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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.
How to Read 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.
The photo above with our blue markings 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.
The photo at left has two black marks drawn right on the sight glass indicating where the building manager wants water level maintained.
On this steam boiler installed in Portland Maine, the black data tag found boiler near the top of the sight glass has these instructions:
Minimum Water Level 4" above the bottom of the gauge glass.
A reader asked us if she should read the water level in the steam boiler gauge glass using the top edge of the water or the bottom edge of the curve that's observed on water in any vertical tube. 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 Portland Maine boiler photo shown immediately above, I would ignore those black magic marker indicators somebody drew on the sight glass and follow the manufacturer's instructions: a water level anywhere 4-inches above the bottom of the sight gauge and below the top of the glass area of the gauge will be just 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 do I 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 you need 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, Repair
Please see STEAM BOILER SIGHT GLASS REPLACEMENT for steps in repairing or replacing a leaky or broken steam boiler sight gauge or sight glass.
These heating boiler leak diagnostic articles may also help troubleshoot steam boiler sight glass crud or leaks:
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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