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Best Vertical Window AC Units for Sliding Windows (Narrow Units)

Homeowners with casement windows often struggle to find casement window air conditioners. However, the good news is that you can indeed put an air conditioner in a casement or tilt window. Casement window air conditioners...

Homeowners with casement windows often struggle to find casement window air conditioners. However, the good news is that you can indeed put an air conditioner in a casement or tilt window. Casement window air conditioners are specifically designed to fit narrow windows, making them the perfect solution for these types of windows. In this article, we will explore the best vertical window AC units for sliding windows and provide some useful information to help you choose the right unit for your needs.

Understanding Vertical Window AC Units

Vertical window AC units, also known as casement or sliding window AC units, are designed to fit into narrow or sliding windows. They are typically around 15" wide and 21" high, making them perfect for casement or sliding windows. These units are smaller in width compared to standard window units, which are wider and shorter.

To install a casement window AC unit, you need a minimum window width of at least 15 1/2 inches. It's important to measure your window length to ensure that the unit will fit properly. Additionally, you will require a special vertical AC kit that usually comes with the unit for installation.

Casement Window AC Casement or sliding window AC units are high and narrow. They are usually 15" wide and about 21" high.

The Best Vertical Window AC Units

While there are only a few manufacturers that offer vertical window AC units, some stand out as the best in the market. Frigidaire, Koldfront, and PerfectAire are known for creating high-quality vertical window AC units. The Frigidaire FFRS1022R1, with 10,000 BTU, and the Koldfront CAC8000W, with 8,000 BTU, are generally considered the top two vertical window AC units available.

To help you choose the right AC unit for your casement window, LearnMetrics has compiled a list of the best casement window air conditioners. You can find the list below:

  1. Frigidaire FFRS1022R1: Overall Best Casement Window Air Conditioner.
  2. Koldfront CAC8000W: Quietest and Smallest Casement Window AC Unit with Superb Energy-Efficiency.
  3. PerfectAire 3PASC10000: Good 10,000 BTU Casement Window Air Conditioner.
  4. Keystone KSTSW10A: 10,000 BTU Vertical Window AC Unit.
  5. Global Industrial MWDUL-10CRN1-BCJ4: 10,000 BTU Slider Window AC Unit.

Understanding the Terminology

When it comes to casement or sliding windows, several terms are used to describe these narrow AC units. Let's clarify the terminology used for these types of window air conditioners:

  • "Casement window air conditioner": The most commonly used phrase for an air conditioner that fits a casement window.
  • "Vertical window air conditioner": A term used to describe window AC units that fit vertical windows.
  • "Sliding window air conditioner": Air conditioners that fit sliding windows and can be installed by sliding the window as close to the AC as possible.
  • "Slider window air conditioner": Similar to sliding window air conditioners, they are designed for windows that slide horizontally.
  • "Air conditioner for crank window": Referring to AC units that fit windows with a crank mechanism.
  • "Thin or thinnest air conditioner": AC units with a high ratio between height and width.
  • "Tilt window air conditioner": AC units designed for tilt windows.
  • "Tall window air conditioner": AC units suitable for taller windows.

In summary, all these terms can be used interchangeably for the same type of window air conditioner. They are perfect for side opening windows or windows that open sideways.

Key Factors to Consider

When choosing a vertical window AC unit for your casement or sliding window, there are five important factors to consider:

1. Cooling Capacity (Measured in BTU/hr)

The cooling capacity of an AC unit is measured in British Thermal Units (BTU). Standard double-hung window AC units range from 6,000 BTU to 15,000 BTU or even more. However, vertical window AC units typically have a narrower cooling capacity, with 8,000 BTU or 10,000 BTU units being the most common. The 10,000 BTU capacity is the most sought-after for its excellent cooling power.

2. Recommended Coverage Area (Based on BTU)

Based on the cooling effect measured in BTU, you can calculate the recommended coverage area for the AC unit. The EPA recommends a cooling effect of 20 BTU per square foot. For example, if you need to cool down a 500 sq ft area, a 10,000 BTU vertical window AC unit is recommended. However, it is always best to check the manufacturer's specifications for the exact recommended coverage area.

3. EER Rating (Energy Efficiency)

Energy efficiency is an essential consideration in selecting a vertical window AC unit. The Energy Efficiency Ratio (EER) measures the energy efficiency of the unit by dividing the cooling effect (in BTU) by the maximum energy input (wattage). A higher EER rating means a more energy-efficient AC unit and lower electricity costs. Casement window AC units with an EER rating of 10 or higher are considered energy efficient.

4. Window Width and Height

Ensure that the AC unit fits properly in your window by considering the minimum window width and height specified by the manufacturer. Casement window AC units typically have a width of less than 15 inches. However, it's important to consider the additional width of the crank window AC installation kit. The height of the window is usually less of a concern for vertical windows.

5. Noise Levels

Nobody wants a noisy air conditioner. When selecting a vertical window AC unit, check the noise levels specified by the manufacturer. Anything below 60 dB is considered quiet, but it's advisable to go for units with noise levels below 55 dB for a super silent operation. Pay attention to the noise levels on the inner side of the unit, as that is where you will be spending your time.

With these factors in mind, you can confidently choose the best vertical window AC unit for your sliding or casement windows.


Choosing the right vertical window AC unit for your sliding windows or casement windows is essential to ensure proper cooling and a comfortable living environment. Take into consideration the cooling capacity, recommended coverage area, EER rating, window width and height, and noise levels when making your decision. The top-rated casement window AC units, such as the Frigidaire FFRS1022R1 and Koldfront CAC8000W, are highly recommended for their performance and reliability. Make an informed choice and enjoy the cool comfort of your home.