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Cleveland HVAC Blog

How a Properly Sized HVAC System Affects Home Comfort

Posted by Paul Wadsworth on Mon, Jul 19, 2010 @ 10:14 AM

Did you know that over half of all home heating and cooling systems in Northeast Ohio are the wrong size?  This is true for both older and newly-installed systems.  If you have an older HVAC system (generally more than 10 years old), it was probably oversized in order to compensate for the fact that your home was not tightly sealed.   Depending on the installer, as well as other factors in your home, your system could be two to four times larger than necessary!

This not only makes the new system more expensive to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Oversized heating equipment can also create uncomfortable temperature swings in the house. Oversized air conditioners and heat pumps do not run long enough to dehumidify the air, which results in that “clammy” feeling and contributes to unhealthy mold growth in many air-conditioned houses.  Compounding the problem is the fact that many homeowners, aware that they need to weatherize their homes to conserve energy, have added new windows, caulking, weather-stripping, and insulation to their homes over the years.  This seals the home more tightly, resulting in an even greater mismatch between the system and what the home actually requires.

So, if you’re experiencing the symptoms of an improperly sized system, consider a replacement.  A more tightly sealed home should allow you to install a smaller system that will offer greater comfort and efficiency.  Here are some things you should know about sizing a new HVAC system:

1.   The label isn’t the last word- Some contractors go by the label, or nameplate, when sizing a new system.  The label on your HVAC equipment offers information such as the BTU per hour output, SEER ratings, etc.  A simple “rule of thumb” determination based on this label is not the correct method of sizing heating and cooling systems.

2.   Manual J and Manual D calculations- Correct system sizing requires considering many factors other than simply reading the nameplate of the existing unit.  There are  several methods and systems to correctly size a system.  One widely regarded approach has been developed by the Air Conditioning Contractors of America (ACCA).  They have developed 2 methods for sizing HVAC systems, and they are among the most recommended methods in use today.  Manual J calculates heat loss/gain from the building through walls and ceilings, leaky ductwork, and infiltration through windows, doors, and other access points, as well as heat gain due to sunlight, people, lights and appliances, doors, walls, and windows, and infiltration though walls.

Manual D is the standard method that contractors use to design residential HVAC duct systems.

Key factors for correctly sizing a heating and cooling system include the following:

  • Local climate
  • Type and amount of insulation
  • Windows- how many, where located
  • Size, shape and orientation of the home
  • Number and ages of occupants
  • Air infiltration rate

Homeowners should insist that contractors use a correct sizing calculation before signing a contract.  Since so many factors are involved, a thorough inspection of all above factors is necessary.  We do load calculations for our customers all the time and can help walk you through the process.

So, if you’re looking at a system replacement, remember—size matters!  Too small a system will not work, but too large is also a problem.  A properly sized system provides more even heating and cooling throughout your home, reduces your energy bills and breaks down less often.

Topics: mold growth, home heating and cooling systems

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Welcome! Read our latest posts or explore the archives. You’ll find tips for how to maintain your HVAC equipment to discussions about indoor air quality, conserving energy and saving money, and information about emerging technologies in heating and air conditioning. Bookmark our blog.

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