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Manual J: Why The Technician Who Sizes Your System Should Know It Inside Out

Posted on August 14, 2012

Many people haven’t heard of Manual J, which is the primary tool that HVAC contractors use to size a home’s cooling or heating equipment. It’s essential when replacing a system to go through the steps this tool requires to pinpoint the right size HVAC equipment for your home. Going by the size of the existing equipment is misleading, especially if you’ve improved your home’s energy efficiency.

When HVAC professionals size your A/C by using Manual J, they take into account a constellation of factors that contribute to your cooling load. Homes that have adequate insulation, little air infiltration and energy-efficient windows don’t need as much cooling power, for example, since they don’t get as hot and stay cooler longer. The layout of your home also will affect the calculations that the contractor makes.

Installing too large a system will result in the equipment not running as long, leaving more humidity in your home. The extra humidity will make you uncomfortable and promote the growth of mold, mildew, bacteria and dust mites, all common allergens. If you turn the temperature down to reduce the humidity, the electric bill will go up and you may feel cold indoors.

A system that’s too large also will put out too much cold air at one time and shut off faster. It also will turn on and off more often, which will put a load on all the parts of the A/C, increasing wear and tear on the equipment.

On the other hand, a system that’s sized too small will have an obvious problem – it will struggle to satisfy your temperature demands.

When you shop around for a new A/C system, make sure that the contractor who comes to your home uses Manual J to size the proposed equipment, as well as the companion protocol for sizing ducts, known as Manual D. Being sure that the ductwork is adequate will increase the life of the A/C and the energy it uses to keep you comfortable.

If you’d like to learn more about the factors that go into Manual J and proper sizing of HVAC equipment, contact Comfort Systems. We’ve provided cooling and heating services for the Wichita area since 1996.

Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems).  For more information about Manual J and other HVAC topics, download our free Home Comfort Resource guide.

Comfort Systems services the Wichita, KS area. Visit our website to see our special offers and get started today!     

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