Steam boiler sight glass showing required water level (C) Daniel Friedman Steam Boiler Sight Glass Replacement
How to change out, clean, fix or replace the sight gauge to set the water level steam boiler

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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.

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Steam Boiler Sight Glass Leaks, Breaks, Repairs or Replacement

Leaky sight glass on a steam boiler (C) Daniel FriedmanWatch 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


Steam boiler sight glass replacement - InspectApedia.comIt'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.

That's if everything goes well. If you encounter corroded fittings that you can't get apart, snap off a fitting at its mount to the boiler, etc. it could turn into a bigger job - not something to attempt on Sunday night when there are no open spare parts stores and you don't want to be without heat.

I'd take a careful look at the condition of the fittings and make sure I had the proper glass and other repair tools and pipe dope, etc on hand. - editor

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

Steam boiler sight gauge opaque with scale (C) Daniel FriedmanSome 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 on the steam boiler sight glass also permits emergency, temporary heating system use if the glass is broken, though of course you no longer know the boiler water level.

Usually you close a sight glass valve by turning it clockwise; some sight glass valves have an ON-OFF direction arrow on them. Or you may need to just try the opposite direction from the position in which you find the sight glass valve when it's found to be open, active, or leaking.

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

The heating boiler leak diagnostic articles given below at "Continue reading" may also help troubleshoot steam boiler sight glass crud or leaks:


Continue reading at SIGHT GLASS, STEAM BOILER - home for this topic, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see STEAM BOILER SIGHT GLASS REPLACEMENT FAQs - questions & answers about fixing or replacing a sight glasss on steam boilers, posted originally at this article

Or see BOILER LEAKS CORROSION STAINS - how to diagnose & evaluate leaks on or in heating boilers

Or see BOILER LEAKS, HOW TO LOCATE - how to determine that a boiler or hot water system is leaking and how to find hidden leaks

Suggested citation for this web page

SIGHT GLASS REPLACEMENT at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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