A few days ago the heat would not kick on so we turned it off and on at the thermostat a few times and it finally kicked on and worked fine for about a day. Then the same thing happened again. We then did the same thing by turning the thermostat up and down until it finally kicked on. The next day it happened again, so we changed the mercury bulb thermostat to a new digital one. It worked fine for one day, now we are without heat again, and this time it hasn't came back on. The digital thermostat is blinking "heat" which means that it is calling for heat but is getting no response. The fan workes when we switch to fan. And when the heat was on, it was working fine. It's like somewhere between the thermostat and the furnace the signal is not getting through. Any ideas please?