Problem turned out (embarassingly) to be dead backup batteries in thermostat unit, I had not realized there WERE batteries since the thermostat is hard wired -- technician handled that extremely diplomatically, I do wish, however, that the person answering the phone, to whom the telltale symptom had been described, had asked "have you checked the batteries" which would have relieved anxiety. The technician also performed a quick diagnostic on the furnace itself, providing useful recommendation.
