I've had a heat gun on the most accessible spot in the pipe for 4 hours now.... and this was AFTER getting up every two hours overnight to run a good amount of water through the pipes. I'm watching it closely in the hopes of actually spotting the likely fire!
Cut a hole in my kitchen ceiling (110 year old plaster lathe, God I wish I had sheetrock) and found the problem: The pipes to the upstairs tubs exist in a large "open" area between the kitchen ceiling and the back porch roof. No wonder the bathtub pipes are subject to freezing.
Trying to decide, (after I presumably get water moving again) whether the problem will best be fixed by cutting a hole in the porch roof to lower someone in or taking down half my kitchen ceiling.
Did I mention frozen pipes really, really suck?