Chapter 6.3.2 - Snap-Action Bimetal And Electronic Line-Voltage Thermostats

Snap-action thermostats are named for the familiar click that is made by the opening
and closing of their electrical contacts due to a permanent magnet that
accelerates final circuit closure and ensures chatter-free electrical contact closure.
Snap-action thermostats were used for almost all electric resistance heat installations,
which were dominant before the advent of the heat pump.A bimetal temper-
ature-sensing plate is adjusted, or calibrated, by means of a setscrew, which is
adjusted so that the contacts are open at the thermostat set point temperature that
matches the actual air temperature reflected by the position of the bimetal plate
(Fig. 3.2). Bimetal thermostats have a long life, although their accuracy usually
declines over time as the bimetal sensor oxidizes, corrodes, gets dusty, and the
springs and contacts become corroded, pitted, and otherwise worn.

The heating system operates in response to current flow through contacts, which
either close (make contact) to enable the flow of current when heat is needed, or
break (open) to stop current flow when the load is satisfied. The sensitivity of the
bimetal sensor to the surrounding air determines how often the contacts open and
close to maintain thermostat set point. It is important to know the contacts’ capability
in relation to current flow so that the correct thermostat is installed. Some thermostats
require a minimum load, or amperage draw, to activate. Others are damaged
by the current surge from a fan, motor, or any inductive load and may only be used
for control of electric resistive loads. Manufacturers’ instructions will detail the ability
of the control to handle resistive and/or inductive loads, and the minimum and
maximum amperage draws for each.

A bimetal line-voltage thermostat is usually designed for use on all common line
voltages. Electronic thermostats are usually voltage specific and will only operate
on the specified voltage. However, some electronic models are designed to operate
on either 120 or 240 V or 208 and 240 V. Bimetal thermostat design includes twocircuit
models that can handle 5000 W in total. Some electronic thermostats can
handle as much as 4800 W on a single circuit. Manufacturers’ specifications will
detail the control voltage, amperage, and wattage capability. As technology advances
are extended to all types of controls, the features and capabilities will continue
to broaden.

Line-voltage controls that are double-pole four-wire controls have the capability
to break both electrical lines, thus providing a “positive” off. Some building codes
require double-pole thermostats. The use of double-pole thermostats is advised for
safety and equipment protection during periods of system shutdown (Fig. 3.3).

Single-pole thermostats are two-wire controls that only break one line.They may
be used on 240-V systems but are most commonly used for 120-V systems to reduce
wiring or deal with an existing single electrical feed. Many electronic line-voltage
controls are only available as single-pole models.

Line-voltage controls have historically depended on temperature-sensing accuracy
in relation to actual room dry-bulb air temperature. In fact, the term droop
was introduced to characterize the deviation from set point experienced by line-
voltage thermostats whose construction and electrical load created heat that
impacted temperature-sensing and operational accuracy (Fig. 3.4). Bimetal line-
voltage thermostats were normally calibrated for droop that occurred between the
20 and 80 percent duty cycle band, to improve performance at maximum load—
normally 16 A. The resulting line resistance heat at maximum load would increase the
temperature of the bimetal sensor by 3 to 7°F or more. As a result, the contact would
open when the actual-space dry-bulb air temperature was correspondingly below set
point. Occupants would be uncomfortable. They would raise the set point to compensate.

In an effort to prevent premature heat shutoff from occurring, the calibration was
offset by the same amount.The opposite problem would occur if the installed heater
load fell below the maximum load by 50 percent to 8 A, because the thermostat
would overshoot the set point due to the maximum load calibration offset. Inaccuracy
would also occur during periods when heat cycles were very short, such as those
that commonly occur during the spring and fall “shoulder” season. Technology and
thermostat redesign has produced models with significantly reduced droop as the
line heat influence has been all but eliminated on newer bimetallic control designs.

A similar problem existed in electronic thermostats due to internal triac or triac–
relay component heat generation. This was often compounded by circuit design
location without regard to sensor impact. Some components in early models also
generated heat due to constant 3- to 5-W current draw that not only generated
temperature-distorting heat but also consumed considerable annual energy, which
was frequently unaccounted for in the energy-saving justification for installation
conversion. As these problems were detected, solutions were developed to
make heat generation and standby current consumption nonissues. However, each
electronic thermostat and control scheme should be evaluated to be sure that
temperature-sensing accuracy and control current consumption are in line with
design parameters.

Snap-action thermostats are named for the familiar click that is made by the opening
and closing of their electrical contacts due to a permanent magnet that
accelerates final circuit closure and ensures chatter-free electrical contact closure.
Snap-action thermostats were used for almost all electric resistance heat installations,
which were dominant before the advent of the heat pump.A bimetal temper-
ature-sensing plate is adjusted, or calibrated, by means of a setscrew, which is
adjusted so that the contacts are open at the thermostat set point temperature that
matches the actual air temperature reflected by the position of the bimetal plate
(Fig. 3.2). Bimetal thermostats have a long life, although their accuracy usually
declines over time as the bimetal sensor oxidizes, corrodes, gets dusty, and the
springs and contacts become corroded, pitted, and otherwise worn.

The heating system operates in response to current flow through contacts, which
either close (make contact) to enable the flow of current when heat is needed, or
break (open) to stop current flow when the load is satisfied. The sensitivity of the
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