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Marvair ModPac Installation & Operation Manual
11/2017 Rev.8
4.4 Electric Heat Controls
Figure 6. Typical Configuration for Single Element Heater
The electric heater assembly can have up to three individual heating elements. Each individual heating
element is protected against overheating by its own dual function thermal cut-out switch. Additionally,
a separate single function thermal cut-out switch protects the entire heater assembly.
The dual function thermal cut-out switch (P/N 70006) is composed of two independent line voltage
snap-disc temperature switches mounted in a single enclosure. One of these switches is an automatic
reset device which cycles off at approximately 145°F and back on at approximately 115°F. Should this
switch fail to open, the second switch will open the circuit if the temperature continues to increase.
This second switch does not reset. If it opens (breaks the line voltage circuit to the heater assembly) the
switch will have to be replaced by qualified service personnel after the source of the overheat problem
is resolved.
In addition to the thermal cut-out switch described above, there is a single function thermal cut-out
switch (P/N 70005) mounted on the heater frame. This switch controls the 24V AC control current
to the heater contactor(s) which powers all the heating elements. This single function thermal cut-out
switch operates totally independent of the dual thermal cut-out switch described above. If the single
function switch senses an overheat situation, it opens the control circuit and turns off all of the installed
heating elements via the heater contactor(s). Because this switch controls the heater contactor(s), only
one switch is required to disconnect power from the contactor(s), regardless of the number of heater
elements. This single function switch is also non-resettable, and must be replaced by qualified service
personnel after the source of the overheat problem is resolved. This switch would typically open if both
elements of the dual thermal cut-out switches failed.
