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Boiler problem - Will not start

JL_3 Member Posts: 7
I have a gas Buderus hot water boiler with indirect hot water storage tank. System will not turn on unless I manually reset it by turning off the power switch and waiting over 1 hour to turn back on. The boiler will then fire up on 1st try run for 10-15 minutes and then completely shut down before reaching aquastat temp setting. Once this happens the boiler LED lights on control go off and it does not attempt to restart unless I reset it manually with the power switch. I can see all the thermostats for hot water and zones are calling properly and circulators are running, but boiler does not try to fire up. We also have GuardDog Low Water cutoff which shows green, so assuming that is not the issue. Vent damper is open. Any ideas on what else is telling boiler to shut down in the middle of the heating cycle and then not attempt to restart. Thanks.


  • pedmec
    pedmec Member Posts: 713
    Yeah, if you have a bad circulator and your shutting down on operating limit. your hour wait could be for the water to cool down. look at the water temperature when it comes on. how fast does it rise?
  • Big Ed_4
    Big Ed_4 Member Posts: 2,080
    First guess you have a control breaking down . I would use a meter to find it
    I have enough experience to know , that I dont know it all
  • JL_3
    JL_3 Member Posts: 7
    Circulators seem to work fine and boiler is shutting down before reaching aquastat high limit and then will not restart. I hear loud buzzing coming from the aquastat which I have not heard before while boiler is heating. I think it is coming from the relay area. Is this normal? I have not noticed before. Then a loud click and it completely shuts down, and boiler LED light shuts off and will not even try to restart. Bad aquastat?
  • HomerJSmith
    HomerJSmith Member Posts: 1,927
    edited November 2022
    How old is your boiler? The buzzing is usually the aquastat relay trying to engage. That could be because of the relay not getting enough power from the transformer. What is the make and model # of your aquastat?

    What happens with a Honeywell aquastat is that the relay opening and closing over a period of time breaks the solder connections on the relay points on the back of the board. Re- soldering the point contacts on the back of the board would fix it, if that is the case or buy a new aquastat. Be aware that the aquastat has high and low voltage sections which must be isolated from one another. It is done with Fish Paper.

    What is happening, I think, is that the relay is making the solder contacts when the boiler is cold and when the boiler warms up the solder contacts separate, shutting the boiler down. The relay is a STDP relay (Single Throw-Double Pole)( two points). One point operates the circulator and the other point the safety circuit, which must be intact for the boiler to operate.

    One other thing, the transformer in the aquastat puts power to the thermostat thru the thermostat wiring before the relay is energized. A problem with those could prevent the transformer from providing enough voltage to the relay resulting in the relay points chattering.
  • JL_3
    JL_3 Member Posts: 7
    Thanks! Aquastat is Honeywell L8148E. Boiler and equipment about 12-years old. Just watching the relay engage I can see how that could happen. Seems to snap pretty hard. Is there anything else that could be causing the safety circuit to break while the boiler is firing before it gets to set limit on aquastat? Water in the boiler is room temp, vent damper manually open, pressure is fine, thermostat calling and circulators running fine. Nothing has changed with thermostats or wiring and everything seems to function as it should after I manually reset with the switch but then it cuts out again. Sounds like loose connection or soldered contact may be right. Would bad or insufficiently powered transformer cause it to shut down after it was already running?
  • neilc
    neilc Member Posts: 2,096
    any chance you changed, or had the thermostat changed ?
    known to beat dead horses
  • JL_3
    JL_3 Member Posts: 7
    Nope. Same since install.