For cold weather operations, engine preheat is always required. Refer to the engine manufacturer’s recommendation and the POH/AFM specific to your aircraft.
“Preheating is required whenever the engine has been exposed to temperatures at or below 20F/-7C (wind chill factor) for 2 hours or more.” Continental Service Information Letter No. SIL 03-1.
“The use of preheat will ease startup during cold weather and is required when the engine has been allowed to drop to temps below -10F/-12C” Lycoming Service Instruction No. 1505.
Many of our customers preheat when the outside temperature drops below 50F/10C.
Preheat may be used at any temperature to reduce run-up times and starting wear but Tanis does not recommend preheating when the ambient temperatures exceed 104F/40C.