My furnace was a coal burner converted to gas. Recently the thermostat went out - it is a Honeywell Powerpile 75V DC with a mercury switch. I have tried to replace it with several systems that say they work with a 750 millivolt but none have worked. The only way to get the furnace to turn on and off is to attach the two wires to each other and the unattach them. At this point I only have "on" & "off" and no auto. Can anyone suggest a thermostat that will actually work?