Much has been written about the energy impacts of control selection and operation.
A survey in Contracting Business found that more than 50 percent of homeowners
were only “somewhat” or “not at all” satisfied with their present comfort systems.
They listed energy savings and comfort as the two most important factors in purchasing
a new home comfort system (Fig. 3.5).
Energy savings potential ranges to 25 percent, but the bulk of the savings are
more related to energy management than to actual control operation (Fig. 3.6). In
fact, the magnitude of savings depends more on how the control is used, the nature
of the heating system design and management, and control features than the actual
operation of the control itself. And, of course, the chart does not reflect the savings
potential of control to operative temperature (OT). However, there are certain basic
thermostat operational characteristics that do impact how closely the thermostat
maintains the temperature set point.
For the sake of this discussion, we are talking about the use of the control in a
convection heating environment where dry-bulb air temperature and mean radiant
temperature (MRT) are equal.The only factor being discussed is thermostat performance.
Earlier discussions defined droop and its effect on set point accuracy.
Although this is very important in heating climates, this may be a relatively insignificant
factor in mild climates, where light loads are being controlled.Where the “on”
cycle is brief, the time period does not result in an internal thermostat heat buildup.
For most electronic thermostats, droop is likely to be between 1 and 2°F at maximum
load if the control is properly designed.
Thermostat cycles occur when the control turns heat “on” or “off.” Bimetallic
thermostat cycle frequency is usually between two and five cycles per hour in the
most common operating range for the heating season (Fig. 3.7). By comparison, electronic
thermostat cycles are frequent and are measured in minutes or even fractions
of a second. Some electronic thermostats have almost instant temperature sampling
that is averaged over the time period selected for cycle frequency. Frequent cycling
is an obvious advantage to maintaining set point (Fig. 3.8).
Differential, normally measured at 50 percent duty cycle, is the difference that a
control tolerates between cycles. Differential is inversely proportional to the cycle
rate. The more frequent the cycle, the closer to set point the dry-bulb air tempera-
ture is maintained. This temperature swing, deadband, or hysteresis is the Achilles’
heel of the bimetal thermostat, because bimetal by its nature is a relatively slow
sensing instrument compared with electronic thermistors controlled by logic and
switching component (Fig. 3.9). One way to measure the deadband on a bimetal
thermostat is to listen for the “on” and “off” click and note the number of degrees
difference or dial space between clicks. Although the best bimetal thermostats have
a 1 to 2°F differential, most old, and some new, units have differentials as large as
from 5 to 7°F, and even larger. Although calibration could reduce the differential in
some units, not all units have this feature.
It should also be noted that the term deadband is also used to define the temperature
band within which a thermostat used for heating and cooling is designed to call
for neither heating nor cooling. The premise is based on the assumption that occupants
will be comfortable within the temperature band applied to the particular circumstances
of the design involved without the use of either the heating or cooling
system. Such a designed energy conservation deadband is not to be confused with a
nonperforming temperature band that results in temperature set point differential
referred to in the preceding paragraph.
Perhaps the most widely perceived test of a thermostat is its accuracy or ability to
maintain average room temperature in relation to set point. It is when you try to discuss
this point that you see how closely intertwined droop, cycling, differential, accuracy,
and yes,MRT are in determining just what the cause of temperature deviation
from set point actually is in real-world applications. And to further complicate matters,
the role of the individual is just that, individual action, which impacts the use
and operation of the thermostat.
A thorough report on performance comparison of thermostat replacement
programs concluded, “. . . energy savings achieved . . . depend greatly on how the
occupants use the products, as well as the performance of the products themselves
. . . and potentially reduce heating energy consumption by about 7 to 10 percent”
(“Advanced Line-Voltage Thermostats for Electric Resistance Heating,” by
J. Gregerson, E-Source, January 1997).
Much has been written about the energy impacts of control selection and operation.
A survey in Contracting Business found that more than 50 percent of homeowners
were only “somewhat” or “not at all” satisfied with their present comfort systems.
They listed energy savings and comfort as the two most important factors in purchasing
a new home comfort system (Fig. 3.5).
Energy savings potential ranges to 25 percent, but the bulk of the savings are
more related to energy management than to actual control operation (Fig. 3.6). In
fact, the magnitude of savings depends more on how the control is used, the nature
of the heating system design and management, and control features than the actual
operation of the control itself. And, of course, the chart does not reflect the savings
potential of control to operative temperature (OT). However, there are certain basic
thermostat operational characteristics that do impact how closely the thermostat
maintains the temperature set point.
For the sake of this discussion, we are talking about the use of the control in...
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