TECH NOTES
Winter 2013
by Tom Trumbull
Promoting Healthy and Comfortable Indoor
Environments Through Precise Temperature and
Humidity Controls
Irregular ambient temperature and humidity are the bane of any building for which
air quality is critical to occupant comfort. Densely populated or special-use facilities
such as office buildings, schools, libraries, museums and healthcare facilities require
temperature and humidity to remain within narrow bands. Excessive temperature
swings and too much variation in humidity can trigger indoor air-quality problems
such as mold growth, condensation, increased sickness and allergic reactions. Such
oscillations also can be nettlesome to research laboratories that rely on precise air
quality characteristics for conducting experiments.
In this issue of Tech Notes, we discuss how innovation in modulating compressors and
modulating temperature and humidity control can be a cost-effective enhancement to a
packaged rooftop unit.
The Fluctuation Problem
For many years, on-off compressors have been the standard used to control temperature
and humidity levels in buildings. Temperature and humidity fluctuations—common
in buildings that use an on-off compressor and on-off, hot-gas reheat air conditioning
equipment—can cause significant discomfort for tenants. For example, when the
compressor turns on, it operates at full capacity, providing cool air to the space until the
temperature drifts down toward, and then through, the setpoint. When it reaches the
lower limit of the dead band, the unit turns off. After some interval, the
AAON’s Digital Precise sensible load in the space causes the ambient temperature to drift up and
Air Control (D-PAC) past the upper dead band, at which point the compressor will turn on
modulating system features and the process will repeat. The temperature band can swing by 5° F to
15° F over a daily cycle, typically causing rapid compressor cycling.
a modulating compressor
and modulating reheat
system that are far better
at controlling temperature
and humidity levels than
traditional on-off, hot-gas
bypass HVAC packages.
H VA C S O L U T I O N S
Perhaps more of a contributing factor to unhealthy buildings than
temperature fluctuation is excess humidity. The most common technique
to dehumidify a building is to use a fan to draw air from the space over
the cooling coil of the unit to condense the moisture. (This moisture
condensing from the air is called latent cooling; the temperature
change of the air is known as sensible cooling.) To increase the amount
of dehumidification, the air temperature must also be decreased,
sometimes past the desired setpoint. As a result, relative humidity (RH)
can rise beyond a reasonable level.
In the past, design engineers were forced to use chilled and hot water, with infinitely
modulating control valves, to maintain constant supply-air temperatures and relative
humidity. Those systems are considerably more costly than a packaged rooftop.
However, a traditional rooftop system, with on-off compressor cycling, hot-gas bypass,
and on-off (non-modulating) hot-gas reheat, do not offer the required temperature and
relative humidity tolerances required in today’s sophisticated HVAC market.
Essentially, the problem with on-off approaches is that no single solution will control
the humidity at all conditions. Humidity fluctuations can move independently of
temperature fluctuations, as the on-off compressor and reheat equipment cycle on and
off without regard to ambient conditions and space loads.
AAON packaged rooftop systems with modulating controls
In our experience, the best solution for improving indoor air quality and occupant
comfort uses modulating capacity control to achieve energy-efficient, load-matching
temperature control.
The major difference between traditional on-off, hot-gas bypass HVAC packages and
the AAON Digital Precise Air Control System (D-PAC) modulating system is that the
modulating compressor and modulating reheat system is far better at controlling
temperature and humidity levels, particularly in densely populated buildings. Improved
temperature control is accomplished by varying the amount of reheat applied to the
dehumidified air. Because AAON’s Digital Scroll™ compressor can adjust its capacity to
match the required load of sensible or latent cooling, the compressor is not required to
cycle on and off as a standard compressor would to match the load. The combination
of the modulating hot-gas reheat and the modulating compressor components can
accurately control the temperature to within +/- 1° F and relative humidity to +/- 5
percent.
Figure 1: AAON return air bypass feature improves airflow
The AAON D-PAC control
systems combines an
optimizing Digital Scroll
Compressor, return air
bypass, and modulating
hot gas reheat installed
downstream of the
evaporator coil to control
the space temperature
and space humidity.
In addition to stable temperature and humidity values, the AAON system also provides
significant energy savings, since the part-load efficiency of a modulating compressor
can be up to 25 percent more efficient than a standard compressor with on-off hot-gas
bypass. This is because in a hot-gas bypass system, the bypassed vapor does no useful
cooling and increases compressor energy usage. The load created by diverting a portion
of hot gas away from the condenser and to the cooling coil requires a longer run time of
the compressor, at full load and thus uses a lot more energy. (see Figure 2).
H VA C S O L U T I O N S
Because the
hot-gas bypass unit
always operates
at full power, it
uses significantly
more energy than
a modulating
compressor.
Figure 2: Modulating compressor energy savings
vs. hot-gas bypass energy usage
Figure 3: AAON D-PAC vs. typical packaged rooftop system
Conditions: Boston, MA
On-Off
Compressor
AAON
D-PAC
EER Total Base Unit (Btu/W-hr)
11.40
11.40
Energy Base Unit (kWh)
VCC Unit
Energy Difference
Annual Electrical Cost ($0.15/kWh)
Percent Energy Cost Savings
77,946
$11,692
39,796
38,150 kWh
$5,970
48.9%
Source: AAON Inc. Values shown are estimates, actual performance may differ due to ambient conditions, load, air flow, and control methods.
Figure 3 illustrates the annual savings of an On-Off Compressor vs. a Variable-Capacity
Compressor (VCC). The AAON D-PAC packaged rooftop system saves approximately
38,000 kWh in energy which, given prevailing electrical costs in the Boston area of
$0.15/kWh, equates to approximately $5,700 per year in cost savings, and roughly a
three-year payback in equipment costs with no electricity rebate. Massachusetts’ utility
companies typically buy the payback period down to achieve an approximate one-year
payback for commercial customers. In addition, there are operating-life benefits that
come from not having to start up the motor as frequently, as the initial inrush current
leads to increased stress on the motor. By stopping and starting less, the motor of the
modulating compressor may provide longer useful life and reduced maintenance costs.
For Additional Information
To learn how AAON D-PAC with modulating compressor and energy recovery wheel can
achieve precise temperature and humidity control and reduce energy costs, please contact:
Trumbull Campbell Associates
39 State Street, Newburyport, MA 01950
978.225.2045 p
978.225.2044 f
trumbullcampbell.com
H VA C S O L U T I O N S