19981007 Ghost in the machine

We recently returned home from a trip to find our new heater/ac system malfunctioning. The heater was turning itself part way on over and over again, overheating the ignition assembly until glowing orange—giving us cause to be thankful that the house was still standing. I immediately unplugged the infernal thing and gave the installing contractor what-for.

He tested everything, then declared that the behavior I described made no sense at all given the multiple system fail-safes—but he knew from previous conversations that I have electrical background and am not given to hysteria—so he believed my story, replacing the furnace board just in case. This did, in fact, fix the ignition cycle.

Unfortunately, there remained a bizarre behavior: after the thermostat main switches were in the cool position for a few days, the heater cycled in an intermittent, erratic oscillation at night. Of course, it would not happen when the contractor who installed it was present, and in fact the problem disappeared again whenever I fiddled with the thermostat settings or whipped out the multi-meter (or even just threatened to). This continued for weeks.

This morning at 3:48 when the heater turned itself on again, I leapt out of bed, determined to find our ghost. After camping out on various zone controller and thermostat wires for more than an hour, I finally caught the thermostat in the act of asking for heat despite that all of its settings should make that impossible. HA!