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This section reviews HVAC units, air handling units, exhaust fans, and window A/C units, how they work, routine maintenance, and troubles associated with them.


How They Work

Operation is based on the principle of using evaporation of a refrigerant to cool the air (see Figure 1).

Figure 1: Basic Air Conditioning Unit

The basic cycle of operation is:

  1. The compressor (B) compresses the cool, low-pressure refrigerant gas (blue), causing it to become a hot (red), high-pressure refrigerant gas.
  2. The high-pressure hot gas runs through a coil and dissipates its heat, and then condenses into a liquid.
  3. The liquid refrigerant goes through an expansion valve (A) and becomes a cold, low-pressure refrigerant gas when it evaporates in the expansion valve.
  4. This cold gas now runs through a coil that permits the gas to absorb heat, thereby cooling the air inside the building/room.


HVAC refers to heating, ventilation, and air conditioning. This is the means a device or system uses to control the temperature and/or airflow in a room or a building. This can be as simple as a window air conditioning unit to a large system used to supply a building.

Window Unit

A window unit is just an air conditioner small enough to fit into a window and plug it into an electrical outlet (Figure 2). It contains the following elements:

  • Compressor
  • Expansion valve
  • Hot coil
  • Cold coil
  • Fans
  • Control unit

Figure 2: Typical Window A/C Unit

Routine Maintenance

Maintenance on window A/C units is simply:

  • Clean the filters monthly; if it has holes in it, replace it
  • Clean the condenser coils annually, and more often if in a dusty environment.
  • Do not short cycle the cooling system. When you turn it off, do not turn it on for at least 10 minutes. This allows the pressure inside the system to equalize.

Unit Does Not Start

  • Check the power to the outlet, fuses, and circuit breakers. If OK, then one of the following may be defective:
  • Thermostat
  • Compressor
  • Overload and/or relay
  • Selector switch
  • Control board
  • Wiring

Fans Runs But There Is No Cold Air

  • Selector switch in the wrong setting.
  • Thermostat not set correctly.
  • Compressor not running.
  • Defective condenser.
  • Defective evaporator.
  • Defective capacitor for compressor motor.

Air is Cool but Does not Seem Cold Enough

  • Check the difference between the air coming in and the air going into the room. There should be at least a 15-degree difference. If there is not, check the air damper. It may be letting in outside air instead of recirculating the room air.
  • The winter cover is still on.
  • The condenser coils are dirty or clogged. They need to be cleaned regularly to prevent buildup of dirt and loss of efficiency.

Unit Never Turns Off

  • The unit is not cooling properly; see above.
  • Thermostat is set too low or not working.
  • Unit is sized too small for the space being air-conditioned.

Water Sloshing Around on the Inside

  • Drain is plugged or clogged.
  • Slinger not working properly.

Unit Tries to Start then Quits

  • Defective overload relay.
  • Defective capacitor.
  • If it continues, let it rest for approximately 5 minutes, then retry. If problem continues, call an A/C technician.

Motor Runs but no Air is Coming from the Unit

  • Fan motor is defective or electrical circuit to motor is defective.

Unit Rattles Loudly when it Shuts Off

  • Unit is not mounted securely.
  • Compressor mounting pads and/or brackets have become worn or broken.

Water Leaking from Front of Unit

  • Unit is not tilted to the rear properly, causing water to collect in the front.

Prior to Installation

  1. Remove the unit from its packaging and inspect for damage.
  2. Use the following cautions when handling the unit:
  3. Wear gloves, as sheet metal may have sharp edges and will cut.
  4. Never use the tubing or coils as handgrips to move or lift the unit.
  5. A/C units are heavy; get assistance when lifting or moving them.
  6. Open the window, locate the center, and position the base in place.
  7. Level the base and mark the screw holes.
  8. Drill mounting holes for the correct screw size.
  9. Fasten the mounting base to the windowsill using rust-proof screws.
  10. Adjust the leveling screw until the base tilts downward toward the outside about 1/4 inch.

Figure 3: Installing Mounting Unit

Installing Unit in Mounting Base

  1. Slide the unit into the mounting base, but do not force it in. Recheck that the mounting base is level and square.
  2. Install spacers between each side of the unit and the frame of the window.
  3. Install rubber gaskets along the top of the unit.
  4. Install a gasket between the upper and lower portions of the double-hung window.
  5. Install a double-hung sash lock.
  6. Use a sealer to fill in all cracks around the unit that will let air/water seep inside.

Figure 4: Installation of Filler Panel Between Housing and Window

Electrical Connections and Testing the Unit

  1. The unit should be connected to an outlet on its own circuit.
  2. The plug for the unit should be polarized with a ground.
  3. Make a final visual inspection and then start the unit.
  4. If it does not start, check items in the Typical Problems list.
  5. When the unit is running, check for leaks inside the conditioned space. If it is leaking, adjust the tilt of the unit.
  6. Check for excessive noise and vibration and adjust as necessary.
  7. Adjust the cutout to between 56 and 60F and the cut-in to between 77 and 80F.