9
Chilled-Water Reset Function
Typically, the supply chilled-water temperature is held constant between 38
o
F and 44
o
F, which is
acceptable for full-load or near full-load operation. Chillers, however, usually operate at part-
load conditions for significant periods of time. Resetting the supply chilled-water temperature to
match the actual building-cooling load leads to energy savings. Generally there is a 2%
efficiency increase for each degree raised. The supply temperature can be reset based on the
outdoor-air temperature, i.e., the supply temperature can be increased as the outdoor-air
temperature decreases. Alternately, the optimum supply chilled water temperature can be
computed based on the current conditions of the valves in the air-handling units (AHUs) and the
supply-air temperature. The implementation details are provided in the 2003 ASHRAE
Handbook for HVAC Applications (Chapter 41)
1
. In addition to decreasing chiller energy use,
resetting the supply chilled-water temperature will usually reduce the amount of energy used for
reheat for a multi-zone environment. In humid climates, the supply chilled-water temperature
should not be raised significantly or should be interlocked with humidity control. Although the
actual upper bound for the chilled-water should be based on the latent cooling capacity
requirements and other criteria outlined in the ASHRAE Handbook, a general rule-of-thumb in
humid climates (e.g., mid-west and southern U.S.) is approximately 50
o
F. In dry climates (e.g.,
western U.S.) the temperature can be slightly higher (55
o
F).
While using reset control strategies, the use of proper sampling intervals is critical to avoiding
oscillation and instability in control. The general rule of thumb is that the sampling interval
between reset commands should be greater than the settling time of the control loop. If you have
cascading resets (i.e. both the supply chilled-water and supply-air temperatures are being reset),
the sampling time between reset commands should be greater than the settling time of the
slowest control loop. Savings from resetting the supply chilled-water temperature can range
from 5% to 20%, as per the 2003 Handbook for HVAC Applications, Chapter 41, page 41.7.
Recommendation: Because resetting (increasing) chilled-water temperatures during part-load
conditions can save cooling energy use, this function is recommended.
Hot-Water Reset Function
Like supply chilled-water temperature, the supply hot-water temperature can be reset to reduce
energy consumption when the boiler is operating at a part-load condition. Besides reducing the
energy used to meet the load, another benefit from resetting the hot-water temperature is a
reduction in the standby losses of the boiler and piping systems. To eliminate boiler damage
from acid formation, a minimum reset temperature should be implemented. As stated in the
ASHRAE, 2003 Handbook of HVAC Application, Chapter 41, page 41.5, “To minimize the
condensation of flue gases and consequent damage to the boiler from acid formation, however,
the hot-water temperature should not be reset below the minimum temperatures recommended by
the boiler manufacturer (typically 140
o
F). Similarly, energy can be saved in steam heating
systems by maintaining supply pressure at the minimum level required to satisfy the largest
heating load”.
1
2003 ASHRAE Handbook of HVAC Application, American Society of Heating Refrigerating and Air-
Conditioning Engineers, Inc., Atlanta, Georgia, Chapter 41 p. 41.1 – 41.39.