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HVAC contractors referred to the sizing process as a HVAC load calculation. They use Manual J software to complete it. The most important element of properly sizing an HVAC system is the overall energy efficiency of a structure. Manual J requires an analysis of:
• Overall insulation levels in the attic, walls and floors.
• Number of windows, their sizes, energy efficiency and solar exposure.
• Air infiltration rates.
• Floor plan design.
• Orientation of the house to the sun.
• Landscaping factors.
• Climate characteristics.
• Cubic footage of the area to be conditioned.
• Family size and ages.
• Number of heat-producing appliances indoors.
• Indoor temperature preferences.
This process takes a while and it has to be done at your home. The HVAC pro will measure your rooms, the windows and exterior doors, and inspect the attic, walls and floors. He or she will ask you about your family and lifestyle. It’s helpful to have copies of your energy bills for the last 12 months to give the HVAC expert an idea about your home’s energy usage.
Once the Manual J calculation is complete, you’ll know how large your next system needs to be. The next step will be to assess the ductwork capacity and configuration using Manual D. The HVAC pro should help you fine tune your purchasing decision by using Manual S to evaluate the system you want and its ability to manage summertime humidity.
Why It Matters
HVAC engineers have designed heating and cooling systems to provide specific amounts of air flow and heated or cooled air. Larger systems provide more, and smaller systems provide less. When an HVAC system heats or cools a home too fast, it won’t be as comfortable, energy bills will be higher, and it won’t remove as much humidity as an A/C system that’s the right size. Excess humidity is harmful to human and home health. It fosters the growth of mold, dust mites, bacteria and viruses. It makes the air feel warmer. If you over-cool the home, the air is likely to feel clammy.
A system that’s oversized will run in short cycles, which damages the parts. The load on HVAC equipment is highest when it first starts up. It takes a tremendous amount of electricity to start the air conditioner that stresses its electrical and mechanical components. An oversized air conditioner will start far more frequently during the day then one that’s the right size. Over time the extra wear and tear results in mechanical problems.
Energy bills also increase because system requires much more electricity at start up than it does during its normal running cycle. In addition, humid air takes more energy to cool than dry air. Not only will you lose comfort with an oversized system, it will also cost you more every time the system runs.
An oversized furnace probably won’t run long enough in each cycle to thoroughly heat rooms that are further from the air handler. As a consequence, your home will either be too hot or too cold in some spaces. While there are remedies for an oversized furnace, they come with a price tag.
Insisting on a Load Calculation
Before you choose a contractor for a new system, be sure that they will perform a load calculation HVAC exercise. It’s essential for you to achieve the greatest energy efficiency from the new system, as well as the comfort that it offers. It does take time, but it’s well spent.
Beware a contractor who goes simply by square footage of your home or the size of your existing system. Over time homes tend to lose their energy efficiency. They can develop cracks in the exterior and around window and exterior door frames. Insulation may not be as effective due to moisture, mold, or compression.
On the other hand, a previous owner may have upgraded its energy efficiency by sealing air leaks or adding insulation in the walls or floors. Windows affect energy efficiency and if the dual pane windows in your home have lost their vacuum seal, they won’t be nearly as energy efficient as they were originally. The only way to learn how large your system should be is through the load calculation process.