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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE DIAGNOSIS & REPAIR
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of HEATING & COOLING TERMS
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
FAN NOISES, HVAC
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
HVAC system controls & switches: this article explains where to find and how to use the switches and controls for air conditioning and heat pump systems.
We list and explain the function of each air conditioner or heat pump control or switch, including providing identification photographs and troubleshooting tips.
HVAC control definitions & photos are organized by where they are found: indoors or outside of the building, and at the air handler/blower assembly or the outdoor compressor/condenser unit, or perhaps in other locations such as wall thermostats or electrical panel main switches.
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Here we list all of the controls and switches on a typical split system air conditioner with indoor and outdoor components include the items listed just below.
We explain these many electrical switches and controls that control an air conditioner or heat pump system. You'll need to check these first if your air conditioner won't start or won't keep running.
If the A/C system won't operate, before requesting a service call check that it is turned on at every control, switch, or circuit breaker, and that the thermostat is properly set.
Article Series Contents
If the A/C or heat pump system will not run check all of these control and safety switches listed here before calling your service technician. If someone or some condition has turned one of these switches off, resetting it may be all that's needed. Not all of these switches will be present on every system; fuses may be used instead of circuit breakers; fuse pullouts may be used instead of a circuit breaker or fuse at some service switches.
Watch out: Safety warning: do not put your fingers or hands inside of a heating furnace or air conditioner blower or blower compartment without making certain that all electrical power to the unit has been shut off. If the blower starts turning you can lose a finger, and there are also electrical shock hazards in these areas.
The pair of photographs just above show the outdoor air conditioner or heat pump compressor/condenser service control switch, in this case a circuit breaker, installed outside at a compressor
for a ductless cooling system compressor.
More photographs of a ductless or split system air conditioning system are
Our page top photograph is an important one to study further. The air conditioning system compressor/condenser service switch for this outdoor unit was a 240V fused circuit with outdoor fuses in the box shown in the photo.
In this close up photograph you can just make out that someone has installed 1/2" copper pipes in place of the original fuses.
This might be a dangerous installation, risking fire as well as a burn up of the air conditioning equipment. But as Mark Cramer pointed out, if the circuit were properly protected by breakers or fuses at the main panel, the insertion of metal pipes in these fuse sockets just converts the device from a fuse box to a simple switch.
In any case, simply installing fuses would restore the proper
safety device but it's likely that further testing and diagnosis of the electrical circuit and the compressor/condenser unit will
be needed to determine why someone installed copper pipes where fuses belonged in the first place.
When someone converts fuses to a switch in this location it may be because the air conditioning system was frequently blowing the fuses --
someone wanted to force the compressor/condenser to run.
Circuit breaker(s) at the electrical panel protect the circuit supplying power to the air conditioning system. Typically separate circuit breakers (or fuses) power the compressor/condenser unit and the indoor air handler/blower assembly.
Watch out: your HVAC equipment may be run out of a sub-panel rather than the main electrical panel. Be sure you have found all of the electrical panels, sub panels, and manual control switches for the equipment both inside the building and outdoors.
Watch out: really watch out: if you re-set a circuit breaker or replace a fuse and the breaker trips again or the fuse blows again, leave the equipment off and call a qualified service technician. You probably have an unsafe condition. Forcing electrical equipment to run can cause shock or fire.
Compressor/Condenser Unit: the "outside" portion of an air conditioner or heat pump, the compressor re-compresses refrigerant gas back into a liquid and in the process, moves heat (in the refrigerant) either from indoors to outside (air conditioning mode) or from outdoors to inside (heat pump mode) if the system is a heat pump.
Compressor service switch: The air conditioning system compressor service switch is located outdoors, at the compressor/condenser unit, typically on a building wall near the outdoor unit,
this switch may be a circuit breaker, a fuse block pull-out, or a simple electrical switch.
Where the compressor/condenser service switch is omitted some technicians are tempted to work on
equipment while it is electrically "live", and a few of them get shocked. These switches are required by current electrical
codes but may be absent on old A/C installations.
A/C and heat pump systems use a contactor relay (circled at left) because the little 12-24V wall thermostat circuit and switches are not capable of handling the higher voltage used by the compressor/condenser motors.
The contactor relay is basically a low-voltage-operated switch [typically 12-14 volts] controlled by the low-voltage room thermostat) that switches a heavier-duty electrical relay to give 120V or 240V electrical power to the compressor/condenser unit.
The "two poles" simply means that the relay switches two electrical wires simultaneously - which is what you'd expect if your heat pump motor is running on 240 Volts.
