The heater should work from about 40deg on up. It will do real good up around 80. The problem with cars with poor thermostats is that not only do they not get to proper temp, they heat up slowly which makes heaters take 3-4 or more times longer to be effective. (I must say I have a poor experience wth heaters as I never see weather lower than 25deg F.).

I would also say that the car will not like running cooler than it is supposed to. Newer cars will set fault codes for not reaching operating temp within a certain time.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician