Comfort Line - Questions and Answers

This quarter our Questions and Answers segment discusses the importance of properly sizing heating and air conditioning equipment.

How important is it to properly size heating and air conditioning equipment?

In the days when energy was plentiful and inexpensive, no one worried much about sizing heating and air conditioning equipment. However, in today’s world of high energy prices and environmental concerns, it has become important because properly sized equipment uses less energy and generates less pollution.

Isn’t it better to oversize equipment so that it doesn’t have to work as hard?

Larger equipment costs more to install and more to operate. Mechanical equipment operates most efficiently in the middle of its run cycle. Oversized equipment cycles on and off more frequently, which reduces efficiency, can create service problems and reduces the life of the equipment. Taxicab engines in New York City are known to log up to 500,000 miles because they run continuously without starting up and shutting down.

How do you properly size heating and air conditioning equipment?

It is a law of physics that warm air (winter) or cool air (summer) inside a building will seek equilibrium with the outdoor temperature. This process is slowed down, but not stopped, by the insulating value of windows, doors, walls and ceilings. The rate at which heat (wintertime) and cooling (summertime) leaves a building can be measured by what is known as a heat loss calculation (winter) or a heat gain calculation (summer).

Years ago, these calculations were done by hand, but today we use computer programs. In order to generate an accurate calculation, we need to know the dimensions of each room, the area of all windows and doors and the insulating value of the building materials. The greater the insulating value of the building materials used in constructing your home, the lower the heat loss or heat gain.

How accurate are heat loss and heat gain calculations?

Heat loss and heat gain calculations are reasonably accurate, but not foolproof. While we can make strong educated guesses, it is not possible to look behind the walls of your home to see exactly what insulation was installed, the quality of the installation and whether any of the insulation has settled, leaving gaps.

Are there any other methods of performing heat loss and heat gain calculations?

With regard to heat gain calculations (cooling), there are no other practical methods other than using the building envelope as described above. However, with regard to heat loss calculations (heating), we have found that tracking fuel consumption and comparing it to degree day (outdoor temperature) information is easier, quicker and more accurate. This fuel consumption method, which was developed at Hart & Iliff, calculates exactly how much energy must be introduced into a building to overcome the heat that is escaping to the outdoors through the walls, windows, doors and ceilings of the building. An article we wrote about this method for sizing heating equipment (see related story) was featured in three major industry publications and has been used by heating professionals all around the country.