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Steam boiler sight glass: this article describes how to find & use the sight glass (or sight gauge) on a steam heating boiler to check or set steam boiler water level. We explain where the sight glass will be found, how to read its water level, and the relationship between the sight glass reading and the procedure for adding make-up water to the boiler. A separate article WATER FEEDER VALVE, STEAM explains the actual procedure for adding water to the boiler.
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The steam boiler sight glass or water level gauge allows the building owner or maintenance person to monitor the required water level in the steam boiler. A mark on the boiler body indicates the desired normal water level and a sight glass shows the current water level.
[Click to enlarge any image]
Steam systems use varying amounts of water. Unlike a hydronic heating boiler, some of the water in the steam boiler is lost through the building's radiator steam vents at each heating cycle. Just how much water the steam boiler will lose varies widely depending on a number of factors.
But if a heating boiler loses its water without also being shut down it will certainly be damaged by the heat of the oil or gas burner (or coal or wood), and it could lead to a dangerous explosion or fire.
Watch out: The sight glass on a steam boiler is an important safety and operating device since it allows the homeowner or inspector to see at a glance when the steam boiler is low on water.
For boilers that lack an automatic water feeder, the steam boiler water level needs to be checked regularly (typically at least once a week) to assure adequate water and safe boiler operation. Low water in the boiler will lead to loss of heat or possibly an unsafe condition that can lead to BLEVE EXPLOSIONS.
Most modern steam boilers are protected against catastrophic water loss by a LOW WATER CUTOFF VALVE. With a LWCO installed, if water level drops too low in the boiler the valve will shut down the burner - turn off your heat.
Our photo (left) shows the steam boiler sight glass or water level gauge on a Weil McLain Model 68 steam boiler. Our blue arrow shows the tag or mark on the boiler jacket indicating the desired water level, while the vertical blue line I drew on the sight glass shows where the water level should be.
Watch out: just as you don't want too little water in the steam boiler you also don't want to over-fill the boiler. Doing-so may prevent steam from rising to the building.
Reading the Steam Boiler Water Level through the Sight Glass
You may also notice that when feeding water into the boiler automagically or by manual valve the water level surges in the sight glass. Just wait a moment for the surging to stop before making your water level reading. If surge is excessive you might try slightly closing the bottom inlet valve on the glass but be sure not to close it fully or you'll get no reading at all.
In fact if you are putting water into the boiler and the level in the sight glass does not move, I'd wonder if one or both of the sight glass control valves were shut or debris-clogged.
The photo above shows a typical mark on the steam boiler indicating the desired boiler water level. Some steam boiler sight glass gauges will sport two index marks: a low-water mark and a high-water-level mark. The boiler water level needs to be maintained between these two levels. You will see two such marks on the steam gauge with red control valve handles earlier on this page.
Don't worry about reading the water level or matching it to the boiler line with high precision. If you are off a half of an inch the boiler won't care. In the photo above, a water level anywhere within the black area of the water level tag on the boiler face is fine.
If your steam boiler has an automatic water feeder you should see the water level always close to the fill-mark and the automatic water makeup valve will normally do this job for you.
When to add water to the steam boiler
If your steam boiler has only a manual valve for adding water, until you know the system's rate of water usage, you should check the water level at least once a week during the heating season.
You'll want to add water whenever the sight glass shows that the water level has dropped to the "add" mark and you should always see some water in the sight glass.
How much water to add to the steam boiler
The sight glass on a steam boiler, combined with a mark or gauge usually placed on the sight glass or on the boiler body behind the glass, will show the proper level to which the steam boiler should be re-filled when it's low on water.
How to add make-up water to a steam boiler
The answer is, it depends. If your boiler has an automatic water feeder it should be doing the job for you and all yo uneed to do is check the boiler water level periodically. If your boiler has only a manual water feed valve you'll need to follow a series of easy steps to check water level and to bring boiler water level to the proper index mark checked against the sight glass.
Details of these water feeding procedures for steam boilers are at WATER FEEDER VALVE, STEAM
Steam Boiler Sight Glass Leaks, Breaks, Repairs or Replacement
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.
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:
These heating boiler leak diagnostic articles may help
Green link shows where you are in this article series.
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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