Window AC - Operation & Maintenance


Window A.C. Unit has following advantages & disadvantages:


  1. Easy to procure.
  2. Easy to install.
  3. Easy to maintain.
  4. Additional place requirement is not there.
  5. No pump required.
  6. Air cooled.


  1. They have less capacity.
  2. External wall with window space is required.
  3. With the increase in ambient above 350C the capacity decreases.
  4. No humidity control.
  5. No fresh air ventilation.
  6. No dust control.
  7. Not suitable for continuous operation.
  8. Adjacent unit has adverse effect on efficiency.
  9. No control for air distribution.
  10. Life is very less compared to central plant.


A typical refrigeration cycle for room air conditioners is shown in figure-I. Compressor takes in refrigerant vapour from the low pressure side, compresses it, and delivers it to the high pressure side of the system at higher temperature. In the condenser, discharged refrigerant vapour passes through a series of coils where cooling air lowers its temperature until it condenses to a liquid refrigerant. Capillary tube regulates the flow of liquid refrigerant from the high pressure side to the low pressure side and reduced pressure of liquid refrigerant. The refrigerant leaves the capillary tube, and enters the evaporator. In the evaporator, the heat is absorbed from the room air by the liquid in the evaporating coils causes the liquid to boil until evaporation is complete. The low pressure gas which results when this liquid evaporates, is drawn through the suction line to the compressor. The gas passes over the motor windings before it entres the compressor cylinder and helps to keep the moter cool. From the suction, the compressor low pressure gas then enters the cylinder, is compressed, and discharged again as a hot high pressure gas to the condenser to repeat the cycle.

Table of Contents

Components of WAC Units

A typical window AC components can be broadly divided into three distinct categories:

  1. Mechanical parts:

    1. Dead plate
    2. Internal partition
    3. Damper
    4. Filter
    5. Cabinet
    6. Grill
    7. Water tank
  2. Refrigeration parts:

    1. Compressor
    2. Condenser
    3. Stainer
    4. Capillary
    5. Cooling coil
    6. Refrigerant
  3. Electrical parts:

    1. Selector switch
    2. Thermostat switch
    3. Overload protector
    4. Starting relay
    5. Running capacitor
    6. Starting capacitor
    7. Blower motor
    8. Heaters
  • Refrigeration Components:

    • Compressor: It is nothing but a pump. It serves two purposes:

      1. It is used to compress the refrigerant vapour forcing it into pressurised state so that excess heat can be liberated at the condenser.
      2. It keeps the refrigerant moving through the system.
    • Compressor Motor: We generally use hermetic sealed compressors. It has a motor and compressor sealed in a dome and welded to together. The compressor motor used is split-phase induction motor. It has a starting /running winding and starting / running capacitor.

    • Condenser: The condenser is a part of high pressure side of the system. It serves two important functions:

      1. It removes the heat picked up by the refrig¬erant in the cooling coil (evaporator) and
      2. It condenses the refrigerant from vapour to a refrigerant liquid state. The refrigerant vapour is passed through a coil that is cooled by a fan. The fan blows air over the coil and since the air is cooler than the refrigerant vapour, the vapour gives off heat to the air trough the coil walls. The condenser removes enough heat to change the refrigerant from a vapour to a liquid.
    • Expansion Valve/ Capillary Tube: It performs two functions:

      1. It reduces the pressure of the refrigerant liquid and there by cools the liquid.
      2. It acts as a metering device between the high pressure side (condenser) and the low pressure side (cooling coil/Evaporator)

      A capillary tube may also be used as a restrictor. The length of the capillary tube and its diameter govern the restriction in the refrigerant.

    • Evaporator: The evaporator (cooling coil) provides a surface over which air from the room can flow and at the same time a passage (tubes) through which refrigerant can flow. Heat transfer takes place here.

  • Electrical Components: 

    • Thermostat Switches: It controls the compressor by sensing the temperature of air drawn into the evaporator. Temperature selection is made by turning the knob towards warmer or cooler setting.

