Honeywell ER90 Owner's Manual page 7

FROST CONTROL
Some Home Ventilation System models have built-in automatic preheat
frost control systems that allow the unit to operate in cold weather regions.
Areas where the winter design temperature does not fall below 5° F [-15°
C] will not require a preheater. The function of the frost control is to ensure
that the intake air brought into the Home Ventilation System is not below
10° F [-12° C]. This ensures proper operation of the Home Ventilation
System. Operation is indicated by the indicator light located on the Home
Ventilation System cabinet next to the PRESS-TO-TEST button. The
indicator light is on when the outdoor temperature is below 10° F [-12° C]
or the TEST button is pressed. The preheat system is functioning if the
indicator light comes on when the TEST button is pressed. Test the
preheat system before the heating seasons starts each year. Follow the
instructions on the cabinet for testing.
IMPORTANT: Release the TEST button as soon as the neon light comes
on. Pressing the TEST button longer can cause overheating of the
Home Ventilation System.
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