~OPERATING CONTROLS - Air Conditioning System Operating Controls~

Basic air conditioning inspection and inspection report information for A/C controls:

1. Thermostats: The air conditioning system is operated by thermostat in the living area. If multiple air handling units and compressors are installed you should find a thermostat for each area served by that equipment.
2. Air conditioning system zone dampers: Some air conditioning system designs may use a single air handler and compressor, but may add zone dampers in the duct work to provide individual "zoning" of cool air distribution. In this case each zone thermostat both calls for the system blower/compressor to operate and also causes a motorized zone damper to open to direct cool air to a particular portion of the building. An ordinary home inspection is unlikely to address proper operation of motorized zone dampers.
3. Manual duct dampers may also be present in duct work to manually balance air distribution among building areas. Be sure to look for these when diagnosing poor cool air delivery to an area.) Also see our note below about the presence of multiple switches and controls.
4. Electrical switches for air conditioning systems will often be found as follows: (some of these may be absent on some systems)
• Electrical panel circuit breakers or fuses will be provided separately to control the air handler (blower system) circuit and the compressor/condenser circuit. Of these the compressor is usually supplied by a 240V circuit and the air handler by a 120V circuit.
• Compressor safety shut off switch outside at the compressor/condenser. The switch may be a circuit breaker, fuse, or a simple "pull-out" disconnect located close to the compressor.
• Air handler service switch inside on or close to the air handler unit itself

  ~SAFETY CONTROLS - Air Conditioning Automatic Safety Controls - Cooling System Fuse or Circuit Breaker Size Requirements~

  ~Electric Power Controls - Safety Disconnects for Air Conditioners~

  Safety disconnects should installed outside next to the compressor/condenser unit and are often also installed next to or mounted on the air handler/blower unit.

  If you cannot find an outside electrical disconnect one should be installed. These controls are recommended for safety to reduce the temptation to open the cabinet and work on the equipment with power on. Working on electrically "live" cooling equipment risks both shock and mechanical injury such as being cut by the fan if the motor starts unexpectedly. Safety shutoffs are required for new equipment.

  ~How to Specify the Breaker or Fuse Size for Air Conditioning Circuits~

  Amperage rating of safety disconnects and A/C circuit breakers: the safety switch on newer equipment may be a simple pull-out fuse-block type power disconnect, leaving circuit protection to be provided only at the circuit breaker or fuse for the A/C circuit where it originates in the electrical panel. Where the actual overcurrent protection is provided (at older circuit breakers used as auxiliary safety disconnects at the equipment, and at the main panel at the origin of the cooling circuit for the compressor/condenser unit) electrical overload protection size (circuit breaker or fuse amperage rating) for modern A/C equipment is specified by the manufacturer.

  The Maximum Fuse or HACR type Breaker: specifies the maximum overcurrent protection or MOP to be used to protect the equipment. The permitted ampacity of the equipment electrical circuit protection (fuse or circuit breaker amps) expressed as MOP or Maximum Overcurrent Protection. If MOP is specified, the breaker or fuse protecting the equipment should match this number.

  As we explained at the beginning of this document, a hermetic compressor draws varying amounts of current (measured in amps) as its internal pressure changes during operation. We said that current draw is higher when starting the motor than when the system is in steady state operation. Current draw is highest if the motor is starting against its highest back pressure such as if the air conditioning system has been turned off and then back on in the middle of operation. Because fusing an air conditioning compressor at the minimum level can result in blown fuses or tripped breakers during these intervals of heavy current draw during compressor startup, compressors are either protected by a slow-blow fuse or a somewhat larger than minimum circuit breaker.]

  Rules of thumb for over sizing air conditioning system breakers or fuses: On some older equipment MOP is not specified. Only when MOP has not been specified can the overcurrent protection required be determined by alternative means such as [RLA OR BCSC whichever is greater x 175%], or if the compressor keeps tripping that device or blowing that fuse, RLA x 225% might be used. The National Electrical Code (NEC) specifies the degree to which a breaker or fuse may exceed the RLA. [For example, if the MOP or fuse size is specified by the manufacturer to be 40 amps, then a 40 amp breaker must be installed with no increase or change in that rating.]

  Multiple switches are often present on cooling systems. As we reminded in the previous chapter, if the air conditioning system won't run, before requesting a service call check all of the switches as well as the thermostat for proper settings.

