The 1000W transformer is not as you describe.  It is in a steel enclosure and no heat-sink.  It is connected to a dedicated 20amp circuit breaker.  To reiterate, it has operated without any circuit breaker trips for about one year, the first one being about a month ago, then again last week.  It has not tripped since then.  Note that the breaker on the underside of the transformer has never tripped.  Could it still be possible that something has gradually changed inside the transformer?

The service panel breaker, not the one on the transformer.  Sorry that I was not clear on that.

My Warmfloor has been installed in our single faminly house for about one year.  It was installed by a local contractor who worked with the local Warmfloor representative in Brookfield, WI.  Included is a 1000W transformer and a Microreg controller.  We have used the system in 2007 during January-March and then again since the end of November.  It has been working very well and we are quite pleased with it.  However, in the past month, there have been two times that the circuit breaker has tripped.  Once I reset the circuit breaker, the Warmfloor operates again.  The time between the first breaker trip and the second one was about 3 weeks.  The Warmfloor installation is on a dedicated breaker (i.e nothing else is connected to this circuit).  If this continues or gets worse, I'm thinking of replacing the Microreg switch myself, before calling in an electrician to check/replace the breaker.  I know of no abnormal load on our electrical system that might have caused a surge, unless it was caused by the power entering the house.