Had my cambelt and pump done.

Drove away with climatronic off.

In traffic, air vents chucking out very hot air.

Went straight back.

Garage say they set climatronic to hot as part of cambelt/pump/coolant change and that its a VW thing.

Sure enough we turn on and set climatronic to 21. Turn off and vents are fine.

How does this work?

I thought if its off, its off!

Not necessarily; AIUI it's software controls, rather than mechanical/electrical linkages that drive Climatronic. So it could be a maintenance setting.

Not over-impressed with the garage that didn't tell you this when you first collected the car though.

Surely it's to allow coolant through the heater matrix for venting.