I have a GW Fuelsaver 25-30 in a fully pumped system. The pump is running, valves are open, pilot light is lit, boiler stat is set but the main burner won't fire up. I have wired out the thermocouples for test purposes and there is 18v ac at the control valve. Is this too low as I believe the valve is rated 24v? Sometimes there is a rapid clicking from the solenoid. Any ideas please?