Roof Top Unit and Economizer Animations

On most commercial buildings you will find roof top units (RTUs), air handlers that provide heating, cooling, and ventilation to the building’s conditioned spaces. Heating and air conditioning account for over 25 percent of the energy use of commercial buildings, and rooftop units serve 40 percent of commercial floorspace.

Cooling and Heating

When the space calls for cooling, a blower in the RTU draws warm air from the space below, through the cooling coil, and then supplies the space below with the cooled air.

A similar process occurs to provide heated air.  



RTUs also ventilate the space, always supplying some outside air. Notice in the animation below that the outside air damper is no longer fully closed, which allows the RTU fan to bring in a minimum amount of outside air. Notice the addition of pressure relief for the added outside air.

A space with people in it requires more ventilation than when it’s unoccupied. An occupancy sensor or a CO2 sensor can respond to changes in ventilation demand, controlling the outside air damper and bringing in more air as necessary.

RTU and Economizer Operation

An economizer is a control system that takes advantage of outside air conditions to save energy when cooling is called for in the conditioned space. When the outside air temperature is lower than the space temperature, the economizer opens the outside air damper of the RTU and brings in cool air. This saves energy, because otherwise the RTU would cool the inside air mechanically.

The following animation shows the standard operation of an RTU with an economizer. It also shows the operation of the compressor that is connected to the cooling coil. In standard operation, both the compressor and the fan run at a constant speed.

The final animation shows optimized operation of the RTU, where the fan and compressor speeds are modulated based on the conditioning demand of the space. Under normal conditions, this approach leads to the most energy efficient operation of the RTU.

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The motion graphics animations were designed and produced by Huma Saqib.