Quote Originally Posted by ctrlfreq View Post
Yeah, but it would have only shut off the backup unit if they were wired in parallel (leaving the primary unit to over-heat the tape). If they are wired in series, then the failure of either unit (ie. one dies and goes dark), would cause the rack not to heat at all.
If the backup comes off the flexxwatt it will ready 78 degrees or whatever your room temp is at, so it won't shut off. If the primary comes off it will turn the heat tape on full bore which should tell the backup to kick in and cut power. Yes, wire them in series.

If they both come off then you are in the same boat is if just one came off, but taping them separately should result in this happening with almost 0 probability.

JonV