Electric Water Heater Controls
Snap-Action Bimetal Discs in Both Controls
The 59T and 66T series Bimetal Thermostat Temperature Controls are designed to meet the high electrical capacity requirements of
electric water heaters. Both use a temperature sensitive bimetal disc to deliver snap-action to the
contacts. The speed and force of contact separation provides long-dependable control life at high
Thermal Response of the 59T and 66T
The bimetal discs of the 59T and 66T are in intimate thermal contact with the backplates of the
controls. Properly mounted, the 59T and 66T series controls provide good thermal response on
storage tank water heaters.
The 59T is available in two switch actions:
Automatic reset SPST – The switch opens the normally closed contacts on temperature rise. The
contacts automatically return to the closed position when temperatures return to the reset point.
Automatic reset SPDT – This operation is the same as the SPST with the addition of an auxiliary
set of contacts, which make circuit upon opening of the normally closed contacts and breaks this
circuit upon automatic reset.
The 66T is a manual reset DPST – The switch opens two sets of normally closed contacts on temperature rise to provide full power disconnect of both conductors. The contacts will reset to the closed
position when the reset button is depressed before the control has cooled to 90°F. Once opened,
until the reset button is pressed, the contacts will not automatically reset at control temperatures
above 32°F (0°C).
The construction is classified as “M2 Trip Free” by the approval agencies. This patent pending
design holds the contacts open in the event the reset button is held in the depressed position in
an attempt to defeat the manual reset function of the thermostat.
The thermostats and wiring diagram (shown on page 31) are typical of a residential electrical storage water heater installation using two heating elements for non-simultaneous operation. The
upper electrical control is a combination 59T SPDT regulating thermostat and a 66T DPST manual
reset limiting thermostat. The lower electrical control is a 59T SPST thermostat. All the thermostats sense the surface temperature of the water heater tank.
The 66T function is to provide full power disconnect (through both power conductors) in the
event of an overheat condition. The 59T SPDT normally closed contacts route power to the upper
heating element. When the 59T SPDT control reaches its set temperature, its switch action breaks
power to the upper heating element and switches power to the lower heating element through
the normally closed contacts of the 59T SPST. When its set point is satisfied, the switch action
breaks power to the lower heating element.