I have a 3 zone hot water baserboard furnace that ocasionly wont turn on. If I turn the AC power off and then quickly back on it works, for a few more days. When turning the thermostat off and on I hear the circulating pump turn on, but no flame. Pilot is ok. When I reset the powere the circulating pump and the furnace gas fires up. What voltage should I have ? any other ideas to check ?