  ~Air Conditioner Fuse or Circuit Breaker Size Details~

  Generally, what is the ampacity we see in the field when inspecting an air conditioning compressor circuit?

  When the air conditioning system is running, if you measured the amperage, it would be roughly 80% of the RLA. The breaker size is typically about 125% of the total of the compressor RLA and the condenser fan FLA (full load amperage).

  The rationale is that the circuit breaker protecting the air conditioner compressor unit should trip in the event of a locked rotor [the revolving axle of a compressor motor, for example] or some significant electrical event, but should not trip during start up loads which, as we know can be significantly higher than the RLA momentarily [as the compressor motor draws higher amperage to get itself started].

  Why can we put an "oversized" fuse or circuit breaker on an air condtioning compressor circuit?

  An air conditioning electrical circuit is different than a general household circuit in that we have a known current load. [There is only one device connected to the air conditioning electrical circuit, and we can read its operating characteristics.] We are not worried about an overload situation where people plug several appliances into receptacles on a single circuit. Generally speaking, the amperage draw is fine or is way too big.

  Section E3602.10 of the IRC says, "Branch circuits for air conditioning and heat pump equipment. The ampacity of the conductors supplying a multimotor and combination load equipment shall not be less than the minimum circuit and capacity marked on the equipment. The branch-circuit overcurrent device rating shall be the size and type marked on the appliance and shall be listed for the specific purpose." In short, do what it says on the dataplate.

  ~How to Diagnose & Repair an Air Conditioner Compressor Which has Lost Cooling Capacity~

  A compressor which appears to have lost cooling capacity can be diagnosed by a service technician who can connect the appropriate test gauges to the system. Lower than normal discharge pressure and higher than normal suction vacuum will indicate this problem. But before assuming that something is wrong with the air conditioning compressor, some basic investigation is in order. Unless there is an obvious indication of a compressor problem (noise, hard starting, compressor won't start), the service technician, to be thorough and economical, will inspect the system in an order, checking the easy and least-costly problems first, such as presence of electrical power, proper setting and operation of system controls, condition of filters, condition of duct work, operation of blower fans, before moving on to check the compressor itself by looking at the air conditioner operating temperatures, pressures, and current draw in Amps.

  ~Cooling Capacity of the Air Conditioning Equipment~

  The cooling capacity of an air conditioning equipment refers to the ability of the compressor/condenser (usually outside) and the air handler/evaporator (usually inside) to deliver cool air to the occupied space. Briefly, the compressor/condenser draws refrigerant gas from the building air handler, compresses it and cools it back to a liquid refrigerant, and the air handler/evaporator coil permits liquid refrigerant to evaporate inside a cooling coil, across which the fan blows building air to cool and dehumidify it. The particular combination of this equipment has a cooling capacity, usually rated in BTUh or thousands of BTU's of cooling capacity per hour, documented on equipment data tags discussed above at "RATED COOLING CAPACITY" - see links at left.

  ~Air Conditioner Long "on" cycle and Insufficient Cooling - Loose or Worn Compressors~

  A longer than normal "on" cycle combined with little or no cool air conditioner output could be due to an inefficient air conditioning compressor or one which has lost its ability to "compress" the refrigerant due to internal wear. This condition can be diagnosed by an air conditioning service technician who will install air conditioning manifold gauges onto the system to check the compressor suction vacuum and discharge or "high side" output pressure. If gauge ports are not installed on the air conditioner compressor unit the technician cannot make this test without cutting the refrigerant lines to install gauge ports (adding to the cost of this diagnosis).

  ~Low air conditioner motor amperage draw~

  Low amperage draw: unlike a high-amp current draw which indicates that the compressor is danged internally in a way that its piston(s) is(are) tight in the cylinder, a low-amp current draw, if below normal, may confirm internal wear on the compressor parts, and would support the diagnosis that the compressor is worn and inefficient. Where there are no gauge ports to actually measure compressor low side and high side vacuum and pressure, this simple electrical test is a useful first step.

