Hvac engineers are starting to realize that nearly anything can be specified into a custom makeup air unit. For example, packaged makeup air dehumidification units with reverse cycle heat pump (RCHP) and enthalpy wheel options were specified recently for a hvac retrofit at the Walker Upper Elementary School (Charlottesville, VA). The packaged units were combined with a closed- loop, water source heat pump concept the 33-year-old school employed to replace its existing two-pipe unit ventilator system.

A water cooled, energy-recovery makeup air unit for a/c isn't exactly a new concept in schools. However, adding an RCHP to eliminate a separate hot water heating system, while also including an enthalpy wheel for energy recovery including dehumidification (summer) and humidification (winter), is unique for a school application.

The consulting engineering firm, Hanover Engineers (Mechanicsville, VA) specified the Dectron Internationale (Roswell, GA) Dry-O-Tron(r) RK-Series of energy-recovery makeup air dehumidifiers with the custom additions. The auditorium and gymnasium use two units each, and the cafeteria uses one for both heating and cooling as controlled by a Novar Controls Corp. (Barberton, OH) building automation system.

"This (replacing unit ventilators with heat pumps and using makeup air dehumidifiers with RCHP and enthalpy wheels) appears to be a good, sound approach that has significantly improved the school's indoor air quality as well as temperature control," says Allen Lambert, vice president of mechanical engineering for Hanover.

Echoing Lambert is Marv Reese, the school district's former facilities engineer who now advises the City of Charlottesville as a consultant for the construction management firm SPN Inc. (Rockville, MD). "I feel this type of system is pretty efficient and there's a good chance the city will use it again in the upcoming retrofit of Charlottesville High School," Reese adds.

RCHP Provides Considerable Savings

The most significant savings in the $3.4 million hvac retrofit, which was overseen by Charlottesville-based VMDO Architects PC, and managed by general contractor, Martin Horn Inc., came from Lambert's specification of Dectron's RCHP option in the makeup air units. The RCHP eliminated the need for new conventional boilers and other heating equipment for wintertime operation. Most of the school's existing heating equipment was outdated and would have needed replacing if the RCHP option wasn't specified. However, mechanical contractor, L.A. Lacy (Charlottesville, VA), did recondition a five-year-old boiler that now serves as the source of heat for the heat pump loop.

The outdoor air dehumidifier delivers dehumidified air to the space at a neutral dewpoint and temperature. This is accomplished by dehumidifying the air down to a 55 degrees F to 58 degrees dewpoint temperature and then using a hot gas reheat coil to reheat the air to 70 degrees for classroom entry.

When air temperatures are below approximately 60 degrees, the energy recovery wheel preheats the air and the Dectron units operate in a reverse cycle, heat pump mode. Brian Cooper, a partner at the manufacturer's representative firm, Robert W. Hayes Co. (Richmond, VA), worked as a liaison between Lambert's specifications and Dectron's custom engineering department. "Because of the relatively heavy density of people and lighting load, schools often air condition when outdoor air temperatures drop to as low as 40 degrees," explains Cooper.

"Between 40 degrees and 70 degrees, classroom heat pumps are typically in the air conditioning mode, rejecting heat into the condenser loop. At the same time, the Dectron makeup air units operate in the heating mode, absorb heat from the condenser loop and warm the outside air up to 70 degrees," Cooper adds. "This creates a degree of balance between the terminal heat pumps and the makeup air units, which is ideal for optimizing efficiency in a water source heat pump system."

"The beauty of this type of system vs. a simple unit ventilator system is each space has better temperature control," adds Fred Huckstep, a Lacy staff engineer. Besides the RCHP, another cost- saving option is the enthalpy wheel, which eliminates the need for humidifiers in winter operation. The enthalpy wheel collects return air humidity produced by room inhabitants and redistributes approximately 70% of it through the supply air to raise seasonably low relative humidity (rh) levels to ASHRAE standards.

In summertime operation, the enthalpy wheel preconditions high outdoor rh levels and reduces the mechanical dehumidification load. "The enthalpy wheel not only reduces the dehumidification load by over 50%, it also eliminates the need for humidifying equipment and does this while requiring less maintenance," adds Cooper.

Most classrooms use seven unmodified RK-Series makeup air units that are combined with each space's own Trane Co. (Tyler, TX) heat pump to provide heating and cooling. Hanover saved the school thousands of dollars by reusing the existing two-pipe system trunk and branch lines for classroom heat pumps, which are a mixture of direct replacement console units, some ceiling-mounted units, and a few large mechanical room air handlers.

Other Savings

Lambert's design saves money in other ways, too. Water cooled equipment is more efficient than its air cooled counterpart because of the constant heat sink provided by a closed water loop. Air-cooled equipment's efficiency however, drops as fluctuating outdoor temperatures drop. The choice of water cooled equipment also offers a longer life expectancy because compressors operate at lower head pressures, according to Cooper.

The units also offer a refrigerant subcooling coil that precools liquid refrigerant with 58 degrees air from the evaporator coil. This increases the refrigerant capacity, which subsequently improves the unit's efficiency.

Hanover also saved the project additional money by renovating and reusing existing equipment such as the on-site cooling tower by Marley Cooling Tower Co. (Overland Park, KS). The alternative to Lambert's chosen retrofit method is retaining the original unit ventilator strategy, but using new equipment that supports ASHRAE's recommended 15 cfm/person of outside air standard for schools.

However, dehumidifying 15 cfm can be problematic for simple a/c equipment in hot, humid areas such as Virginia. Using dehumidifiers combined with heat pumps is more cost efficient and allowed Lambert to fulfill the school board's request of bringing in 20 cfm/person as a guarantee against IAQ problems associated with schools. "Indoor air quality in schools across the country has many times been suspect in sick buildings, infectious virus spreading, or disease outbreaks, so the extra 5 cfm is a safeguard against that," says Lambert.

The City of Charlottesville school district has already approved a similar hvac retrofit approach for its Buford Middle School. "The first phase that's on-line at Walker right now seems to be working fine," added Lambert. "This is good concept that doesn't fit every application, but it's certainly a good, sound approach that might become more popular in future school hvac design." ES