"lp13-30" wrote in message
...
If the polarity of the incoming power is reversed you will still have
120 volts between black and white and the furnace will do exactly as the
OP described. I have seen it happen before-- more than once. Make sure
the black is hot and the white is neutral. Around here, about 99+% of
furnaces are connected with a pigtail and receptacle, so the polarity
can be accidentally switched easily if anything is done anywhere in the
circuit. Most furnaces will run ok during a/c use that way-- it will not
show up until the heat is fired up. Larry

My furnace installation manual specifically warns about this possibility.
Larry's suggestion and method of testing are both valid.

Don Young