, ‘cool’, or ‘auto’) when the room temperature rises above the set-point, the thermostat will close the connection between the ‘R’ and the ‘Y’ terminals completing the circuit to the compressor and condenser unit that provides the cooling for the evaporator coils mounted in the discharge ducting of the system.

The next step up from the older home systems was the introduction of the ‘fan on / auto’ switch. This function allows you to circulate the air in the house by using the fan in the furnace without heating or cooling the air moving through the system. In the most basic system, this functionality is provided by use of a fan center relay, and the low voltage wiring to the thermostat now will require a minimum of three wires (for heat only units) and four wires (for heat / cool / fan) for control. This additional terminal is labeled ‘G’ in the thermostat. When the ‘fan on’ setting is selected, the contacts between ‘R’ and ‘G’ are closed turning on the relay that powers the blower motor directly regardless of a call for heating or cooling. If you are replacing an old thermostat with a new digital thermostat, the electronics in these units may need yet another wire for a terminal labeled ‘C’. This terminal is for a common connection from the control transformer that will provide steady 24 volt power between terminal ‘R’ and ‘C’ to power the thermostat itself.

If adding wires to your thermostat location is not possible, or very difficult, look for a thermostat that is battery powered which won’t require the terminal ‘C’ for operation. The obvious down side of this is that if the battery goes dead the thermostat will no longer function. Climate control systems have evolved a great deal in the last several years with the evolution of electronic control. This has made the units more efficient, with added safety features, and with this comes increased difficulty for the average home-owner to repair or replace any parts of the system that may break down (and they will, but only when you really need it!) However, the terminals of the thermostats of today are still labeled the same, and provide the same functions as what we have discussed thus far. They just get a little more sophisticated so if you are pre-wiring a new home, check with the contractor providing the HVAC system as to what thermostat wiring is required. A new system may need as many as ten wires (like a two-stage heat, two-stage cooling system, heat pump reversing valves, fan control, etc.)

If the system had two-stage heating, and/or cooling, the 2nd stage termainals would be labelled W1 and W2 for heating, Y1 and Y2 for cooling.  These additional terminals are not shown in this diagram. Now that you are armed with a basic understanding of the temperature control system in your home, you should now be able to identify what kind of system you have, and what type of functions you will need if replacing or up-grading your existing thermostat. I hope this article helped with your question about thermostat wiring.  Thanks for taking the time to read it!  If you have any questions or comments on your furnace situation, feel free to add a comment below. Not sure about doing this type of project yourself?  Have you been stumped by a three-way switch, struggled installing a new receptacle, or basically put off doing any wiring project yourself because you didn’t feel confident working around electricity?  Electricity can be downright scary.  However, armed with the proper information you must have to work safely and completely, home owners like yourself CAN complete most simple home wiring projects.

Rather than scour the internet looking for the information you need to work on a wiring project or problem, I can recommend a tremendous resource that pulls together virtually everything you will need to know.  I don’t endorse many projects, but this is one that I fully endorse – “The Basics of Household Wiring” DVD or e-book. As I licensed, journeyman electrician, I have pretty high expectations for a DIY resource.  It must be professional, it must be technically correct, and above all, it must focus on safety.  I found all of that in this DVD, and I am confident that you will be impressed with the quality of the information contained in this excellent resource. This easy to understand tutorial (and no – you won’t need to understand technical “jargon”) breaks down the majority of home wiring projects you’ll encounter around the house and provides detailed, step-by-step instructions on how to complete them – safely and competently.  This comprehensive, professionally produced DVD will give you the confidence you need to work on any of those home wiring projects!