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Steam Boiler Sight Glass will not fill

bigel6 Member Posts: 2
edited December 2020 in Strictly Steam

I have a relatively new Weil Mclain boiler (EG-35) and I randomly noticed that the sight glass was completely full when I went to do my normal drain of the sediment that collects. Assuming that the boiler somehow got over filled I began to drain some of the water and expected the sight glass level to lower, it didn't. I then drained some more water in hopes that it would lower and still no luck. The low water cut off kicked in which indicated the boiler was at the correct level initially, the sight glass however was not. I then decided to clean out the sight glass by tightening both valves and loosening the petcock below the glass. The water drained correctly and I proceeded to refill the boiler. The boiler refilled correctly and the LWCO light went out, letting me know that it was back up to the correct level. I fired the boiler up and everything functioned, but the sight glass still remains empty and has not filled back up. I tried opening and closing the gauge valves in different sequences (both open, both closed, one closed one open, etc.) Am I missing something blatantly obvious or is there any recommendations anyone has? My guess is that the valve to fill the sight glass must be clogged somehow......


  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    Your guess is probably right, and you may have to take it pretty well completely apart to fix it.

    It looks easy. It's not. You'll need new gaskets at the very least -- and you should have a replacement glass on hand as well, as they break very easily.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 11,060
    You could avoid working with the glass part by;
    When off and cool....drain the boiler again
    Unscrew the valve bonnet nuts just behind the handles.
    The handle will unscrew the stem out of the port.
    You can then use one of the guard rods to poke into both the valve ports into the boiler.
    Start to fill the boiler and the water should come out of the lower opening, flushing out crud into a bucket, screw the valve back in before you get too wet.
    You can over fill the boiler and flush the upper port the same way.
    You then check that the small drain under the glass is not plugged also. When steaming you can close the valves, open the little drain and give it a slow opening shot of steam with the top valve, this may clean the glass somewhat.
    Close the top valve and the drain plug, then open both valves and see what happens.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,826
    edited December 2020
    Those gauge glasses usually don't come in the correct size. You will need to cut the glass to the proper size. If you get a nice guy at the supply house, he may cut it to size for you. but that is rare today. Not many steam gauge glasses sold to homeowners nowadays.

    If you are very careful you may not need to replace the glass tube. try to remove the top gasket or at least slide it down the glass in order to get the glass to move up high enough to get the bottom of the glass out.

    You will need to replace the rubber gaskets and the brass washers. Once the glass is out of the way, you can try to poke a coat hanger thru the valve opening. I have had limited success with that. Mostly You will need to remove the valves and poke the coat hanger thru the pipe openings in the boiler and in the valves.

    But that leads to the next observation. if the gauge glass tappings are blocked, What is the condition of the openings to the pressure gauge and the pressure control(s)? They are most likely in need of cleaning also!

    Or you could bite the bullet and get a pro with the proper tools to make the glass the right size and also clean all the other small openings, and maybe even flush the bottom of the boiler out. I'll bet the boiler drain at the bottom is also clogged.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • bigel6
    bigel6 Member Posts: 2
    edited December 2020
    Thank you both for your comments! @Jamie Hall I will order another sight glass to have on hand just in case and then when received I will give your (@jughne) instructions a try!
  • The boiler tappings to the gauge glass are plugged. The entire assembly needs to be removed from the boiler. Boiler tappings and gauge glass valves cleaned, then reassembled. If you break the gauge glass, not catastrophic. Just close the gauge glass valves and fill water using LWC light as guide until you can replace. 

    If gauge glass plugged, then likely pressuretrol and boiler drain as well. Proper annual maintenance prevents having these issues. 
    New England SteamWorks
    Service, Installation, & Restoration of Steam Heating Systems