Air Conditioning, Heat Pump & Refrigeration System Pressure Controls & Safety Switches
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Description of refrigeration pressure control switches - cut-in/cut-out controls for the compressor unit
Description of refrigeration system safety control switches that protect against over pressure or under-pressure on air conditioners, heat pumps, etc.
Key switches and controls to check if your air conditioner or heat pump is not working
Questions & answers about cooling system & heat pump system switches & controls

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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Air Conditioner or Heat Pump Operating Pressure Control Switch & Electrical Sensor Safety Switches

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. Also see THERMOSTATS and THERMOSTATIC EXPANSION VALVES.

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

Commercial refrigeration equipment pressure control, Ranco

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.

Similar pressure control switches are used in many mechanical system operations including steam heat - see Pressure Switch, Steam Boiler and even on water pumps - see WATER PUMP PRESSURE CONTROL

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.

Ranco pressure safety switch 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.

Sources of pressure switches for refrigeration equipment include the

Behr A/C pressure Switch (less than$30). and other automotive switches.
Micro Pneumatic Logic pressure switches (wide range of applications).
Ranco and Grainger - pressure control switches and pressure safety switches for air conditioning and refrigeration systems, protecting against overpressure and under pressure
Sensata automotive air conditioning (A/C) pressure switches [dual function pressure switch provides high pressure cut-out (HPCO) with fan function or high side low pressure (HSLP) cutout functions

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A/C - HEAT PUMP CONTROLS & SWITCHES
  Outside Switches, Fuses, Breakers
  Inside Switches,Components
  Air Handler / Blower Switches
  Blower Door Switches
  Backup Heat Controls
  CAPILLARY TUBES
  CIRCUIT BREAKER SIZE for A/C or HEAT PUMP
  Compressor Condenser Switches, Controls
  Condensate Overflow Switch
  CONTACTOR RELAY DIAGNOSIS & REPAIR
  CONTROL CIRCUIT BOARD, A/C
  FAN, COMPRESSOR/CONDENSER UNIT
  Duct System Switches
  Duct System Filters
  OPERATING CONTROLS, A/C & HEAT PUMP
  Starter Capacitors
  Thermostats & Controls
  Thermostatic Expansion Valves
  Motor Overload / Overheat Reset Switch
  Pressure Controls & Safety Switches
  Zone Damper Controls

Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Lester Richer, a professional home inspector, for the reminder that a bad air conditioner condensate drip tray switch can shut the whole system down.
Behr Hella Service A/C pressure switches,
Grainger, http://www.grainger.com/
Ranco Corporation, http://www.invensyscontrolseurope.com/ranco/ Tel (Europe) +44 (0)845 130 5522
Sensata Technologies, 529 Pleasant Street, B41 Attleboro, MA 02703-2964 Phone: 1-248-692-5600 Fax: 1-248-692-5630 Email: auto-mktg@sensata.com Web: www.sensata.com, pressure switches for automotive air conditioning systems
Micro Pneumatic Logic, Inc., Pompano Beach, Florida Tel. (954) 973-6166 - pressure switches

How to diagnose and fix an air conditioning system that is not working

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.

OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
LOST COOLING CAPACITY: what to do when not enough cool air comes out of the system
- What to check first if there is no cool air or not enough cool air
- Compressor failure diagnosis: basic checks of the air conditioner compressor
- Ducts & Air Handler diagnosis: basic checks of the indoor air handler (blower), air ducts, and filter systems
COMPRESSOR CONDENSER: problems with air conditioner compressor/condenser units
AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself
DUCT SYSTEM DEFECTS: problems with the air duct system, air filters, supply registers, return air registers
A/C REFRIGERANT LEAK DETECTION: how to use a TIF5000 to detect air conditioning refrigerant gas leak
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
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Air Conditioner or Heat Pump Operating Pressure Control Switch & Electrical Sensor Safety Switches

Operating Pressure Control Switches for Air Conditioners, Heat Pumps, Refrigeration Equipment

Pressure control switch sensor tubing warnings

Adjusting the refrigerant pressure control switch

Refrigeration System Pressure Safety Switches: protect against over pressure or under pressure in air conditioning or heat pumps

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Technical Reviewers & References

How to diagnose and fix an air conditioning system that is not working

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair