Self contained or ptus (packaged terminal units) are labeled as such because all
of the machinery, (condenser, furnace, evaporator coil, and blower) is contained in one
large cabinet. The most common are the rooftop units. Heating only, cooling only, or heating
and cooling can be obtained from these machines. Some manufactures design them with interchangeable
panels that allow ductwork to be attached to the bottom for rooftop applications or to the
sides for outdoor location of the unit and through the wall ductwork.
Heat can be from any fuel (gas, oil, electricity, or heat pump), and air conditioning
will be electric. Designed originally for commercial applications, their versatility has
created a demand for residential use; and smaller, quieter machines are produced for the
home. With the help of a fresh air hood and enthalpy controls, air conditioning demands
can be met when outdoor air temperatures are low by means of drawing fresh air in and ducting
it to the space, reducing the demand for electricity by keeping the compressor idle. As
the air temperature rises, the enthalpy control will mix fresh air with re-circulated air
as it turns the compressor on for needed cooling.
Rooftop installations that are ducted out of the bottom require a curb, which not only
raises the unit up off the roof but has framework to support and fasten the ductwork to.
The heat loss and gain calculator in this site can be used to size a unit, but in commercial
applications other major factors such as lighting, fresh air, delivery doors, and machinery
operating within the space are critical, and therefore not recommended.