    • Start Relay: The single phase induction motors are not self starting. For starting purpose in addition to main winding, auxiliary winding is also provided to get phase difference field. One of the methods used is to provide a capacitor in series with main and auxiliary winding. Such motors are called PSC (Permanent Start Capacity) motors. The capacitor remains in the circuit both at the time of starting and running. The starting torque of such motors is relatively low and the motors take a start only when they are under no load condition. In the compressor this amounts to compressor starting with no pressure differential between suction and discharge side. To provide a higher starting torque, an extra capacitor is provided at the time of starting. This capacitor has to be disconnected as soon as the motor reaches about 80 % of its rated speed. Such motors are called CSR (Capacitor Start and Run) motors and are provided with :

      • Run Capacitor
      • Start Capacitor
      • Relay

      The relay used for disconnecting the start capacitor is a voltage operated device, with silver contacts to give it is prolonged life.

    • Voltage Relay: The voltage type relay operates as a function of increasing voltage across the auxiliary winding as the compressor picks up speed, the relay has normally closed contacts and the start capacitor is connected in series with the contacts.

      Precautions to be taken:

      • Relay should be mounted in the position indicated.
      • Relay should not be mounted near source of dust, water spray, etc.
      • Terminal fixed to relay should not be loose as this can lead to arcing and malfunctioning of relay.
    • Overload Protector: Thermal protection of compressor is designed primarily to prevent thermal deterioration of electrical insulating materials and sub sequence contamination of the system of electrical failure, on hermetically sealed refrigeration units. A disc heater combination motor protector is very common. Such a protector is designed to meet the following conditions:

      • To prevent motor windings exceeding permissible temperature limits.
      • Operate satisfactorily without trips in ambient temperature range.
      • To operate successfully within the voltage range of compressor.
      • Precautions to be taken for satisfactory operation of OLP:
      • The bakelite cover on the OIP body should be removed prior to fitting on the compressor shell.
      • Physical contact of the OLP on shell should be adequate, otherwise there may be air gap between the OLP and the compressor shell, leading to malfunctioning of the OLP.
      • There should not be excessive air flow/draft on the OLP.
      • If the lead lengths provided by the OLP are not adequate for doing the electrical wiring, it is suggested that during lead cutting and soldering adequate care should be taken so that stands are not removed and a joint is made.
      • It is desirable to use imported terminal clips for making electrical connections; otherwise loose connection will lead to arcing and malfunctioning of the unit.
      • We have get different circuit diagrams for PSC, CSR and RSIR operation of OLPs. It is suggested to connect the OLP correctly in the Electrical circuit otherwise motor burn out of the compressor is possible.

      It is essential that only recommended of OLP should be used along with the compressor, otherwise motor burnout/nuisance tripping of compressor is possible.

    • Start / Run Capacitor: 

      • The function of run capacitor is to improve the power factor of the motor and to give adequate starting/breakdown torque to the compressor.
      • Start capacitor is used only to improve starting performance of the compressor.
      • The run/start capacitor are specified by the following parameters:
      • Capacitance
      • Voltage rating
      • Duty Cycle
      • Use of capacitor of voltage rating lower than specified will lead to reduce life of the capacitor and subsequent burn out of the compressor.
      • It is essential that only capacitors of rated capacitance should be used along with the compressor as it is optimize from running as well as starting performance points of view. Use of a capacitance higher as well as lower than specified will lead to reduce life of the unit, as well as deterioration in the immediate performance of the compressor.

Maintenance Schedule

  • Monthly: 

    1. Clean the Air Filter with brush and Air blower.
    2. Clean evaporator coil, condenser coil fine with brush and Air blower to remove the dust and dirt.
    3. Lubricating fan motor with high grade SAE 20.
  • Annually: 

    1. Complete over-hauling of the unit including cleaning the condenser and evaporator coil fins with weak solution of caustic soda (1 kg of caustic soda for 10 litres of water).
    2. Cleaning of air filter with water under pressure, replacement of faulty electrical parts, replacement worn out insulation, painting and patch work to base tray, etc. It is necessary to check the currents after over loading.

