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Sizing a New Ductless System Ensures Energy and Comfort Is Maximized

Many home and business owners across the country are taking advantage of the benefits ductless systems offer for a plethora of applications: versatile installation, great energy efficiency and whisper-quiet comfort. If you’ve decided a ductless mini split or multi split system is right for your project, make sure you maximize your investment by sizing a ductless system correctly.

Advantages of Right Sizing

Like central air-source heat pumps and cooling-specific systems, ductless systems provide efficient heating and cooling by transferring heat energy with refrigerant between indoor and outdoor coils. Electricity is needed to power the components that manipulate and move the refrigerant, rather than burning fuel for heating like a furnace.

Using refrigeration principles for heating and cooling is quite efficient, and ductless systems take efficiency and comfort to new heights compared to their ducted counterparts. These are the advantages ductless systems offer if they are sized correctly:

  • Even heating and cooling -- Ideally, ductless heating and cooling cycles are long lasting, rather than frequent short-cycling common with single-stage conventional systems. Longer cycles promote better air distribution through the entire zone.
  • Dehumidification -- In parts of the U.S. with high humidity problems, such as the Southeast U.S., the long cooling cycles help reduce zone humidity, which boosts comfort, reduces cooling costs and helps protect valuables from damage caused by excessive moisture.
  • Air quality -- Longer cycles mean that more air volume is pulled through Mitsubishi’s advanced three-phase filtering system for cleaner indoor air.
  • Energy efficiency -- If a system is too large, it will quick cycle and never reach peak efficiency. If a system is too small, components endure excessive wear, which can lead to malfunctions, and hot and cold spots through the living spaces are prevalent.

Getting Started

To determine the size of the ductless system needed for the project, you need to determine the heating/cooling load of the zone or zones. Since the load influences ductless size (and cost), it may be to your benefit to seal up your home and add insulation if needed.

Sizing a Ductless System

The industry standard method for load calculations is called Manual J, which was developed by engineers at the Air Conditioning Contractors of America (ACCA). Manual J, and other publications by ACCA, are used by responsible companies throughout the country, and are often required procedures by local municipalities.

However, if you are installing a ductless mini split (one room/zone), and you’ve decided to go by the square footage, this is how it is done for a ductless system:

  • To find the square footage of a room, multiply the width and length of the room.
  • Do not include closet space, unless you are installing a concealed slim duct with an air supply and air return vent.
  • If the ceiling in the zone you are conditioning is taller than eight feet, ask your HVAC contractor to perform a load calculation using Manual J.

This example highlights the deficiencies of using the rule-of-thumb method for sizing a ductless system:

  • Zone is 250 square feet. According to rule-of-thumb sizing charts, the ductless system should be 6,000 BTUs.
  • However, rule-of-thumb does not take into consideration how many people will normally occupy the room.
  • It does not calculate heat gain/loss from large windows (or no windows).
  • Is the room a converted attic, or converted basement space?
  • What is the local climate -- mostly hot, humid, moderate, cold?
  • Is the zone well sealed and insulated?

Working With Your Contractor

There are just too many variables for sizing a ductless system to not use Manual J. Work closely with your HVAC contractor to ensure the heating/cooling load of the zone or zones are correctly calculated. Your contractor should be well versed with Manual J, and have sought education and licensing for installing and sizing a ductless system correctly, which very few actually are.

If you are installing a ductless multi split (multiple zones), you may well benefit with home-efficiency upgrades of air sealing with caulk and weather-stripping, and boosting or replacing insulation if needed.

A professional home energy evaluation can determine if home-efficiency upgrades are recommended to save energy and boost comfort. And, home-efficiency upgrades are good for the life of the home.

If you want the very best service, product and assistance, please contact us to locate a licensed Mitsubishi Electric Contractor in your area.

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