A pressure switch is designed to check that there is a differnce in pressure. This is need to be sure of the direction of air flow.  Air will flow from a higher pressure area to a lower pressure area. It is very important to make sure the burner box is in a negative pressure area. The burner box needs to be in a negative pressure to safely burn all the fuel, and then remove the combustion gases from the home safely.

The pressure switch makes sure it is safe to advance to the ignition step of lighting the furnace. The pressure switch prevents a roll out flame and possible resulting home fire.

Pressure switches are the most accurate way to verify there is a pressure difference. The air pressure is measured in inches of water water column. One inch of water column is approximetly 1/27 of a PSI.

There are two major types of pressure switches; a one port, and a two port.  The one port switch can only compare the pressure difference form the point where the switch is mounted, to the point at the end of the hose that is mounted to it. The two port switch can compare the pressures from two locations, the two locations at the end of each of the two hoses.

one port pressure switch	two port presssure switch	one port pressure switch