When Size Matters

When doing an addition or renovation, remodelers often have to reconfigure or even replace the home's heating, ventilation and air conditioning (HVAC) system. If you want your customers to be really happy with the job, sizing the system correctly is key. "Properly sized HVAC systems lead to satisfied and comfortable customers; lower initial and operating costs; reduced callbacks; an...

August 31, 2006

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When doing an addition or renovation, remodelers often have to reconfigure or even replace the home's heating, ventilation and air conditioning (HVAC) system.

If you want your customers to be really happy with the job, sizing the system correctly is key.

"Properly sized HVAC systems lead to satisfied and comfortable customers; lower initial and operating costs; reduced callbacks; and healthier indoor air quality," says Carl Seville, former owner of SawHorse Inc., in Atlanta and currently president of Seville Consulting, a company that helps remodelers, builders, and homeowners incorporate sustainable practices in their projects.

In every job, Seville recommends comparing the size of the system to the needs of the house. Many homes have been built with oversized HVAC systems that cost more to operate and leave the home feeling damp and drafty.

Through proper sizing, you may also be able to reduce your equipment costs.

"We've been in situations where we could take a ton or more off the HVAC system by doing a careful and accurate Manual J-load calculation," says Seville. The Air Conditioning Contractors of America's(ACCA) Manual J is the industry standard for HVAC sizing.

Rules made to be broken

Too often, builders and contractors rely on rules of thumb to determine HVAC size. The accepted estimate is that the HVAC unit should provide 1 ton (12,000 Btu) of air conditioning for 400 to 500 square feet of building area.

However, this rule fails to take into account how well the home is insulated, how well it's sealed and the local climate. In a very efficient house, 1 ton could condition 800 to 1,000 square feet of space.

Use the ACCA's Manual D to calculate the proper size of ductwork in newly remodeled spaces. 

"Many HVAC contractors don't understand that the system has to be right-sized to dehumidify well," Seville says. "They are afraid of undersizing the system because they don't want their customers to complain about being hot in the summer. But a house simply won't be dehumidified properly if the HVAC is oversized. We've seen situations where the owners have to keep a house at 65 degrees in the summer to keep it dry."

"Air conditioners cool a house first, then dehumidify it," he adds. "If an HVAC unit is too large, it will short cycle. That is, it will turn on, cool the house down, and turn off again before removing much humidity from the air. Frequent starting and stopping increases energy consumption, makes the home uncomfortable, contributes to mold and indoor air quality problems and wears down the equipment faster.

"A right-sized system combined with proper air sealing creates a healthier house. Together, the two will maintain a comfortable humidity level, reduce the occurrences of allergy-causing mold, and help keep dust and pollen outdoors."

A better way

The Partnership for Advancing Technology in Housing (PATH) provides guidance on HVAC sizing in "HVAC Package for New Construction — Forced Air System," a Tech Set available at www.pathnet.org.

HVAC systems should be sized using ACCA's manuals J and S, the industry standards for residential load calculations and equipment selection required by most U.S. building codes. These load calculations will ensure that the system achieves optimal efficiency.

The accepted estimate of 12,000 Btu of air conditioning per 400 to 500 square feet of space doesn't take into consideration how well the home is insulated and sealed.

"Using Manual J, a contractor calculates heat loss from the house through walls, ceilings and leaky ductwork, and infiltration through windows, doors and other penetrations," says Seville. "Manual J also helps calculate heat gain into the house from sunlight; people; lights and appliances; doors; walls; and windows."

Most of the information needed for sizing cooling loads can be taken directly from house plans. Essential information includes solar gain, which is a function of window area; orientation of the house; window type and glazing (such as Low-E, low solar heat gain coefficient, gas-filled); shading from landscaping and building overhangs; and shingle and siding type and color.

"Most remodelers don't own the software and don't perform the loads themselves," Seville says. "But there is always the risk that an HVAC subcontractor who knows how to use the software may not correctly size a system for a particularly well-sealed and efficient house. That's why the remodeler or builder needs to be involved. Make sure the inputs are accurate."

There are also simpler versions of Manual J — Manual J-8 Abridged Edition for instance — that make it easier to learn the procedures and do the hand calculations.

If you need to replace or install ducts, they should be designed to ensure proper air distribution using ACCA's Manual D. Do load calculations room by room to properly size the ductwork. Also, place the plenum in a central location to minimize duct lengths. This will reduce material and operation costs.

PATH's Tech Set recommends sealing all ducts, including plenum junctions, with foil tape that meets UL 181, or with mastic, the only sealant approved by the ACCA.

If you are replacing the HVAC unit, place the outside condensing unit out of direct sunlight and in a location with consistent airflow.

Old home with modern air

In remodeling a historic Atlanta home, Seville used proper HVAC installation and sizing practices that significantly reduced the home's energy use and improved indoor air quality.

Although the home more than doubled in size — from 2,300 square feet to 4,700 square feet — the energy bills remained the same. And the home was more comfortable than before.

"The real pay-off for the remodeler is counted in customer satisfaction," Seville says. "When the equipment functions properly, the home is more comfortable and the air quality is better. The homeowners notice that immediately. Then they start to see the impact on their utility bills. Rising energy prices make this improvement even more valuable."

"Load calculations cost approximately $100 or $200 per house and take between one and two hours for an average home. However, this cost is often recouped immediately because the system can typically be downsized. If a number of homes with similar plans are being calculated, costs are even lower."

In the end, Seville's decision comes down to quality, not cost.

The right person for the job

If you have to replace, install or upgrade an HVAC unit, you'll probably rely on your contractor. But not all HVAC contractors are equally informed about sizing issues. Ask these three questions to determine whether the contractor is as qualified as he thinks:

  1. What are your duct sealing methods?
  2. What software or calculations do you use to size your HVAC system?
  3. In which previous projects have you used this software or calculations?
  • "Contractors should know Manual D and J," says Glenn Hourahan, ACCA's vice president for research and technology.

    "A lot of times a contractor will say they know them — and they should because they are required by code — but by asking them how they are doing it, you can discover that they don't even have the correct tools."

    Although spending the extra time to find the right contractor may seem like a chore, Hourahan says it's nothing compared to the time lost to a callback or work lost because of a bad reference.


    Author Information
    Scott T. Shepherd writes about better building practices on behalf of PATH. The Partnership for Advancing Technology in Housing (PATH, www.pathnet.org) is administered by the U.S. Department of Housing and Urban Development.

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    Resource Information

    To learn more about HVAC sizing, consult the links provided online. Visit www.ProRemodeler.com and click on "When Size Matters."

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