I have a 1974 Hydrotherm hot water boiler Model HC85 that uses a millivolt gas valve and an Auqastat to control the circullating pump. My problem is that lately if the boiler is not used for a day or longer because no heat is required I have to jump start the boiler when it calls for heat by shorting out the thermostat connections at the boiler with a jumper wire until the gas valve opens and then the furance runs normally even after I remove the jumper which indicates that the thermostat is now providing the close circuit to the furance.

I changed out the thermostat with a new one. I rewired my thermostat wiring at the boiler to the gas valve. I changed out the thermocoupler. Still the same problem. Only thing left to change out is the millivolt gas valve but if that is defective then the pilot lite woundn't stay lit or the valve wouldn't work properly once I jump start it. By jump starting, the millivolt generated by the thermocoupler, is taken the path of least resistance and operating my system and stay operating all day with the jumper removed. Everything works normally until the boiler is not used for 24 to 48 hours then it starts all over.

I am at a lost on this problem and cannot replace the boiler because of the expense so hopefully someone can help me on this??

You need to do some research and find out how many MV the generator is supposed to put out. The pilot generator is responsible to supply voltage for the system operating controls. If the output is within specs then I would suspect that the thermostat wire or something in the control line is not allowing the required millivolt voltage to pass to the gas valve.

You may also check the system by measuring the millivolt at the valve during a call for heat. Just disconnect one wire from the valve and take a reading. You might find that all the milivoltage is available but the gas valve due to age is requiring move milivoltage to open. If this is the case you might have to replace the gas valve.

First check if the pilot is working properly. If you have a nice and powerful flame. Sometimes the connection to the pilos is loose and the flame is smaller than required, also it can be some dirt in de pilot. Also check for bad connections to the valve.