Gas Boiler not heating

Discussion in 'Boiler Forum' started by wglassi, Oct 14, 2013.

  1. wglassi

    wglassi New Member

    Messages:
    2
    Location:
    Illinois
    I have a gas boiler that uses hot water heat in cast iron radiators. The boiler is running. Pilot light is on. Circulating pump is running and responding to the thermostat. Pressure reads approximately 13. Problem is the temp gauge is as low as it goes, and there is no HOT water exiting the boiler. Does anyone know what the problem might be. Ive run out of ideas. Thanks
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,051
    Location:
    New England
    Air in the lines can prevent the water from circulating. In most systems, the pump is not strong enough to push the water up without some falling down the other side (i.e., the system is full of water). In this manner, they can use a smaller pump which uses less energy and is also quieter.

    It's likely that you need to bleed the system.

    Now, it's possible that the circulating pump has a problem, but try bleeding before looking at a hardware issue.
  3. wglassi

    wglassi New Member

    Messages:
    2
    Location:
    Illinois
    I was mistaken when I thought that the boiler burners were actually firing up. All I am seeing is the pilot light. What might prevent the burners from running? Thanks again for your help.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,051
    Location:
    New England
    There are all sorts of interlocks on a modern boiler! Low water pressure, some have a sensor to determine there's a clog in the air intake, failure to sense the burner is on (flame sensor), and there may be more. If it was a transient fault and there's no reset button, sometimes just turning it off then back on will work. But, other than that, if it doesn't have any indicator lights or signals, you need to know and understand how to trouble-shoot the interlock system. Could be a bad relay, but it's probably an interlock from a fault. It could also just be a bad sensor (part of the interlock system). The manual often has some trouble-shooting. If you aren't comfortable, you may need to call a pro. Zapping a control board can get costly (if it has any!), and there's at least 120vac in there somewhere (and likely a bunch of 24vac for the interlocks).

    The manual will have a theory of operation section that describes things that may help.
  5. Dana

    Dana In the trades

    Messages:
    2,862
    Location:
    01609
    Blue car won't go. The starter motor seems to crank, the windshield wipers work, but no vroom-vroom.

    I'm not sure how it's possible to diagnose anything without even the most basic information on the boiler other than the fuel is gas, and that it's pumped hot water, not steam.

    Model numbers are sometimes useful, since the manuals are often available in online archives, even for some real antiques. The odds of hitting the real solution to the problem shooting in the absolute dark are pretty remote.
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