New, more efficient furnaces and advances in load calculation techniques have changed the way furnace sizes are determined. Having the wrong size furnace can lead to high energy costs and low comfort. It is best to let a trained professional handle sizing. If you do it yourself, you need to know more than the size of your home to determine the size of your new gas furnace.

Vocabulary

Just like any other industry, HVAC (Heating, Ventilation and Air Conditioning) has many terms and abbreviations that you'll need understand. BTUs (British Thermal Units) are the measure of the heat output of a furnace. Residential furnaces usually range from 40,000 to 160,000 BTUs. Efficiency describes how much of the burned gas is turned into usable heat. A gas furnace works by burning the gas to heat a metal plate known as the heat exchanger. Air heats as it blows across the heat exchanger and flows to the home. Some of that heat is lost--10 percent in a 90-percent efficient furnace. Some furnaces have a variable speed fan, making them more efficient at heating and allowing for a smaller furnace in some situations.

Size and Layout of Home

Begin by calculating the heated square footage of your home. It is important that the square footage used is the heated--or occupied--square footage of the home. This number is often different from the total square footage. If the heated square footage is unknown, simple measure the area of each room, hallway, foyer and any area where people will be, and then add the measurements together. Multiply that square footage by 45 to find the average size. Adjust the number higher if you live in a colder than average climate or lower if in a warmer climate. A split or multilevel home will also affect the size of your furnace. Consult a professional when dealing with one of these types of homes.

Air Infaltration/Escape

The most often overlooked criteria when sizing a furnace is the "tightness," or air loss, in a home. Any opening to the outside--doors, windows, holes for wires to enter--creates a place for warm air to escape. Plus, the types of the doors (wood or metal) and windows (single, double or triple paned), along with the amount of insulation, also affect the size of furnace needs of your home.

Calculating

Professionals use the ACCA (Air Conditioning Contractors of America) Manual J to calculate the load for a home. You can purchase the manual online, and it even comes in a software version that allows you to plug in numbers without having to look up charts. The Manual J takes into consideration size, layout, door and window area, along with insulation and geographic region. It is the next best thing to allowing a professional to calculate your new furnace size.

Source:

Sizing a Furnace

How to Size HVAC Systems Correctly

AC Direct.com