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Steam boiler sight glass repair or replacement: this article describes how to fix a leaky or broken steam boiler sight glass to obtain an accurate reading of the water level in the boiler. We discuss emergency steps such as simply shutting off water into and out of a leaky sight glass as well as sight glass replacement.
A separate article STEAM BOILER SIGHT GLASS explains how to read the boiler sight glass.
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Watch out: partly because of crud that develops inside the steam boiler, debris can ultimately occlude the view of water level in the sight glass. Your heating service technician can clean or replace the glass if needed.
If some Klutz (like me) breaks the sight glass you may be able to keep the boiler going and stop water from pouring out of the broken glass by closing the two valves found at the top and bottom of the glass tube - those round black handles in my photo.
In an emergency, if the sight glass is leaky try closing the control valves at the top and bottom of the glass (that's one of the reasons they're there). Don't use so much force that you break the valve or you'll be sorry.
A drip like the one in our photo (left) is annoying but not a catastrophe - it's not dripping onto a control.
Ask your heating service company to replace the leaky parts and while you're at it, if the glass is etched and horrible just replace that as well.
Reader Question: is replacing the steam boiler sight glass difficult?
Is replacing the sight glass on a Weil McClain steam boiler difficult? - Carl 4/17/11
It's not mechanically difficult to replace the steam boiler sight glass, but you have to shut down power to the boiler, let it cool down, and drain down the water level in the boiler low enough to be able to disassemble the fittings and install the new glass.
I should add that on some steam boiler sight glass setups the shutoff valves are working and you might be able to just shut off water there provided the boiler's not too hot to work on. The ease of slipping in the new sight glass depends on its fittings and their condition.
Cleaning the sight glass on a steam boiler
Some steam boiler additives intended to avoid frothing and to control the corrosive chemistry of water in the boiler will also help reduce soiling and scale formation inside the sight glass. But in a badly obscured sight glass actual cleaning (or replacement) may be in order.
To physically clean an otherwise intact sight glass that has become impossible to read due to scale and debris (like the sight gauge in our photo at left) you'll need to remove the glass (as described above) and use a small diameter bottle brush (or make-shift brush) to clean the tube interior.
As the sight glass crud is probably going to include a hard mineral scale, you may need to soak the sight glass in vinegar over night - making this a task to do when you don't need the heating system. Frankly given the trouble involved I'd probably just replace the glass with a new one of the proper diameter and length.
Reader Question: what are those red knobs above and below the sight glass on my steam boiler and which way do I turn them
What are the red knobs above and below the sight glass for and how should they be turned? - Anon 10/15/12
The red control valves at the top and bottom of the steam boiler sight glass are water shutoff controls used to isolate the sight glass from the boiler to permit glass replacement should it break. Closing both valves also permits emergency, temporary heating system use if the glass is broken, though of course you no longer know the boiler water level.
Relationship of the boiler sight glass to adding boiler water
Water is added to the steam boiler manually by opening a water feed valve, or it is added automagically by the automatic water feeder (if your boiler has this device).
Even if your steam boiler has an automatic water feeder (WATER FEEDER VALVE, STEAM), you should still check the water level (and clarity) in the sight glass frequently, but the risk of a ruined boiler from lost water is of course much less.
Watch out: one more warning: the LOW WATER CUTOFF VALVE itself needs to be regularly flushed on most steam boilers - during that weekly check, more or less often depending on the rate at which your particular steam boiler consumes water. Failure to flush the LWCO (using the "blowdown valve") means that crud accumulating in the LWCO can prevent its proper operation - an unsafe condition.
Sight Glass Clues to Boiler Water Feed or Heating Boiler or Piping Leak Problem
In the FAQs we discuss the concern with boilers having mysterious increases in water level beyone what is called for. For manual-feed steam boilers we suggested these checks:
Where to Buy Steam Boiler Sight Glass Repair Parts
These heating boiler leak diagnostic articles may also help troubleshoot steam boiler sight glass crud or leaks:
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Frequently Asked Questions (FAQs)
Question: water is rising in boiler sight glass but I'm not adding any
sight glass on steam heat furnace with indirect heater water rising
without adding it I have a manuel feed I took off union checked
Harold, if your system does NOT have an automatic water feeder and you see water level apparently rising in the boiler anyway some explanations could be:
Question: boiler sight glass problems?
I had a fill sight glass replaced by a service technician because it crack. Before that repair, my boiler was automatically feeding water. After the repair, it seems that I manually have to open the valve. There are three valves that are close together and seem to be connected to the same pipe. Now I have to turn my boiler on much higher than before to get it warm in the house. - Anon 10/23/12
steam boiler's sight glass is rusty and every time the boiler comes on the automatic water feeder,feeds water to the boiler filling the sight glass above the required level..i have been reading the other problems on this site ..is there a required height for the auto feeder to be installed in relation to the sought glass ? - Phil J 11/2/12
Anon: you want to call your heating service tech to have him/her show you the water feed valve setup and to check the condition of the automatic water feeder. Perhaps the sight glass repair stirred debris that has entered a valve or control. Try flushing things out.
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