Annual Servicing: In case of compressor fault, it is advisable to be repaired is workshop of genuine spares are available, otherwise go for replacement through original manufacturer. Faults like coiled leak, tin work, blower fan rewinding, electrical faults can be quickly rectified in the workshop. Details of servicing procedure is as below :

  1. Unit is to be completely dismantled. Condenser coil, evaporator first clean with brush blower, then spray weak solution of caustic soda (1 kg for about 10 litres of water) wipe the Aluminium fins with wire brush, then spray water with force and then allow the coil to dry. For some makes the Alluminium fans are very thin, so in due course of usage they will be corroded particularly condenser coil. In such case the coil can be got refined. This is a skilled job and so advisable to get it done through the specialised agency.
  2. The condensor, evaporator coils and capillary after cleaning are flushed with dry nitrogen gas with CTC at a pressure of 30 to 10 PSIC to blow out the oil, dirt etc. (instead of CTC clean petrol can be used for flushing along with Nitrogen and finally using little CTC at the end. But case should be taken about fire while using petrol). After flushing seal each part with rubber plug.
  3. Patch work to be down to tin work wherever damaged for base tray, front grill etc. Paint the base tray, apply thermal insulation to grills etc.
  4. Replace the air filter with new one.
  5. Check the fan motor, apply lubricating oil SAE 20, Check up for current, the no load current should not exceed 1 ampere.
  6. Then assemble fan motor, blower, fan section, see that they are not touching the housing.
  7. Assemble condenser coil, evaporator coil, compressor and inter-connect the refrigerating piping by braising.

    The braising is a skilled job and has to be done by an expert using an appropriate brazing material. It is always advisable to use new capillary and strainer.

  8. Simultaneously complete the electrical wiring. it is always preferable to replace the over load protector and relay with now one and use original parts.


Troubleshooting: Refrigeration System
DefectSymptomReason for defect
No Capacity

Compressor runs continuously

No cooling of evaporation

Minimal current & Power consumption no pressure difference when started and stopped

Leaks of every kind on the high side defective valves
Less CapacityThe evaporation pressure will be abnormally high and hence it surface temperature. Compressor may run continuously

Leaks of every kind

Defective valves

NOTES: The capacity defect if casually checked will be mistaken for ‘Lost charge’. Easy way to see the difference is as follows: If the former case, the evaporator pressure will be high where as in the latter case, it will be low.

High condenser pressureCondenser pressure too high, Compressor capacity reduces current & power consumption increase & OLP trips (some times overload is such that it does not trip OLP: This is more dangerous as ultimately motor burns out) Valve checking or carbon deposits on valves.

Dirty or clogged fins

Defective fan

Block Evaporator circuit (this defect is only rate)Uneven frosting/cooling too high temp. of evaporator surface. 
Block in capillaryEvaporator not sufficiently cooled, sub-cooled, liquid in more areas of condenser.

Dust or brazing material or burns checking capillary

Moisture in system plugging the capillary at low ambient low loads.

Refrigeration Charge

Current & Power consumption increased, front formed on suction tube, storability reduced on overloads.

Compressor runs longer low suction pressure, low current/power consumption, compressor abnormally hot.

Inadequate compensation for ambient or carelessness while charging.

Possible leaks in

NOTES: The impression of under charge can also appear if the capillary resistance is increased and gradually if it is partly checked. Hence to confirm ‘Less Charge’ the thermostat must be shorted so that the compressor runs continuously and stabilised conditions are obtained.

Cut out temp. lowThe temp. in chiller will be too low and the running time will increase.Supplier’s setting incorrect.
Cut out temp. highThe temp. desire is not obtained.-do-
Thermostat not cutting inNo cooling comp. & fan do not startThermostat has lost its charge or contacts pitted or coated.
Differential too smallStand still cycle too short or protector trips.Supplier setting incorrect.
Differential too bigEvaporator temp. is high-do-


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