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What Size System Do I Need?

Correct sizing of a ductless system is a critical element in the process that ensures that you get the performance and efficiency results promised by the system's specifications. Ductless heat pump sizing refers not to the physical dimensions of the unit, but to the unit’s BTU-per-hour capacity. In order to effectively cool an enclosed living space, a ductless air conditioner must be capable of extracting a certain minimum number of BTUs of heat energy. Normally, this figure is expressed in BTU tonnage. One BTU ton equals 12,000 BTUs per hour.

Why Size Matters

Even the best ductless air conditioning systems will not be able to meet expectations if they are not correctly sized to the rooms they are supposed to condition. If a ductless unit is under-sized, its BTU tonnage is below what is required to cool the room. That means it can’t extract enough heat from the air to produce the kind of consistent comfort level you expect. An under-sized unit operates in extended “on” cycles as it struggles to meet thermostat settings, consuming excess energy and increasing cooling costs. However, an over-sized unit also has drawbacks. Because it cools the room so quickly, units that have too much capacity cycle on and off frequently. This effect, known as “short cycling,” puts extra wear and tear on expensive components, particularly the compressor, possibly leading to premature repairs. In addition, because they typically run in such short “on” cycles, over-sized units don’t extract humidity from the air as effectively as units with a capacity that more closely matches the BTU requirements of the room. The guideline for ductless air conditioner sizing should always be, "not too little, not too much."

Getting a Professional Load Calculation

A qualified Mitsubishi Electric contractor can perform a professional cooling load calculation on any rooms in your home that are designated for ductless split system installation. Utilizing industry standard software called Manual J, the contractor will input data such as the square footage of the room as well as information relating to the number and size of windows, the type and amount of insulation in the walls and ceiling, the number and kind of light bulbs and the usual number of occupants in the room. With this information, the software calculates the cooling load—the precise amount of BTUs of heat energy that must be extracted from the room to maintain a comfortable temperature, usually in the mid-70s. The contractor can then use the cooling load figure to recommend a ductless unit with the BTU capacity range to most effectively and efficiently cool that space.

In the absence of a cooling load calculation performed by a professional, homeowners asking, “what size mini split system do I need?” can make a ballpark estimate of the capacity required to cool a room by using a simple DIY formula. Although it’s not a replacement for a load calculation by a trained technician with sophisticated software, this method can at least serve as a basis to conduct preliminary comparison shopping among different Mitsubishi Electric ductless air conditioning systems. Here’s how to make a rough estimate of the cooling load in any room in your home.

  1. Measure the room where the air handler will be installed. You’ll need to know the total square footage, which means multiplying the length of the room times the width. Don’t include closets in the calculation. If the room is not a perfect square or rectangle, divide it into rectangular areas, figure the square footage of each, then add all the square footage results together for the total.
  2. Figure the BTU requirements to determine how much cooling capacity will be needed to keep the room comfortable. The general formula is to multiply the total square footage of the room by 25 to determine the BTU capacity of a ductless unit to cool the space. Therefore, a room that is 200 square feet in size would require a cooling capacity of 5,000 BTUs per hour.
  3. The baseline BTU-per-hour figure above should be modified to incorporate factors that will increase the cooling load. Make allowances for such variables as typical number of occupants and any use of major appliances. As an approximation, when estimating the cooling load for bedrooms add 450 BTUs to the total for each person who will normally occupy the room. To accommodate major appliances when sizing a ductless unit for a kitchen, add 4,000 BTUs.
  4. When calculating the cooling load for a multi-split system where one outdoor heat pump will serve multiple indoor ductless air handlers, add the BTU estimates for each room where an air handler will be installed to determine the total system requirement.

Consult Your Local Mitsubish Electric Contractor

Keep in mind that this BTU figure is only a rough ballpark figure. For a more exact result that ensures that you receive a ductless unit with the optimum capacity, contact your local Mitsubishi Electric contractor for a professional cooling load calculation.