Sizing Residential Heating and Air Conditioning Systems

The size of the heating system is directly related to the amount of heat lost from the house or building. All structures lose heat to the outdoors or to adjacent unheated or partially heated spaces when the temperatures of the outdoor air or adjacent spaces are colder than those inside the structure. The heat within the building is normally lost by transmission through the building materials and by infiltration around doors and windows.

The loss of heat from a structure must be replaced at the same rate that it is lost. Consequently, determining the correct size of the heat system and the rated capacity of the heating plant required by the steam are very important. Unfortunately, many heating and/or cooling systems are either undersized or oversized, with the latter being the most common mistake.

Undersizing means that the heating and/or cooling equipment does not have the capacity (output) to meet the heating and cooling requirements of the structure. Oversizing means that the heating and/or cooling equipment has more capacity than required. Both undersizing and oversizing are caused by using guestimates or rule-of-thumb sizing calculation methods.

The Results of Oversizing and Undersizing

Oversized heating equipment is more expensive to install, operates inefficiently, uses more energy resulting in higher fuel bills, creates uncomfortable indoor temperatures by providing more heat than the structure requires, and produces wide temperature swings.

Oversized heating equipment also breaks down more often. Oversized units require larger air flow, resulting in noisier operation.
Undersized heating equipment lacks the capacity to provide sufficient heat, especially during extreme cold spells.

Oversized air conditioners and heat pumps create higher than normal humidity levels indoors because they do not run often enough to dehumidfy the air. This is known as short-cycling. The dampness in the air can also result in unhealthy mold growth indoors.

This article describes several methods for calculating heat loss, ranging from rule-of-thumb methods to the more precise method of using overall coefficients of heat transmission (U-values) computed for the various construction materials and combinations of construction materials through which heat is commonly transmitted.

Rule-of-Thumb Methods

There is nothing wrong with using a rule-of-thumb method when making an initial rough estimate of the heating/cooling load requirements for a structure. But it should be used as nothing more than a benchmark from which the true and precise calculations are made.

It provides the equipment installer/contractor and the homeowner with some idea of how much the heating and/or cooling system will cost. But correct sizing will involve the consideration of many different factors, including but not limited to the following:

  1. Size, shape, and orientation of the structure
  2. Local climate
  3. Type, location, and number of windows
  4. Type and amount of insulation
  5. Number and ages of occupants
  6. Structure design
  7. Construction materials
  8. Planned use of the heating/cooling system
  9. Condition of the distribution system (ducts or pipes)
  10. Air infiltration rates

There are three common rule-of-thumb methods used by contractors/ installers to size heating/cooling equipment.

Upgrade Method.

A common rule-of-thumb method is to install a furnace or boiler the same size or larger than the original one in an existing house or building. The problem with this approach is that the original equipment may have been incorrectly sized.

Furthermore, many changes have probably been made to an existing structure over the years, and these changes will have changed the load requirements for new equipment. The house has most likely had its insulation levels increased, because adding insulation in the attic, caulking around windows and doors, or installing double-glazed windows are relatively inexpensive upgrades.

If the HVAC contractor looks at the metal furnace or boiler tag (nameplate) specifying its output, Btus per hour, and so forth, and advises you to purchase and install one of the same or higher output, get another estimate. Unfortunately, this is an all too common method used by many to size equipment. Correct sizing involves the consideration of many different factors.

Sizing by Square Footage Method.

Another rule-of-thumb method is to size by the square foot area of the house or building. This is called the “sizing by square footage” method. It is one of the most commonly used of the inaccurate sizing methods.

It involves taking the square footage area of the structure and multiplying by a specific value. For example, a typical value assigned to air conditioning equipment is 1 ton (12,000 Btu/h) per 500 square feet of space (46 m2).

It fails to take into consideration any of the variables listed above.

Chart Method.

The so-called chart method of sizing heating/ cooling equipment involves filling in the blanks on a prepared chart. The chart lists the following:

  • Floor area of each of the heated rooms and spaces
  • Insulation levels in the floors, walls, and ceilings
  • House category (closest description of the type of house)

When all the required information is entered on the prepared form, multiply both the upper and lower values for heat loss in Btu per hour per square foot (from the data table used in conjunction with the chart) by the floor area of the house to estimate the required heating range. This estimating method does not take into consideration house location, design, or many of the other factors listed above.


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