Cold wheather warm-up

Hi.
Recently my car ('91 Civic 1.6, 270K km) is having trouble warming up. Where
I live the temperature is now below freezing point, but nothing extreme. Hot
air comes out of the vents in less than 5 minutes, but the engine temp gauge
needs more than 20 minutes of driving to reach the middle, i.e. normal
engine operating temperature.

I realize that engine will warm up much slower during winter, but 20
minutes? Last winter it was also warming up kinda sluggish, but not this
badly. During summer the car usually takes 3 or so minutes to warm up.

What do you think the problem could be? My mechanic tells me it's probably
the thermostat, but don't bad thermostats usually cause engine to overheat?
Also, can one do permanent damage to the engine driving this way? Should I
change the thermostat ASAP?

Thanks.

> It's the thermostat. Very common problem, and cheap to fix.

Woohoo, that was it. Now it warms up about 4 times faster LOL. Thanks.