After receiving some great questions from our last newsletter, I wanted to dive into another energy-efficient hotel design solution, one that is often overlooked. This solution involves utilizing your hotel’s dedicated outdoor air system (DOAS) for no-cost cooling.
A DOAS is a unit typically mounted on a hotel roof that is used to condition (heat, cool, humidify, dehumidify) all of the outdoor air brought into a hotel for ventilation and then deliver it to each occupied space at neutral (70°F) temperature. A separate local HVAC unit (such as a fan coil, water-source heat pumps, or PTAC) in each space then provides cooling or heating to maintain the desired space temperature.
Though the mechanics of a DOAS remain complex, a few simple points offer valuable insights:
- A DOAS cools outdoor air to approximately 55°F to dehumidify the air first. In other words, even if you’d like for your DOAS to deliver 70°F to guestrooms it first cools air to 55°F to remove moisture.
- A DOAS then reheats this air from 55°F to approximately 70°F before delivering air to the hotel. Heating of this air is done using heat recovered from the hot refrigerant gas cycle, making it quite efficient.
- If designed with modulating control of the hot gas reheat, a DOAS allows the building owner to set the dry-bulb temperature delivered by the DOAS based on their preference. In other words, if an owner wants to deliver 65°F rather than 70°F to the space, they can.
- Since the outdoor air is cooled to 55°F for dehumidification already (regardless of the DOAS set point), it is delivered at a lower temperature than 70°F — requiring less cooling load from the local units.
- This method reduces the load of local HVAC units in your hotel, which would otherwise be providing this cooling in the guestroom and public spaces. The reduction not only lowers electrical costs but also extends equipment life.
- There are several energy-efficient options that can be implemented with DOAS. Examples include Modulating Hot Gas Reheat, Energy Recovery Wheels, and Variable Speed Supply and Exhaust Fans. Depending on the application, these options can drastically reduce a building’s energy use.