*3.4.1. Rooftop units*

Rooftop unit is one type of unitary air handler designed for outdoor use, usually on the roof. There are two types of configurations: packaged unit and split unit. A typical packaged rooftop unit has a refrigeration system delivering cool air into the space. Therefore, it is also called the direct-expansion (DX) unit. Meanwhile, most RTUs provide heating to the space using either a gas heater or an electric heater.

In an RTU, the supply fan (or indoor fan) and compressors are usually running at a constant speed. At partial load conditions, excessive fan and compressor power are consumed due to constant speed operations. With the installation of VFDs on the fan and (or) compressors, significant power consumption could be saved. In addition, compressors account for the largest part of power consumption in an RTU. With the reduction of compressor speed, both the demand and energy consumption are reduced greatly. In an RTU, the supply fan (or indoor fan) and compressors are usually running at a constant speed. At partial load conditions, excessive fan and compressor power are consumed due to constant speed operations. With the installation of VFDs on the fan and (or) compressors, significant power consumption could be saved. In addition, compressors account for the largest part of power consumption in an RTU. With the reduction of compressor speed, both the demand and energy consumption are reduced greatly.

Figure 12 Single‐stage DX rooftop unit Figure 12 is a typical single‐stage RTU. The supply fan circulates air through the evaporator and heater. Traditionally, the supply fan and compressor run at a constant speed. Initially, the VFD is used on the supply fan to modulate the fan speed and maintain the space temperature set point. This type of RTU can be called variable capacity RTU. Later, the VFD is used on the compressor as well. Both fan and compressor speeds can be modulated to

Heat pump units are very similar to RTUs as both systems utilize a refrigeration system. However, heat pump units can use the refrigeration system to produce heat as the first stage. At colder weather conditions, auxiliary heat is turned on to provide additional

A VFD can be installed on heat pump units as well. The schematic diagram is very similar to

**Figure 12.** Single-stage DX rooftop unit

control the space temperature.

the RTU as shown in Figure 12.

*3.4.3. Computer Room Air‐Conditioning Units*

*3.4.2. Heat Pump Units*

heating capacity.

Figure 12 is a typical single-stage RTU. The supply fan circulates air through the evaporator and heater. Traditionally, the supply fan and compressor run at a constant speed. Initially, the VFD is used on the supply fan to modulate the fan speed and maintain the space temperature set point. This type of RTU can be called variable capacity RTU. Later, the VFD is used on the compressor as well. Both fan and compressor speeds can be modulated to control the space temperature.
