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Phil, Mechanical Engineer.
Category: Plumbing
Satisfied Customers: 7497
Experience:  Retired mech. contractor, shop owner, 51 yrs experience.
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I have a Utica Boiler. The water in my sight glass does not

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I have a Utica Boiler. The water in my sight glass does not move even though the low level light came on. I unscrewed the nut on the underside of the tube and water started coming out, but the water in the sight glass did not move. I loosened the knobs at the top and the bottom of the glass about a quarter turn and the water drained from the sight glass. I replaced the nut and refilled the boiler and I was able to get the tube to be about half filled with water. I tightened the knobs back the way they were, which I'm assuming is closed. I filled it enough so that the low water indicator light turned off and left it that way.

My question is, how should those knobs be turned for the sight glass to work properly?
Welcome to Just Answer!

The knobs should be left closed at all times except when you are checking the water level, thats a safety measure in case the sight glass breaks.

You should check the low water cut out switch every 30 to 60 days by opening the sight class valves, and then opening the drain valve on the boiler slightly and noticing how low the water gets in the gage glass before the low water cut off switch opens and shuts the gas off to the burner. or the make up water valve fills the boiler back up to half glass again. Then close the boiler drain valve at the bottom of the boiler.

Do not let the water level get closer than 1 inch to the bottom of the glass.

If you get me the model number of the boiler and tell me if it is supply stream or hot water to the radiators i can be a bit more specific.

Customer: replied 3 years ago.

The boiler is model number PEG112CDE and is supplying steam to the radiators.


So, if the knobs are supposed to be both closed, how is the sight glass supposed to indicate the water level in the boiler? (top and bottom - as pictured here):

utica boiler site glass


Right now, I have no idea how much water is in there because the water in the sight glass doesn't move and I don't want to overfill it.

Hello again, virtually all steam boilers ever built have an automatic water feed float system so that when the water gets a few inches low the water feed valve opens and fills the boiler to about half a glass.. From your reports that is how you boiler is operating.

When you want to read the sight glass turn both knobs on the sight glass 1/4 turn open and read the glass, then open the boiler *drain valve slightly* and you will see water in the drain where the drain valve goes.

You will see the water level in the sight glass drop, proving it works, then close the drain valve, and you will see the sight glass fill up again, proving the city water feed works... you should do that test every 30 days. 60 days between tests maximum.

After you do the test turn the knobs on the sight glass to the right.. that closes the valves and the sight glass will not read again until you open them for another test.

The sight glass is fragile, that is why it is left closed between tests.


The sight glass is ONLY an indicator it does not control the water level. The water level is monitored by a float valve. The checks mentioned above are to insure the float valve works... and that the switch it is connected to that kills power to gas valve stops the burner if the water in the boiler gets low.

Let me know if that makes any sense or not on your end, we keep going until you have full and complete answer you need, and understand it, there are no time limits here.


Customer: replied 3 years ago.

Let me make sure I understand properly and have the correct procedure down for when I need to do this.


Right now, the red light on my low water cut off is off, so I don't need to do anything.


every 30 - 60 days, I should open the knobs a quarter turn, drain a little water to see if the level in the glass goes down. I'd imagine that I should turn off the heat and let the boiler cool before doing this.


Otherwise, when the light goes on, after letting the boiler cool (I know that cold water+hot boiler = bad Frown ), I should turn both knobs about a 1/4 turn open. that will show me the water level. Then, use the water feed into the boiler to fill it and when the sight glass is at an acceptable level, stop the water, close the knobs on the sight glass and turn the heat back on.


Is that correct, or am I missing a step.


Thank you for your patients, I just don't want to do something to mess up a very expensive part of my home.

Hello again, thats close.

Letting cold water into a hot boiler IS bad if you do it on a hot drained boiler, in this case the boiler water feed should kick in after you let just an inch or two of the boiler water out, so letting that small amount of water out and letting it refill is part of a normal operation.

If you do this when the boiler is on and firing, it should stop firing and the red light should come on when you let an inch or two of the water out... the burner should go off (thats part of the check and you should hear the water feed adding water to the boiler and the water level in the sight glass start to rise again.

Let me know if you more information, we continue with this until you are quite clear on the process... it will be good try it a few times, then check back with me if there are any questions.

Phil, Mechanical Engineer.
Category: Plumbing
Satisfied Customers: 7497
Experience: Retired mech. contractor, shop owner, 51 yrs experience.
Phil and 3 other Plumbing Specialists are ready to help you
Customer: replied 3 years ago.

Thank you, that was very clear.


you are welcome however to stay in touch in this thread if you need any further clarification.


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