Reheat is one of the largest end uses of energy in many hospitals. There are many ways to minimize reheat, as documented in other chapters in this guidebook. Reducing reheat, particularly when associated with combustion-based heating sources, is a primary goal in decarbonization of hospitals. Where reheat is associated with the need to dehumidify, this technology uses energy recovery in an air handling unit to reduce the reheat load.

The traditional method of dehumidification at an air handling unit is to cool the mixed air to the desired dew point temperature. Often, this air is cooler than is needed to satisfy the sensible cooling load of hospital spaces, given the minimum airflow rates that are required by code. So, traditional systems rely on reheat to raise the supply temperature and avoid over-cooling. This technology uses the return chilled water to achieve a portion of the reheat effect, simultaneously reducing the load on the cooling system (because warmer water is returned to the chillers) and the heating system. The system uses ostensibly standard components, except that cooling coils must be larger and deeper than in traditional handling units and air handling units must be zoned to take advantage of supply air temperature reset strategies.

Cooling and reheat is only one of several ways to dehumidify supply air (see chapters on Dedicated Outdoor Air Systems, Desiccant Dehumidification), but it is the most energy- and carbon-intensive, especially when reheat is done with fossil fuel. This section explores the use of return chilled water as a kind of energy recovery medium that provides “free” reheat. This will not always be the lowest-energy approach to dehumidification, but it improves on the traditional method and may be the right choice in some applications. It may also be applied in a Dedicated Outdoor Air System as a lower-cost and lower-maintenance alternative to desiccant systems.

Technical Description

How does it work?

In a traditional air handling unit design, mixed air (return air + outside air) passes through a chilled water cooling coil, which removes both sensible and latent heat. Leaving air temperature may be a fixed value, common in older designs, or it may be reset to satisfy the cooling needs of the spaces served by the unit and minimize reheat (See Supply Air Temperature Reset). However, if the outside air carries too much moisture, the supply temperature must be reduced to remove moisture. This dehumidification process overrides the supply air temperature reset strategy and delivers air that is colder than the spaces need. When this happens, air is reheated, typically with a hot water coil, to maintain comfort in the space. Reheat energy also becomes a cooling load when air from the space is returned to the air handling unit.

Figure 1 below shows a traditional AHU design diagram and details how the system takes humid outdoor air, conditions it, and delivers supply air at the desired temperature and relative humidity. Figure 2 below shows that process on a psychrometric chart.

Figure 1: Design Diagram of Traditional Cooling/Dehumidification AHU with Terminal Reheat

Figure 2: Psychrometric chart for Traditional Cooling/Dehumidification AHU with Terminal Reheat

Dehumidification using CHW return for reheat works by transferring heat absorbed from the mixed air by the chilled water back to the supply air to provide “free” reheat. The cold supply air absorbs sensible heat from the warmer chilled water return while maintaining the same dewpoint. This, in turn, reduces the chilled water return temperature, reducing the load of the chiller. Figure 3 below shows the AHU design diagram for a dehumidification system that uses CHW return for reheat. Figure 4 Psychrometric for dehumidification AHU with Chilled Water reheat.

Figure 3: AHU Design Diagram Using Chilled Water Return for Reheat

Figure 4: Psychrometric chart for Dehumidification AHU with Chilled Water Reheat

To best leverage this concept, there are a few changes needed to the design of the air handling unit:

First, the chilled water coil must be designed for a high chilled water temperature rise (expressed as “delta T”), so that the return chilled water temperature is high enough to provide useful reheat. The chilled water coil may have more rows of tubes, the fin spacing may be tighter, and the coil face area must be increased - or some combination of these features to optimize the coil delta T without increasing air resistance, which would increase fan horsepower.

Second, a chilled water heat recovery coil is positioned downstream from the cooling coil. Because reheat is being done with return chilled water, which is at a low temperature compared to a heating hot water system, the heat recovery coil will also be larger, deeper, and have more fins than a traditional hot water reheat coil. A space should be maintained in the unit between these two coils, with an access door for cleaning of the cooling coil and drain pan.

Chilled water leaving the chilled water coil is piped to the heat recovery coil, then back to the chilled water return line. A 3-way control valve is used to divert some or all of the chilled water leaving the cooling coil to either the heat recovery coil or the chilled water return.

Design considerations

This system requires a chilled-water-based cooling system. Other cooling systems such as DX cooling will not be compatible.

This system is only appropriate where there is a significant need for dehumidification, that is, where the dewpoint of the outside air is significantly above the design dewpoint of the space for a significant number of annual hours. This is a function of both the local climate and the design dewpoint of the space. Dehumidification may be needed more often in hot and humid climates, but it is also needed in milder climates for spaces such as operating rooms, especially for orthopedic operating rooms, pharmacy clean rooms, and other spaces where the the requirement for outside air is high and the indoor temperature is low. In mild climates, general hospital spaces such as exam rooms, patient rooms, and most clinical space, back of house, and administrative space will not need active dehumidification, so this system is not beneficial.

