Wales-Darby Blog: Rightsizing Your Heating and Cooling System

Rightsizing Your Heating and Cooling System

Published by National Grid

Modern heating and cooling systems are more reliable and much more efficient than older space conditioning systems, especially if they are more than ten years old. When the time comes for replacement, choosing a properly sized unit is critical. Both heating and cooling output should be taken into consideration to ensure optimal efficiency, maximum comfort, and lowest maintenance and operating costs during the unit?s life span.

When it comes to inadequate heating and cooling the most common mistake is oversizing the heating and cooling system. Oversizing not only makes the system operate inefficiently, it costs more to operate, has a tendency to break down frequently, and it is more expensive to install. Oversized heating equipment leads to temperature swings in the building, which creates an uncomfortable environment. Oversized air conditioners do not dehumidify?a building?s air?enough which?can promote mold growth and lead to?feelings of clamminess?by occupants.

Incorrect Sizing Methods

It is very important to perform the correct sizing calculation for the building. However, most contractors only perform a label nameplate check of the existing system and install a similar unit or a larger one. Another method contractors use is to install a system based on the size of the building and charts from the unit?s manufacturer. These methods do provide a good first size estimate, but they should not be used alone to determine the size of the heating and cooling system.

Why Most Older Systems are Oversized

In the past, when homes were not designed and built with the tight construction that is customary today, it was normal to install furnaces and air conditioners that were two to four times larger than necessary. If a building has been updated with new windows, weather-stripping, caulking, and insulation; referring to the nameplate is going to lead to oversizing the system. These types of improvements help reduce heat loss in the winter and heat gain in the summer and permit the use of a smaller system in the building. Not only will a smaller system maintain comfort levels, it will save large amounts of energy as well.

What to Consider When Sizing a System

In order to correctly size a heating and cooling system, a contractor must consider many factors other than just reading the nameplate of the existing system. Factors that should be considered include the following:

• Local climate conditions
• Size, shape, and orientation of the building
• Current insulation levels
• Location and types of windows
• Air infiltration rates
• The number of occupants
• Occupant comfort preferences
• Lighting and lighting efficiency of the building
• The types of major appliances that give off heat

Methods of Sizing

Building owners should request that a sizing calculation be used to determine the proper size of a new heating and cooling system.

Manual J?the Air Conditioning Contractors of America?s (ACCA) Residential Load Calculation is the recommended method for use in the United States. Manual D?the ACCA?s Residential Duct Design should be used if ducts are part of the installation. Manual S?ACCA also provides a comprehensive guide called the Residential Equipment Selection for choosing home heating and cooling systems.

There are a number of aspects that affect a building?s heating and cooling requirements?or load. In order to determine the correct system size to satisfy the load, walls, ceilings, and floor space should be measured to determine the building?s volume along with assessing the R-value of the building?s insulation, windows, and construction materials. Average outside temperatures and humidity levels affect the demand on the heating and cooling system along with the orientation of the building and overhangs.

Air leakage also has an impact on a building?s load requirement and a blower door test is the most accurate way to measure leakage. Furthermore, an inspection of the location, size, joint seals, and insulation of distribution ducts and placement of supply and return registers of the forced air system is needed. All these factors must be considered in order to properly size the heating and cooling system.

To size a heating and cooling system correctly, the contractor, builder, or developer must consider all aspects of the building. Far too often?the current unit?s information is taken from the nameplate and a similar sized unit, or worse, a larger unit is installed. This will not provide the highest efficiency, best comfort level, or the lowest maintenance for the building owner.?

Source: http://blog.walesdarby.com/2012/07/rightsizing-your-heating-and-cooling.html

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