Update May 20, 2014
We have finally received a reply from Caframo. They have offered to replace one of the fans but did not address the question of why they might smoke and overheat even if not seized. One of the fans did slow down a bit but the other seemed to run at normal speed. Here is the reply...
Dear Mr. Baier,
I am sorry for the bad experience you had with the Caframo model 747 fans. Please be assured that we take these issues seriously.
With your fans we only us special Fire Retardant grade plastic for the housing so that should the motor seize there is not risk of fire. However, with older units, should a motor seize heat can get sufficient to cause some smoke and smell.
Our new fans have added thermal protection so that should the blade get jammed or the motor seize current will be cut off to the unit ensuring all components keep at low temperatures. We will gladly send you a new unit at no charge. Please confirm your postal address and I will arrange for a unit to be sent to you.
Sincerely, Mike Tettenborn