Here is a list of the general types where it is specifically intended to apply:

• Gas fired (natural and propane) furnaces up to about 200,000 BTUH
• Vertical, horizontal, upflow, or downflow makes no difference.
• Electric heaters usually from 5 to 30 KW (or more)
• Heat Pumps:
• Heat pumps are regular air conditioning units that have controls added that reverse the flow of the refrigerant for the heating mode. Instead of the hot air blowing into the air outside the building and the cool air inside, it is the reverse and the building is heated instead of cooled. In some locations of the country, these units are more efficient users of fuel.
• These are more commonly used with the Electric heaters for the auxiliary and supplemental heat; however, more often now are they being used with the gas fired furnaces as well.
• Other Types of air moving devices can be used, of course, but are not considered here. The above mentioned units are standardized in air volume and velocity, etc. for the specific need of home heating and cooling applications.