Air Conditioning - Mystery appliance

There's a white, rectangular box on the wall in my living room, but I'm not sure what it's supposed to do. It draws in air from outside and blows it out through a hole in the side. There's a dial on the side which allows you to regulate the airflow. There are valves - like you'd find inside a transistor radio - on the dial, so I doubt that it's just an ordinary fan. The guy from the estate agent said that most people just leave it on all the time and that he thought it helped prevent condensation. I have attached some camera phone pics. Apologies for the poor quality.

Can anyone tell me what it does?

http://download.yousendit.com/2447920D0390663C

http://download.yousendit.com/7514E456412076AE

http://download.yousendit.com/6450EBCA4CB66831

Never seen one like that. But if it is for condensation id remove it. Condensation in winter time can be removed with a bath fan or buy cracking a window.

It looks like an Air Exchanger. With a tight house or if you want to exchange the air without losing the heated or air conditioned air this is what would do it.

Looks like an air exchanger to bring fresh air into the home.. Have you taken the covers off to see inside? Some unit has an air filter that needs to washed.