Hi, I have a Hydrotherm HC-100 Nat Gas Boiler that is not igniting the burner. The thermostat shows that the house is at 60f and is set for 68f. If I turn the thermostat down and back up occasionally the burner will ignite and heat the house for a few hours and then cold again. As I mentioned in the subject the standing pilot is on, also when I turn the thermostat down below the room temp and then back up I hear a relay click and the circulation pump appears to be working because I hear the flow of water if I am standing next to the boiler. I can't seem to find a damper switch or/and safety thermostats, where would they be located as I know there needs to be some. I checked for voltage at the gas valve and through the temperature selector and they are not getting power. What should I be looking for and where would these safety thermostats be located so I can check voltage or continuity.

I have one of these (tankless coil) with 2 zones. Suppose for argument sake, one of the thermostats is set to 90 and is always on. Also suppose hi limit is 180, low is 150, diff is 10. 1) The circulator is on and the boiler is running, then someone takes a shower. What temp does the circulator shut off? 2) The shower stops, what temp does the circulator come back on? 3) Leaving the setpoints alone, what does changing only the diff do? 4) If you want the circulators to run more when the boiler is off, what changes do you have to make? Is this even possible without sacrificing too much DHW? Thanks a lot.