A furnace should only have about 300-500W running the fans/oil pump at 120v - and with an induction motor, frequency should not be a problem, unless it's so low it causes motor overheating, [which is not immediate anyway]. It may be under or over voltage rather than frequency - many modern electronic burner controllers have voltage protection both sides of the norm and may also baulk at square waves or spikes. Her's the Danfoss spec -
http://heating.danfoss.com/PCMPDF/OBC_82-10_X009114_VDACY202.pdf
