To have an HVAC (heating, ventilation and air conditioning) system run at optimal efficiency, homeowners and business owners need to know the heat load of the building. This metric, measured in tonnage, determines which size of HVAC system to purchase. Selecting the right size system is important for efficiency. For example, over-sized systems don’t properly de-humidify the air during the summer. Under-sized systems may not be able to effectively heat or cool a building. Homeowners and business owners want the right size to have efficient heating and cooling.
The most common way to determine the heat load of a building is a Manual J calculation. A licensed HVAC contractor can complete this calculation for a fee. However, there are some online Manual J calculators that allow home and business owners to complete their own calculations. The key piece is not the calculation, however, but the inputs. Owners must know many details about the building in order to complete a Manual J calculation. The checklist below contains the inputs required to do a Manual J heat load calculation.
Materials for Walls, Ceiling and Floors
A heat load calculation takes into account the insulation properties of the house. For this reason, it’s not enough to know that a home is “brick.” Homes with brick walls also may or may not have insulation. The insulation can be various types. The same principle applies to ceilings and floors. A building plan or an HVAC contractor may be required to find out this information.
The insulating properties of windows can also vary, depending on whether the window is single or double paned. Even the frame type (usually metal, wood or vinyl) has an impact on the heat load calculation.
Surprisingly, there are differences in insulation properties between wood doors. In fact, there are multiple types of wooden doors in the heat load calculation. The insulation depends on whether the door is hollow or solid. Metal doors also have drastically different heat properties.
Ceiling and Window Height
The ceiling height helps to determine how much air needs to be warmed or cooled. Window height also has an impact on the heat load calculation.
Length x Width x Height of Walls, Windows and Doors
This information helps determine the size of the home or business and how much air needs to flow through the building for proper heating and cooling.
Orientation of the Building
The orientation of the front door and windows can be north, south, east or west. Southern exposure tends to be warmer in the northern hemisphere, since the sun hits that surface year-round.
With this checklist, home and business owners can begin to complete a heat load calculation. There are online resources available to assist. However, it’s important to note whether the calculation assumes the house is a “block” or it asks for the inputs for “room-by-room.” Block calculations are simpler to complete, but they don’t tell owners how much heating and cooling is required for each room. To properly size ductwork and maximize efficiency, a room-by-room heat load calculation is preferred.