SL 320 Heater question

[Old Merc]

My SL 320 (1997 straight six)The cooling system,heater and air con works perfectly all year,except on the odd hard frosty morning?? I pop out,start it up,everything on max and leave it running to thaw things out.Ten minutes later,the engine is getting warmed up but the heater is still blowing stone cold? I then give it a few good revs and suddenly the heater starts blowing hot as if a switch has been turned.Advice please lads.

[Tobeman]

I can't imagine they're related, as my C63 is some 11 years newer, but I'm having a very similar issue right now: http://www.pistonheads.com/gassing/topic.asp?h=0&a...