Honeywell T4398B - Manual (Page 7)

T4398A,B PRECISION ELECTRIC HEATING THERMOSTAT
7 69-1604
Frost Protection (T4398A)
When the ambient temperature falls to 40°F (4°C), the vapor
in the diaphragm changes into liquid. The diaphragm collapse
to a fraction its original thickness, turning on the heat,
regardless of setpoint. This feature guarantees minimum, yet
reliable, operation of heaters in spaces near freezing.
NOTE: To prevent heat from turning on below 40°F (4°C),
turn off the circuit at the breaker panel, or install a
T4398B.