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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
This article explains where to find and how identify and adjust the refrigerant pressure operating and safety pressure controls for air conditioning and heat pump systems and similar equipment. These same controls are used on other refrigeration systems such as commercial coolers and freezers.
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See A/C - HEAT PUMP CONTROLS & SWITCHES for our complete list of HVAC controls and switches. We include photographs to assist readers in recognizing cooling system defects.
Other articles at this website discuss certain of these controls in greater detail. See SYSTEM OPERATION for a discussion of thermostats, zone dampers, and circuit breakers on air conditioners and heat pumps.
If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS. See How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building. CONTACT us to add HVAC system switches or controls or diagnostic suggestions you do not find here.
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 here.
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 at REFRIGERANT METERING DEVICES TEVs
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.
Among these refrigeration system pressure controllers, the Ranco Type "O" single function pressure controls are most common. These pressure control switches actually are designed to operate either as normal operating controls or as protection devices (see Safety Switches below). On these controls a "multi-capsule" pressure element operates a single or double pole electrical switch that typically will turn the compressor motor on or off.
Watch out: some pressure controls use a manual reset on their cut-out pressure feature. That means that your equipment may be shut down until you locate and press the reset bar on the equipment. Typically the reset bar is located on the top of the pressure control switch.
Pressure control switch sensor tubing warnings
Also for proper installation of one of these switches you should avoid sharp bends or kinks in the capillary tube sensor tubing, and don't let the tubing be exposed to vibration due to improper mounting or installation. Tubing that is too loose and vibrates can rub and wear through; tubing that is too tight can vibrate in what Ranco calls "violin string" vibration that can cause the tubing to break. And of course all refrigerant lines in the system will need to be purged when installing a new control.
Adjusting the refrigerant pressure control switch
You should not mess with the switch adjustment unless you are a qualified HVAC service technician since doing so risks damaging the equipment or causing an unsafe condition.
But FYI, in general, turning the RANGE adjustment screw inwards or "clockwise" (the right hand screw on the control top) will decrease the control pressure setting, and turning the screw out or counterclockwise will increase the pressure setting.
The DIFFERENTIAL setting screw (the left hand screw of the pair as you face the front of the control) turns in or clockwise to increase the differential and out or counterclockwise to decrease the pressure differential.
Note: you won't normally see this type of refrigeration pressure control switch on residential air conditioners, heat pumps, nor residential appliances.
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.
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.
Ranco also produces air conditioning safety controls: their G60 pressure controller (small photo at immediate left) is a cut-out switch that protects the refrigeration system compressor against abnormally low suction pressures that could otherwise cause liquid refrigerant to enter (and destroy) the compressor.
The Ranco G63 pressure controller is a high pressure cut-out switch that protects the refrigeration system against excessive compressor outlet or discharge pressure.
These air conditioner safety switches are offered in a range of models, depending on the cut-out and cut-in pressures desired. Cut out/in pressure ranges extend from 0.14 psi cut-out and 8 psi cut in up to 362 psi cut-out and <286 psi cut-in with variable differential settings.
Typically a low pressure safety switch to protect the compressor against liquid refrigerant will automatically reset when conditions return to normal. A high pressure safety switch may include an automatic reset and (for pressure decrease) a manual reset mode.
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.
Where to buy pressure control switches for refrigeration equipment
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Technical Reviewers & References
Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.
If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.
CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.