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:

• Size, shape, and orientation of the structure

• Local climate

• Type, location, and number of windows

• Type and amount of insulation

• Number and ages of occupants

• Structure design

• Construction materials

• Planned use of the heating/cooling system

• Condition of the distribution system (ducts or pipes)

• 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.


Always insist on a correct sizing calculation before signing a contract with an HVAC contractor or installer. See the sec­tion Manual J and Related Materials Used for Sizing Heating/Cooling Systems later in this chapter.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers