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Why a Load Calculation is Important to Furnace Installation

A furnace installation for new-home design or a system replacement can make or break energy bills and your home-comfort experience for the life of the new install. It's important to get it right the first time, which calls for an accurate load calculation by an HVAC professional who does not cut corners or fudge numbers. Use these commonly asked questions and HVAC-industry best practices and answers to guide you through the calculation and installation process.

Why is a load calculation important?

A heating load calculation is necessary to determine the correct size of furnace to install in the home. Any other criteria, such as the old rule-of-thumb square-footage calculation, is ultimately a guess. The same applies to just going with the same size system you're replacing. When it comes to energy efficiency and home comfort, accuracy counts in energy dollars and comfort sense.

What is involved with a load calculation?

A load calculation includes a careful analysis of a home’s size, levels, design elements, construction materials, age, weatherization (insulation and sealing), and the number of occupants, their energy-usage habits, comfort preferences and schedules. That’s a good start. Many more factors are considered, and data collected, such as sun orientation, number and size of windows and more. Moreover, your HVAC contractor should ask pertinent questions to help calculate the actual heating load, and install the right-sized furnace for peak performance and comfort.

Can I just install a new furnace that's the same size as my old furnace?

Probably not. Odds are that when your old furnace was installed, an accurate load calculation using HVAC-industry best practices was not performed. Also, since that furnace was installed, technology has greatly increased furnace efficiency, while at the same time most homes have become more airtight and better insulated. Even today, many contractors determine furnace size and installation factors solely by the square footage of the home. When you consider the differences of one home to the next, the load calculations are likely to be very different, even if the two homes have the same square footage.

A furnace that's too large is subject to short-cycling -- powering on and off in short bursts. This creates discomforts for the occupants, wastes energy, and creates extra wear on system components. Also, rust and corrosion are issues for an over-sized furnace, because increased moisture becomes a factor. Inadequate airflow through the home creates issues with moisture, too, and indoor air quality suffers.

Is there a manual for all this stuff?

The Air Conditioning Contractors of America (ACCA) developed guidelines for forced-air installations, including load calculations, ductwork design and system (furnace) sizing, known as Manual J, D and S, respectively. These manuals have been instituted into building codes of local governments across the country. The U.S. Department of Energy recommends homeowners seek contractors who use ACCA’s Manual J: Residential Load Calculation for system installation.

The process for calculating a home's load is long and tedious. Manual J software is used to streamline the process, help prevent errors, and automate the process as much as possible. Manual J also helps prevent number fudging, which some contractors do to be on the safe side. However, following Manual J to the letter and by the numbers is the only accurate means to accurately determine your home's heating load, and, in turn, select the right size furnace to install.

How do I find a reliable contractor?

Ask potential contractors if they use ACCA's Manual J for calculating the heating load of the home. Your contractor should also be fully licensed, insured and eager to provide references of its work. Ask your contractor if it employs NATE-certified technicians– the standard of HVAC certification across the country.

Customer service is important since you may be calling the same contractor for system tune-ups or service later. Find a contractor you trust and feel comfortable inviting to your home. Ask for service and workmanship guarantees and product warranties. Your contractor should be knowledgeable and helpful regarding manufacturer rebates, and federal energy-efficiency tax credits that might apply.

For more information about heating load calculations, and furnace installation and sizing, please contact us at Comfort24-7.com to locate a reputable contractor near your Chicago area home.

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