Some areas of a hospital are occupied 24/7 and have ventilation requirements to ensure thermal comfort is maintained. However, there are other certain areas of a hospital that are not occupied during nights and weekends, including administrative areas and some patient care areas (ORs, Diagnostic treatment areas, laboratories, Exam Rooms). These areas can be as much as 30% of the hospital footprint (see figure below). By zoning these systems with separate air handling units and exhaust fans, entire systems can be shut down during unoccupied times, saving energy for heating, cooling, and fans.

Technical Description

Design considerations

In new construction, building HVAC systems should be designed such that the areas that can be scheduled are fed by the same air handling unit (AHU) and that the air handling unit does not feed any areas that will require air 24/7.

In existing buildings, during renovation projects, if at all possible, splitting of these systems should be considered. Solutions include installing a dedicated AHU to serve only non-24/7 areas, or installing variable air boxes for each zone and a variable speed drive on the fan so ventilation can be turned down or off to those unoccupied zones.

One of the areas that can have the largest impact on unoccupied energy savings is Operating Rooms. Because of their high air change requirements, ORs utilize considerable energy, and they are seldom used during the night. A common strategy is to reduce the air quantity in the rooms from 20+ air changes to as little as 6. It is recommended that this is done while still maintaining the temperature and humidity setpoints so that the room can get to the required setpoints quickly when needed. In order to reduce the air, there are some design requirements that must be considered. Since positive pressure has to be maintained, as the supply air quantity is reduced, the return air quantity has to be reduced as well. Ideally, this is done by providing a return/exhaust tracking box in the room return/exhaust that will maintain the offset between supply and exhaust. In addition, scheduling of the rooms and occupancy sensors in each OR are required to control the spaces. If it is an existing facility where the ductwork layout and ceiling space do not make it feasible to add tracking boxes, there is another option that can be done if the ORs are served by a separate air handling unit. This would be to have the OR’s occupancy sensors control the air handling unit rather than the room boxes. Only when all rooms are unoccupied will the AHU reduce to some predetermined volume that needs to be carefully set through testing and balancing all of the ORs to determine how low the unit can go while still maintaining the pressure relationship in each OR.

In 2021, ASHRAE 170 added a column to Table 7.1 which clearly defines which spaces can have unoccupied turndown. Except for the rooms which require specific pressure relationships, the air changes in the rooms can go down to 0 (as long as unoccupied temperature and humidity is maintained). A summary of ASHRAE 170 supply air requirements is shown below in Table 1.

Table 1: Typical peak supply air and occupied minimum supply air in hosptial spaces

Control strategies

Air handlers that serve only spaces with unoccupied periods should be scheduled to shut down and operate only when needed to maintain unoccupied space setpoints. The only concern is making sure that the spaces meet required indoor design conditions once the spaces are occupied. This can be done utilizing morning warmup and cooldown cycles which optimizes the start time of the system to bring the spaces to temperature. Time-of-day scheduling is a simple way to control these spaces and can be done on even the most basic control systems. Occupancy sensors can also be utilized to ensure ventilation and heating/cooling be fed to an unoccupied zone in the case of an unexpected occupancy, outside the normal time-of-day schedule. Operating room setback is one example of an effective use of occupancy sensors.

How does this decarbonize?

Accounting for over 40% of total building energy, and over 75% of total natural gas use in an average hospital, zone reheat is the largest share of energy consumption and a large driver of scope 1 emissions. Excessive or over-ventilation results in increased consumption of natural gas and electricity through chilled water and fan use. By shutting down the AHUs at night, and reducing ventilation to unoccupied zones where it is not needed, heating, cooling, and fan energy is saved and results in a reduction of Scope 1 Natural Gas emissions and Scope 2 Purchased Energy emissions. Savings can vary by site. We have found that it can be as little as 0.5% to as much as 8.5% of the total hospital energy costs.

Implementation

Barriers: Codes

The national ventilation standard ASHRAE 170 and the CA standard Title 24, Part 4, California Mechanical Code, require some critical zones to maintain pressure relationships regardless of occupancy. This measure is limited to the spaces where the ventilation codes allow a turndown or full shutdown.

Barriers: Culture

Operating rooms have very high minimum ventilation rates and are among the highest in a hospital. Both the national code (S170) and CA code (T24, P4, CMC) allow operating rooms to control to an unoccupied mode, which allows operating rooms, when unoccupied, to have ventilation reduced from an occupied minimum rate of 20 air changes per hour to an unoccupied minimum rate of 6 air changes per hour. However, many facilities face challenges in finding a schedule and programming a control system to ensure the fool-proof operation of this strategy. Time-of-day schedules do not allow for emergency procedures and occupancy sensors are often too sensitive and ramp ventilation back up when a cleaning crew or other occupant enters the room for a reason other than an operating procedure. Push buttons for doctors and surgeons to hit when an emergency procedure needs to happen can condition the room quickly, but it may take up to 15 minutes which is often deemed too long, or in some cases, the doctors and surgeons forget to hit the button. It is part of a solution, but it is not foolproof.

Strategy:

It is important to think about what areas can be turned down and what limitations exist. The best systems are often designed with a combination of advanced controls, time-of-day schedules for each AHU, and in operating rooms, occupancy sensors and push button starts, and educating the medical staff to operate them in emergency situations. Another design to ensure a safe, conditioned OR in an emergency situation is to keep 2 operating rooms at full ventilation even during unoccupied times.

Financial analysis and business case

We have found that the simple payback for doing unoccupied turndown typically ranges anywhere from 0.1 to 1.2 years.

Hospital in Franklin, WI (518,000 sf)

The focus was on unoccupied turndown of the Operating Rooms and other spaces that could be turned down at night that were previously scheduled for 24/7 use (administration, exam rooms, surgical preoperative and postoperative spaces, etc.). The biggest savings were due to the OR setbacks. Other spaces, if they didn’t have dedicated air handling units, were done by using the Building Automation System and programming the VAV boxes.

Results: Saved $62,110 or 8.2% of total energy

Outpatient Clinic in Milwaukee, WI (190,000 sf)

The focus was on unoccupied turndown of the Operating Rooms and other spaces that could be turned down at night that were previously scheduled for 24/7 use (administration, exam rooms, surgical preoperative and postoperative spaces, etc.). The biggest savings were due to the OR setbacks. Other spaces, if they didn’t have dedicated air handling units, were done by using the Building Automation System and programming the VAV boxes.

Results: Saved $48,027 or 8.3% of total energy