Chilled water reheat cannot provide all of the energy needed for reheat. This system is essentially an advanced form of supply air temperature reset, but with dehumidification and energy recovery. Terminal reheat will still be required, so the designer should focus on decarbonizing the heating system even if this technology is adopted.

This system can result in a higher chilled water delta T, which reduces chilled water pumping energy. This effect is most noticeable at times when the cooling load is such that less reheat is used, so that the return water temperature is higher. Some proponents of this technique advocate for chilled water delta T of 25- 30 degF, which can significantly reduce pumping costs compared to a more standard 12-15 degF. However, the designer of the system must balance the advantages of reduced pumping energy against higher fan energy, resulting from deeper and denser coils.

Coil face area must be increased to lower the face velocity through both coils to counter the air pressure drop that would otherwise be incurred by more rows and tighter fin spacing. This means higher cost for air handling units and more space required in mechanical rooms.

Dedicated Outside Air System (DOAS) units may be an appropriate place to consider this system, potentially in lieu of rotating energy wheels. Because outside air is the primary source of moisture in buildings, directly treating outside air, prior to mixing with return air, can be beneficial. See the section on DOAS.

This technology is a simple heat recovery system with a minimum amount of maintenance. No rotating energy recovery or desiccant wheel, no distributed refrigerant piping circuit at the unit so leaks and failure of these potentially high maintenance items are not applicable to this technology. Based on existing installation, very little maintenance is required besides changing out air filters on a predetermined schedule.

Control strategies

Control of the chilled water cooling coil and chilled water reheat coil must be coordinated to simultaneously provide dehumidification and minimize zone reheat.

A standard supply air temperature reset strategy minimizes cooling energy by raising the cooling coil leaving air temperature, but high space relative humidity overrides the reset strategy to remove moisture by reducing supply air temperature. In a traditional system, zone reheat would make up the difference. In this system, the strategy is to use the chilled water reheat coil in the reset sequence, raising the supply air temperature.

Refer to the section on Supply Air Temperature Reset within the Retrocommissioning section.

How does this decarbonize?

This technology can reduce the use of fossil fuel for reheat when dehumidification is required. Heat is provided by the chilled water system without the use of a gas fired boiler.

In a conventional system that relies on fossil fuel for reheat, the reheat energy becomes a cooling load at the air handling unit. With this system, most of the heat used for reheat comes from heat recovery and requires little to no additional energy for reheating purposes. This system effectively reduces both the cooling and heating loads in the building. Design for higher chilled water delta T reduces chilled water pump energy, resulting in a net reduction in electrical consumption as long as the gains are not offset by higher fan energy.

Implementation

Barriers: Culture

Lack of familiarity of this technology among engineers and owners. Engineering design has standardized on air handling units designed for 400-500 feet/minute face velocity. At such velocities, this system is impractical. Design teams will need to consider these systems early in the design process in order to align budgets and space requirements with the potential energy and carbon savings.

Strategy

Overcoming these barriers will require public awareness of installed system successes and third party verification of the results.

Financial analysis and business case

The following analysis should be considered as projected information for first cost, energy savings and maintenance cost. Simple payback information is projected and will need to be confirmed when the installation of the system is completed. The information provided is based on air handling unit replacement at the UCSD Thornton Hospital in La Jolla, California. This pilot project has received funding from the CEC.

For an energy-based equipment change out, the simple payback is estimated to be in the 5-15 year range. However it does depend on many variables such as site installation complexity, annual load profiles, ambient conditions, utility rate structure and base case system efficiency, design and operation. Based on the analysis for this project, this system offered the lowest first cost of the energy-recovery systems considered.

First cost and energy savings:

1. The hospital air handling unit replacement project consists of three air handling units. One serves the ORs and support spaces and the other two units serve general hospital areas.
2. Incremental installation cost: $538,000, less Southern California Edison incentives of $403,000. Net incremental cost = $135,000.
3. Annual energy cost savings: $120,000
4. Simple payback, excluding incentives: 4.5 years
5. Simple payback, with incentives, and 1.1 years

Maintenance cost

1. This technology is a simple heat recovery system with minimum amount of maintenance.
2. No rotating energy recovery or desiccant wheel, no distributed refrigerant piping circuit at the unit so leaks and failure of these potentially high-maintenance items are not applicable to this technology.
3. Based on existing installation, very little maintenance is required besides changing out air filters on a predetermined schedule.

Timken Museum in Balboa Park, San Diego, California uses a 12,300 cfm unit to provide HEPA-filtered air and tight control of temperature and relative humidity (RH). Construction was completed in May 2022.

Department of Defense in Fort Bragg, North Carolina uses a 10,000 cfm unit. This project has been successful in operation and reported to require very low maintenance. The Department of Defense reports energy savings of 57% to over 70% for the humidification and reheat process.