  ~Abnormal air conditioner compressor pressure readings~

  Refrigerant line pressure readings which are abnormal (probably too low) on the high pressure side (compressor output) or on the low pressure side (compressor input or suction line) can indicate a problem with the compressor's ability to develop normal operating pressure ranges and thus will affect the cooling capacity of the air conditioning system. For more details on air conditioner refrigerant line pressures and how they are examined see COMPRESSOR PRESSURE READINGS

  ~Cooling Capacity of the Air Conditioning Duct System~

  Even if a very high BTUH capacity cooling system is installed, if the duct system is defective the ability of the system to deliver cool air to the occupied space can be severely or even totally lost. Duct System Efficiency (in percent) describes the percent of cooled air produced by the A/C equipment which is actually delivered to the occupied space. This number is less than 100% because of air flow restrictions and losses in the duct work. Duct Delivery Effectiveness is the percent of cooling capacity which is delivered through the registers into the occupied space. Registers themselves restrict air flow. See "DUCT SYSTEM DEFECTS" - see links at left.

  Cooling Capacity of the Whole Air Conditioning System

  So the ability of the entire A/C system to cool a building or rooms in it requires that both the cooling equipment and the duct system be in proper working order.

  These Simple Checks May Enable Inexpensive Repair of Lost Cooling Output from an Air Conditioner

  Before ordering an expensive air conditioner service call to restore lost cooling capacity, here are a few simple steps to perform. Some of these can be done by any homeowner, others may require a bit more expertise.

• Clogged filters can lead to lost cooling capacity first, because the clogged filter reduces the air flow through the system, meaning that you'll feel less air flow at the supply registers than was previously present.
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• Damaged cooling ducts such as ducts which have been improperly routed and are crimped, crushed, or have excessive bends can reduce cool air flow in an otherwise properly functioning system. One client had us drive a considerable distance to repair her apartment's central air conditioning system after having had several unsuccessful service calls. Apparently no one had managed to get into a rather tight and hard-to-enter attic crawl space where the cooling ducts were routed. We found that the main cooling duct had become disconnected. The attic was nice and cool but no cool air was being blown into the living area. Check the condition of the duct system for blocked ducts, loose leaky connections, closed dampers, crimps and bends, before calling your service technician.
• Cooling Coil Ice-Up A second result of the reduced air flow due to a dirty air conditioning filter can cause the evaporator coil (the cooling coil) in the air handler to become blocked by ice, stopping or significantly reducing cool air output from the system. A visual inspection of the cooling coil in the air handler can quickly show whether or not it's ice-covered. If the coil is iced-up and blocked, turn off the cooling system entirely until the ice has all evaporated and cleared. Be sure that your condensate drain is not blocked and that the water from melting ice will be properly disposed-of. If you replace all dirty filters on the system and remove ice from an iced coil and the coil ices up again when the system is turned back on, you may have the next condition in this list. For a detailed discussion of air conditioner or other refrigeration (or dehumidifier) cooling coil ice-up diagnosis and cure, see Air Conditioning Cooling Coil or Evaporator Coil Ice-up
• Improper refrigerant charge - too little: an air conditioner system which has lost some (but not most) of its refrigerant will sometimes run too cold at the evaporator coil, leading to coil icing and loss of cool air delivery in the home. If the filters are clean and the coil ices-up this condition may be present. A service technician will need to evaluate and test the system and if needed, adjust the refrigerant charge.
  Watch out for refrigerant leaks that lead to a repeat of this problem. If the system is low on refrigerant because it has a leak, it is much smarter to find and fix the leak than to simply keep adding refrigerant. If you keep adding refrigerant to a cooling system you're leaking possible contaminants into the environment as well as wasting money.
• Improper refrigerant charge - too much can also lead to improper air conditioning system operation and in some cases can damage the compressor. A service technician will need to evaluate and test the system and if needed, adjust the refrigerant charge. In this case the cooling coil is probably not going to ice-over, it just won't get cool.
• Loss of most refrigerant from an air conditioning system means that the cooling coil will not get cool at all. A service technician will need to evaluate and test the system and if needed, find and fix the refrigerant leak, evacuate the system, and install the proper refrigerant charge.
• Compressor problems such as an aging cooling compressor that is at or near the end of its life may be unable to properly compress the returning refrigerant gas to a sufficiently high pressure. A service technician will need to evaluate and test the system and if needed, replace the compressor. Since this is a costly repair, be sure to ask why the compressor failed and to correct any underlying cause (such as low voltage). Variations in line voltage can lead to improper compressor operation and loss of cooling output.