Tips for changing out or installing a new magnetic contactor relay on an air conditioner or heat pump
Match the part numbers correctly when installing a new contactor relay in an air conditioner or heat pump. If the coil that activates the relay is not the right one for the compressor motor, you can have either of these problems:
Coil strength is too low: the coil won't reliably energize the start circuit in the motor and the compressor will be hard to get started
Coil strength is too high: the contactor relay coil won't let go of the start circuit: it will keep the start circuit active in the system even after the compressor motor has started - resulting in burning out the starter circuit.
Also locate a magnetic contactor relay switch in the proper physical position (mount it like the original was mounted) don't just hang the relay by its wires (as in our photo above). Some relays may be affected by gravity, either holding points closed too long or not holding the points closed. A current-operated contactor relay (most new units) can be mounted in any position.
Watch out: Some of our readers report successfully replacing minor electrical components such as switches, relays, and contactors. But unless you are qualified to do so we do not recommend trying to do work on electrical systems and components in your home as there are potentially fatal electrical shock hazards. Because air conditioner compressor/condenser units include start/run capacitors even when you have turned off power you can get a nasty shock if you're not careful.
Air Conditioner, Heat Pump, Refrigeration System Operating Pressure Control Switches & Electrical Sensor Safety Switches
We distinguish between operating control pressure switches and safety switches that monitor against too-low or too-high refrigerant pressures in a refrigeration system. Both are described in detail
Operating Pressure Control Switches for Air Conditioners, Heat Pumps, Refrigeration Equipment
Operating pressure control switches set the normal compressor cut-in and cut out pressures in commercial refrigeration systems including air conditioners, coolers, refrigerators freezers.
Commercial refrigeration equipment such as refrigerators (coolers) and freezers use a pressure control switch to set the cut-in and cut out pressures at which the compressor shall operate.
The pressure control switch and the thermostatic expansion valve may both need to be inspected, tested, and set to cooperate with one another, as I describe in an old field service call report described
Shown at left is a Ranco™ single pressure control switch with an operating range of 12-50 psig and a differential range of 5-35 psig. Ranco produces a wide range of switches and controls including air conditioning controllers. This particular Ranco switch shown at left "opens" on low.
Available from Grainger and other refrigeration equipment suppliers.
The Ranco Type "O" single function pressure controls are very widely used on refrigeration systems and can operate either as normal operating controls or as protection devices
Refrigeration System Pressure Safety Switches: protect against over pressure or under pressure in air conditioning or heat pumps
Some HVAC systems (air conditioners, heat pumps, refrigeration equipment) include other sensor switches that may include an electrical pressure or temperature transducer/sensor (photo at left) or an air conditioning or heat pump pressure sensing switch that detects improper (too high) or [in some systems including automotive air conditioning] too-low refrigerant pressures in the system.
Details about these safety switches are
For safety the air conditioning or heat pump pressure switch can shut off the system. When pressures return to normal the pressure safety switch normally auto-resets and operation can continue.
Watch out: As Ranco and others warn, both under pressure (that can damage the compressor) and overpressure (that can damage or eve blow something up) at an air conditioner or heat pump system - can be dangerous need to be guarded against.
See A/C - HEAT PUMP CONTROLS & SWITCHES for details.
Air Conditioner or Heat Pump Controls In or On the Air Handler Unit or Blower Compartment & At the Cooling Coil
Air Handler: the air conditioner or heat pump air handler is the "indoor" portion of the cooling (or heating) system whose job is to condition air from the living space by blowing air across a cooling coil (air conditioning) or heating coil (heat pump), sending the conditioned air on through supply ducts into the occupied space. The primary parts of the air handler unit include:
Thermostatic expansion valves & other refrigerant metering devices: An air conditioner thermal expansion valve or "TEV" or just "expansion valve" (tan colored device in the photo) is a device located at the cooling coil and connected between the incoming refrigerant line and the refrigerant
inlet to the cooling coil in the air handler.
There is a variety of refrigerant metering devices and they're not all called "TEVs":
Blower compartment door switch: an air handler blower compartment access door switch is present on newer units, and can be seen as a button or switch which is depressed when the blower compartment door or cover is properly in place.
[Click to enlarge any image]
If your air conditioner won't run and someone has been fooling with it, be double sure that the blower compartment door is properly closed and that the interlock switch sensor button or lever is properly depressed to convey that fact to the system.
Quoting Trane's typical equipment instructions:
Details about air handler, furnace, or air conditioner blower compartment door safety switch installation, testing, repair or replacement are
Other switches found inside the air handler unit
(Feb 9, 2014) firstname.lastname@example.org said:
ducted gas heating triggers safety switch
We'd like to help but don't understand the questio. Which safety switch?
(May 10, 2014) William said:
air conditioner wont run it will kick the house breaker out when you turn the power on
William, leave the unit off. The problem could be a bad compressor - we don't want to damage other components or overheat wiring (a fire hazard) by trying to run it.
(May 30, 2014) Hi, I'm Georgia, my goodman all in one heat pump will not come on at all . We put in new thermostat and it shut whole system down, we replaced transformer, 2 capacitors and contactor it did not help. Nothen comes on. Can someone help.? said:
Hi, I'm Georgia, my goodman all in one heat pump will not come on at all . We put in new thermostat and it shut whole system down, we replaced transformer, 2 capacitors and contactor it did not help. Nothen comes on. Can someone help.?capacitors and contactor it did not help. Nothen comes on. Can someone help.? I hope I'm put this in right place don't see anywhere else. Thanks
(June 4, 2014) Namecarhy said:
outside unit does wrk plz help
A good place to start diagnosis of inoperative heat pumps is right at LOST COOLING CAPACITY where we give a list of steps to take
(June 24, 2014) justice dithole said:
hi I have a problem with window type air conditioner, it is cooling even when I have turned the adjuster to heating mode. it only cools and regrets to warm me up in my office
(June 24, 2014) DAVE said: What does it do Justice?
Dave and I face the difficulty of guessing at what's wrong on an air conditioner with very little information. But I'll add that if a unit will not change from cooling to heating mode, and presuming it is designed as a heat pump capable of both cooling and heating, then an internal control or switch needs repair or replacement - possibly the reversing valve in the unit.
(June 28, 2014) terry wycoff said:
i have an old arcla? brand ac that runs water outside,but as soon as you turn the dial only runs seconds
(July 28, 2014) Anonymous said:
Hi. I have brand new American Standard split system. Runs fine until there's a storm. Power doesn't need to go off, nothing weird happens in the house, but the unit loses all power, even the thermostat is dead, which is wired to the air handler in the crawl space. I've had utlility company out and everything on their side is good. I've had electrician out and all on my side is good. Unit is under warranty for now, but I'm frustrated.
I have to call them out on average once a month when a storm goes through. I have two other places and this doesn't happen, ever. Was thinking about installing whole house surge protector, but both air guy and electrician tell me that won't help my problem. This is not good and I need some type of idea on what to do. I'm really upset, since my air company can't seem to fix it. Thank you.
I agree you need better diagnosis, and from here can only guess.
Has anyone checked for a water or moisture problem in controls or wiring?
8/18/14 sleepless in Seattle cause no cooling said:
I have an American Standard Heritage 16 Heat pump. I did the terrible deed of not cleaning the air filter located above furnace and it shut off the whole system. I cleaned the filter and ran through some test modes on the thermostat and the furnace comes on fine, but not the heat pump. I read that a motor may have a reset button that needs attention, but I opened up the side of this thing and it looks like a motor covered with orange insulation is under the fan, but I can't see how to get to it. Yes. I know nothing about this stuff but can read directions, only I'm not able to get any schematics on this thing. Any help would be appreciated.
Sleepless - Seattle
We're flying blind here but possibly a fan motor might overheat due to reduced air flow. However If the furnace and blower run in response to a call for heat then the fan motor is working. If the fan is inoperative the furnace limit control should shut the system down.
I'm not sure how an indoor fan problem will keep an outdoor heat pump from turning on. I'd look for a thermostat mode setting problem or a control board or relay problem first.
The furnace fan comes on during test mode from the thermostat. Once the thermostat calls for heat or cooling, it shuts off and heat pump does not come on. Thermostat flashes "wait" like the heat pump is in some setup or testing mode?
If the blower runs properly in test mode then we're not looking at a problem with the blower fan motor itself. More likely there is a problem with the compressor/condenser unit or a control board, relay, or theremostat problem.
First check your thermostat's instructions and manual. Some therm air conditioner and heat pump thermostats such as some Honeywell models will introduce a five-minute delay before starting the compressor when the thermostat is switched "on" or set higher in short intervals. That's because rapid turning off and on of a call for heat at a thermostat can cause compressor motor short cycling. Because some compressors may have trouble starting against high refrigerant head pressures that remain at the moment of switching "off" the five minute delay is intended to give time for refrigerant pressures to equalize in the system - giving an easier start to the compressor. If the compressor still won't start I'd suspect a bad start relay or start capacitor.
It's time to ask your HVAC service company to send out a trained service tech.
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