What Size Air Conditioner do I Need?

A room air conditioner is a compact self-contained air conditioned air condition unit which is normally installed in a window or wall opening of the room and widely known as window type air conditioner.

Working of an Air Conditioner – Theory of Operation

Common household air conditions work on what's called the "vapor compression system." The parts of a window air conditioner are:

  1. Refrigeration system
  2. Control system (thermostat and selector switch)
  3. Electrical protection system
  4. Air circulation system (fan, motor, centrifugal blower, evaporator blower)
  5. Ventilation (fresh air damper)
  6. Exhaust system

The refrigeration system consists of a hermetic type compressor, forced air cooler, finned condenser coil, finned cooling coil, capillary tube used as throttling device, and a refrigerant drier. The refrigerant used is R-12 or R-22.

The condenser is a continuous coil made of copper tubing with aluminum fins attached to it to increase the heat transfer rate.

Room air conditioners are installed so that the evaporator faces the room. A centrifugal blower is installed behind the cooling coil which sends cool air in the room.

A filter is installed on the fresh air entering side of the evaporator to remove any dirt from air. A damper inside the cabinet regulates the fresh air intake of the room air conditioner. The quality of fresh air may be varied by adjusting the dampers.

The window air conditioner performs two functions: one is to cool the room and the other is to dehumidify the room as required for comfortable living conditions. Normally they are available in different sizes which means they are available in different cooling capacities. Always the air conditioning cooling capacity will be expressed in BTU’s (British Thermal Units). Cooling capacity normally depends on the volume of the room to be cooled along with sensible cooling factors.

Window Air Conditioner

window Air conditioner

Before selecting an air conditioner, one needs to consider the various factors that determine the size of the conditioner. There are different components of a cooling load.

Sensible Heat Gain

When there is a direct addition of heat to an enclosed space, a gain in sensible heat is said to occur. This additional sensible heat should be removed for proper air conditioning. The sensible heat gain may occur due to the following sources.

  • Heat flowing into the building by conduction through exterior walls, floor, ceiling, doors, and windows due to the temperature difference on their two sides.
  • Heat received from solar radiation. Heat from solar radiation includes heat transmitted directly through glass of windows, ventilators, or doors and heat absorbed by the walls and roof.
  • Heat conducted through interior partitions from rooms in the same building that are not conditioned.
  • Heat given off by lights, motors, machines, and cooking operations.
  • Heat liberated by occupants.
  • Heat carried by outside air leaks through cracks in doors, windows, etc.
  • Heat gain from the fan work in ducts.

Latent Heat Gain

When there is an addition of water vapor to the air of an enclosed space, a gain in latent heat is said to occur.

It occurs due to the following sources:

  1. Heat gain due to moisture in outside air entering by infiltration.
  2. Heat gain due to condensation of moisture from occupants.

The sensible heat gain plus the latent heat gain are the total heat load to be removed by air conditioner.

Psychometric chart

The psychpmetric chart helps us to trace out the physical properties of humid air and outlines the connections between six different parameters involved. These properties are either given in American or Metric units. It may be vital to know that the data expalined in the chart is always displayed per unit mass of dry air. Though the lines in the chart may look quite haphazard and not sensibly directed, it actually makes the necessary obsevations and calculations much easier and favorable.

A more comprehensive explanation is provided HERE.

Psychometric chart Image

psychometric chart


So first calculate the sensible heat gain of the room and the latent heat gain of the room.

Sensible Heat Gain q = (h2 –h1)

The heat absorbed by the air during sensible heating may be obtained from the psychometric chart by the enthalpy difference.

Sensible Heat:

The heating of air, without any change in its specific humidity is known a sensible heating.

So we have to do cooling effectively to remove the heat gained.

Sensible Cooling:

The cooling of air without any change in its specific humidity is known as sensible cooling.

Let us consider the air to be cooled is sent through the evaporator of the temperature td3. Inlet air temperature is td1 and air leaving the cooling coil is td2, which will be more than td3.

Heat rejected by air during sensible cooling may be obtained by psychometric chart.

q = (h1- h2)

Humid specific heat = CPm

And it's value is taken as 1.ρ022 KJ/kg k

The heat rejected q = CPm (td1- td2).

For air conditioning purpose the sensible heat per minute is given as:

SH= Ma × CPm × Δ t

Ma= Mass of air flowing.

Ma = V × ρ.

V = Rate of dry air flowing in m3/ min.

ρ= density of moist air at 20o C and 50 % relative humidity = 1.2 Kg/ m3

Δ t= td1 – td2.

SH = V ×1.2 1.022 x Δt

SH = 1.2264 x V x Δt (KJ/ min)

SH = 0.02044 V x Δt (KW)

Therefore for sensible cooling, the cooling coil may have refrigerant, cooling water, or cooling gas flowing.

Sensible cooling can be done only up to dew point temperature (tdp) cooling below this will result in condensation of moisture.

Selecting Correct Size of Window Air Conditioner

Finally the approximate size of the window air conditioner can be calculated by this simple technique:

As the volume of the room to be cooled is directly proportional to the cooling capacity of the air conditioner in BTU’s, first calculate the volume of the room to be cooled – V ( square feet).

Sensible heat gain due to the solar radiation – SH

The greater portion of the wall exposed to outside in the room faces which direction? This gives you the approximate value of the Sensible heat gain. (SH)

  1. North -16
  2. East -17
  3. South -18
  4. West -20

Thus the approximate size of the window air conditioner (Cooling Capacity) = (V x SH) / 5 (BTU’s).

Thus this simple calculation gives you the approximate size of the widow air conditioner to be fitted in the room. If you fit an improper size of the air conditioner it may result in power loss and improper humidification.

Fitting a bigger air conditioner for a smaller room, will result in frequent short running of the compressor. It will start and stop very frequently. This may result in more power consumption and this also creates problems in humidification. Due to the frequent start and stop interval of the compressor, the air that is taken in by the blower will not have a sufficient time to be dehumidified, which may result in an uncomfortable situation.

As for the other case, fitting a too small compressor for a bigger room will also result in constant running of the compressor as it has to cool a larger volume. So it will result in less efficient air conditioning, but with power being wasted by running the compressor.

So before buying the air conditioner, make sure that you properly calculate the cooling capacity in BTU’s and select the correct size of the compressor to avoid operational problems.

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