How To Replace A Steam Boiler Sight Glass

Apr. 20, 2021

Despite its small size, a boiler’s sight glass has big importance. In this post we’ll cover how to replace an old or malfunctioning sight glass so you can keep your boilers working and your workplace safe. 

It’s important to note that different types of glass and different lengths of glass will provide you with different readings. Always make sure that the glass you’re using is rated for your boiler. Now let’s get started!

How To Replace A Steam Boiler Sight Glass

How To Replace A Steam Boiler Sight Glass


Check to make sure the shut-off valves are tightly closed, or make sure your boiler is completely empty of pressure and water. Be positive that there is no pressure in the line before replacing your sight glass, or you risk serious injury. In our sight glass replacement demonstration video, we use a boiler that is not firing or hooked in. 


Once you’re positive the line has no pressure, valve the water column off, and drain the water from the gauge through a valve at the bottom. Remove the protective rods, crack the nuts on the top and bottom of the sight, and spin them off. 

Some sight glass enclosures have a “seat” or “stop” where the bottom of the glass rests -- be sure to account for that seat when measuring your new glass. Slide the glass up and out of the enclosure, then you’re ready to measure for your new glass. 

AS EASY AS 1-2-3

Measure from valve face to valve face, and be sure to factor in the depth of the seat. This measurement is how tall you should cut your replacement glass. You should now have a new piece of glass, new brass and rubber washers, and the nuts you removed earlier. To each end of the glass, slide your parts on in this order: 

  1. Nut

  2. Brass washer

  3. Rubber washer 

The brass washer prevents the nut from gouging the rubber washer, so make sure these three pieces are on in the correct order on both ends of the glass to avoid any potential damage. 


Slide the top of the glass through the bottom entrance of the top valve (you won’t be able to slide the glass through the top, since the nuts and washers are attached) and nestle the bottom of the glass into its seat or stopper. Hand-thread the nuts to start, and maneuver the washers into position. If your glass has a line in it, be sure to face it towards the back. 

Now that your sight glass is in place, use a wrench to finish tightening the nuts. When tightening, the goal is “slightly more than snug” -- tight enough it’s secure, but not so tight it breaks the glass. 


Now you’re ready to put the protective rods back in place, tighten the drain valve at the bottom of the sight, and open the valves back up. Your sight glass is the window to your boiler once again. 

You can find sight glass and valves here, and if you want to learn more or need assistance, contact us

  • Tel.: +86 318 5261 900 / +86 318 5261 902 / +86 318 5261 906
  • Fax: +86 318 5261 912
  • Address: 1-1-901 to 908 Fuxing International Park, 161# Ronghua Street, Taocheng District, Hengshui, Hebei, China. 053000
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