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Do I Really Need a Manual J Load Calculation?



Do I really need a Manual J load calculations done on my home?

Let's discuss the issues and the benefits of a load calculation, then you decide!

Proven Facts

  • National surveys have determined that well over half of all HVAC contractors do not size heating and cooling systems correctly.
  • When it is time for a new hvac system, is critical that the system is sized correctly for the best efficiency, comfort, and lowest maintenance and operating costs over the life of the new system.
  • Older hvac systems (10 years old or more) are often unreliable and are definitely less efficient than a newer system.
  • Hvac contractors habitually oversize equipment. They prefer to err on the large side. This costs the homeowner in higher installation costs, inefficient operation costs more on utility bills, and the equipment breaks down more often costing more for service maintenance.
  • Oversized air conditioning equipment does not operate long enough to control excess humidity, making the home clammy and uncomfortable. This can also create a mold issue.
  • Many contractors / installers only check the nameplate of the existing system for sizing. This method does not account for today's increased equipment efficiencies, or home improvements which undoubtedly have been done over the years. How does this contractor know that the equipment was properly sized for the home originally? He doesn't!
  • Older structures were not sealed well. This has led to hvac systems sized 2 to 4 times larger than necessary. These homes have since had windows replaced, weather stripping added, insulation added, and gaps and cracks sealed. A much smaller system will operate efficiently, saving large amounts of energy.
  • Many municipalities around the country are now requiring a Manual J calculation and a Manual D duct design be performed before a building permit is issued.

The Manual J and Manual D calculation method

Correct hvac system sizing requires looking at many factors of the home.

  • The local climate
  • Size, shape, and North orientation of the house
  • Insulation levels in the attic, crawlspace, and walls
  • Window area, window locations, type and u-value
  • Air infiltration rates
  • Occupant comfort preferences
  • Lights and major home appliances, which emit heat into the home
  • Ceiling heights and window shading
  • many more factors

Homeowners should insist that a proper Manual J load calculation be performed before signing a contract.

According to ACCA (The Air Conditioning Contractors of America), Manual J is the recommended hvac load calculation method for use in the United States. Make sure that the procedure used by your contractor follows Manual J.

If ducts are part of the installation, they should be sized using the ACCA's Manual D.

ACCA's Manual S is used for proper Residential Equipment Selection.

I couldn't say it any better. Following are quotes from the DOE (Department of Energy).

Steps a Good Contractor Should Take to Size Your System

Many factors affect a home's heating or cooling requirement, or "load." A good estimator will measure walls, ceilings, floor space, and windows to determine the room volumes, and will assess the R-value of the home's insulation, windows, and building materials. A close estimate of the building's air leakage is also necessary. A blower door test is the best way to measure air leakage.

A good estimate will also include an inspection of the size, condition of seals on joints and insulation, and location of the distribution ducts in forced air systems. The placement of supply and return registers should be appropriate for the system type and size.

The orientation of the house also affects heat gain and heat loss through windows. Overhangs can reduce solar gain through windows. Make sure the contractor uses the correct design for the outdoor temperature and humidity in your area. Using a higher summer design temperature results in oversizing air conditioners.

When the contractors are finished, get a copy of their calculations, assumptions, and the computer printout or finished worksheet. This is your only proof that they did the job right. To summarize, when designing your new heating and air conditioning system, the contractor you choose should do the following:

1) Use a computer program or written calculation procedure to size the system
2) Provide a written contract listing the main points of your installation and includes the results of the heating and cooling load calculation
3) Give you a written warranty on equipment and workmanship
4) Allow you to hold the final payment until you are satisfied with the new system.


Visit "Design Calculations": Benefits and Uses for the Homeowner, Builder, and the Do-It-Yourselfer

Read about "The Hvac Purchaser's Dilemma: Design First or Regret It Later.

Read more about Wrightsoft Manual J calculations

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