Mechanical - HVAC Engineering


Window air conditioners are the space cooling solution with the lowest price.

Simple Installation

Window air conditioners can be installed without opening walls or ceilings.


Window air conditioners provide independent cooling for each building area.

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Window Air Conditioners

Window Air Conditioners

Window air conditioners are one of the most common space cooling solutions for residential buildings. They are characterized by their low upfront cost and simple installation, while allowing independent cooling for each room in your property. Professional MEP engineers can specify window air conditioners of the right capacity for each indoor area in your building.

Unlike other space cooling systems, which use separate components installed in different locations, window air conditioners offer a packaged solution for cooling and ventilation. A window  AC unit includes all the basic component of a space cooling system within a single housing: thermostat, control panel, condenser coil, compressor, evaporator coil, expansion valve, air filter and the fans that blow air through the condenser and evaporator coils.

Some models also include space heating functions, providing an HVAC solution that is useful for the entire year. Window air conditioners also offer intuitive and simple controls, which can be understood by any occupant regardless of professional background. For instance, window air conditioners are much simpler to use than chilled water systems or packaged rooftop units.

Packaged terminal air conditioners (PTAC) are very similar to window air conditioners, since they have a packaged housing with all the elements of a space cooling system. However, PTAC units are normally larger than window AC units, and they are designed for installation on wall slots with specified dimensions, instead of windows.

Advantages of Window Air Conditioners

Window air conditioners are among the most affordable space cooling systems, making them an excellent option when there is a limited budget for mechanical systems.

  • Due to the simplicity of these AC units, they are available from a wide range of manufacturers and there is fierce price competition.
  • You can often get window air conditioners with excellent discounts, and in some locations you may get a rebate for purchasing a high-efficiency unit.

Since a window air conditioner contains all the necessary components to deliver space cooling, it can be installed with minimal disruption of your building. For example, there is no need to install air ducts above a suspended ceiling, and there are no hydronic piping lines crossing walls and floor slabs. Window air conditioners can be installed on an open window, as their name implies, and all they need is an electric receptacle with a suitable voltage output and socket type.

Another advantage of window air conditioners is that each unit operates independently from all others in the building, making them a suitable option when each area has different space cooling schedules. Large air conditioning systems with a centralized configuration normally lack the flexibility to serve multiple areas with independent cooling loads and schedules.

Window air conditioners can be an excellent option for historic buildings, which often have walls and ceilings that are not suitable for bulkier space cooling systems. These buildings typically have steam radiators for space heating during winter, and window AC units offer a simple cooling solution during summer.

Important Considerations When Using Window Air Conditioners

In spite of their low cost and installation simplicity, window air conditioners have limited energy efficiency compared with other space cooling options. They are outclassed by ductless air conditioning systems, chiller plants, and variable refrigerant flow (VRF) systems. However, there are many ways to compensate for the lower efficiency of window air conditioners:

  • In general, space cooling systems operate more efficiently when you use the right equipment capacity. Undersized window air conditioners waste energy because their compressor never stops. However, oversized units suffer from a reduced service life because they run intermittently in short cycles.
  • The US Department of Energy has created the Combined Energy Efficiency Ratio (CEER) to rate the performance of window air conditioners. The CEER is similar to the gas mileage value of a car - just like a car with a high MPG value uses less fuel for a given distance, a window air conditioner with a high CEER value uses less electricity for a given cooling output. Window air conditioners with a high CEER are highly recommended.
  • The ENERGY STAR program from the US Environmental Protection Agency includes window air conditioners, and certified units are characterized by a blue square-shaped label. Any product that bears the ENERGY STAR label has been subject to rigorous laboratory tests that validate its energy efficiency (CEER) and overall performance.

Window air conditioners achieve a lower operating cost when they are properly sized and labeled by ENERGY STAR. Other air conditioning configurations can offer a higher efficiency, but they come with an increased price tag and involve a more complex installation.

Safety is also an important consideration when dealing with window air conditioners; consider that they are heavy and most of their bulk is located outside the window. A poorly-supported air conditioner can fall out and damage surrounding property, not to mention the risk of severe injuries for pedestrians. If your building uses window air conditioners, you must make sure they are properly supported.

Window air conditioners also have a significant visual impact on building facades, and their installation may be subject to regulations if the building is part of a historic district in your city. HVAC engineers who are familiarized with local requirements can clear your doubts, ensuring you meet any applicable landmark laws.

Since window air conditioners create an opening in your building envelope, it is important to make sure that the gap around them is properly sealed. Air leaks can cause unwanted summer heat gain and winter heat loss, driving up electricity and gas bills.


Final Recommendations

Window air conditioners provide a very affordable space cooling solution, and they can be installed on existing windows with no need to disrupt walls and ceilings. However, consider that they have a lower efficiency than other air conditioning systems, so you must make sure adequate units are purchased.

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