In 1990 Damark International, a national mail order firm based in Minneapolis, was advertising for $699.99 ("manufacturer's suggested retail price $1,199.00") the Bionaire Cold Front Portable Air Conditioner, a "free-standing portable unit" which "does not require outside venting."

This appliance was intended to be rolled from room to room by the user, who could simply plug it into an ordinary 110-V AC outlet and enjoy the cool air from its "Cold Front." It was claimed to provide 5,500 Btu/hour cooling capacity for 695 watts by drawing 6.3 amperes of electric current.

This advertisement can be used to think about energy principles and calculations:

1. It can be checked whether 695 W is consistent with 6.3 amperes and 110 V.

2. The heat removed from the air blown out the front, 5,500 Btu/hour, can be calculated to be 1,612 W in SI units, making use of the conversion factor 1 Btu = 1055 joules.

3. The coefficient of performance (COP), the ratio of heat removed to energy supplied, is then

Although the COP is a dimensionless quantity, air conditioners in the U.S. carry labels giving the COP as an "Energy Efficiency Ratio" (EER) in mixed units of Btu per watt-hour, so the EER of this unit is

This is on the low side, since efficient window air conditioners can have EER values of 10 or higher; in fact, it does not meet current national minimum standards.

4. Heat must be rejected at the rate of

Presumably this is not out the "Cold Front," so it must be out an unadvertised "Warm Back."

5. This air conditioner has the net effect, while its compressor is running, of providing

to the room in which it is used. So this air conditioner, because it does not vent the exhaust heat outside, is really a heater! Perhaps during the winter the unit could be sold as a portable heater, with users facing its "Cold Front" away from them! Of course, its price of $739.99 (including shipping) is an order of magnitude greater than that of commonly-available electric resistance space heaters.