Steam heat boiler basic controlsGuide to Steam Boiler Controls & Gauges

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Steam heat controls: here we provide a photo guide to all types of controls found on residential & light commercial steam heating systems.

We tell you what each control does, where it is located, what it looks like, and we link to in-depth information about setting controls or diagnosing & repairing problems with steam heat controls.

This article series provides an illustrated inspection and repair guide to Steam Heating Systems. The page top photo shows a modern steam heating boiler.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Steam Heating Systems Guide: Controls Found on Residential Steam Heating Systems

Steam boiler schematic (C) Carson Dunlop Associates

All of the steam heating system controls described in our introduction are illustrated below and described in greater detail in individual articles found in this steam heat series.

[Click to enlarge any image]

If you don't know what kind of heat your building uses, we explain how to figure out the answer

If your heating system is not working properly,

Sketch of a one-pipe typical steam heating system (left) is provided courtesy of Carson Dunlop Associates.

[Click to enlarge any image]

In a one pipe system each radiator is served by a single pipe: steam rises into the radiator and condensate (water) returns from the radiator through the same pipe loop, as you can see at left.

A two-pipe steam heating system is similar but provides a separate loop of piping that collects condensate and returns it from the radiator to the boiler.

List of Definitions, Uses & Photographs of Steam Heating System Controls

If your heating system looks like a heating boiler but your heating radiators have valves which hiss and let air escape as heat is coming on your heat is probably being delivered in pipes which circulate steam from the steam boiler up through radiators in the occupied space.

Although both hot water - hydronic heating systems (discussed at BOILERS, HEATING) and steam boilers (discussed here at STEAM HEATING SYSTEMS) use a boiler that heats water, the heat distribution method and most of the heating boiler controls are different between these two systems.

Both steam boilers and hot water boilers may share certain controls that monitor and manage the heating source such as an oil burner or gas burner. Heater burner controls make sure that there is a safe proper combustion flame, control draft, and protect against flue gas spillage.

Examples include the CAD CELL RELAY SWITCH (oil burners) or SPILL SWITCH, FLUE GAS DETECTOR (gas burners).

Below we provide a master list of links to articles about individual controls found on steam heating systems, but before jumping off on one of those links I suggest reviewing the rest of the article below this list.

Master List of Steam Heat Controls, Gauges, Valves

Five Common Safety Devices on a Steam Heating Boiler

  1. SIGHT GLASS, STEAM BOILER described below is just one of the safety devices found on a steam boiler. The sight glass tells us the level of water in the boiler; a required boiler water level mark is normally visible on the boiler jacket behind the sight glass, or it may be marked on the sight glass itself.
  2. RELIEF VALVE, TP VALVE, STEAM BOILER, a pressure/temperature relief valve - a protection against BLEVE EXPLOSIONS
  3. LOW WATER CUTOFF VALVE - abbreviated as LWCO - that turns off the burner if water level falls dangerously low
  4. HARTFORD LOOP (to prevent loss of water out of the boiler by siphoning out of boiler water from a piping leak), and on many modern residential steam systems, and on modern systems
  5. WATER FEEDER VALVE, STEAM - an automatic water feeder valve may also be installed to maintain boiler water level. Water feed valves on steam boilers include a manual flush valve that must be opened regularly, often weekly, to flush accumulated sludge from the water feed control and LWCO.

All of these steam heating system controls are illustrated and described below, along with all other controls and devices used to operate residential and most commercial steam heating systems.

Hartford loop piping schematic for a steam boiler - adapted from ITT's The Steam Book - (C) InspectApedia Daniel Friedman

Hartford Loop safety piping arrangement on steam boilers

As you can infer by looking at our drawing at above left Boiler Without a Hartford Loop, a leak anywhere in the wet return portion of the condensate return piping that is below the boiler water line risks siphoning out all of the water from the heating boiler.

Our illustration at above right titled Hartford Loop shows where the Hartford Loop is located on a steam boiler. Details about the Hartford Loop, why it's needed, how it works, and the history of the steam boiler Hartford loop are found

Watch out: at CONDENSATE RETURN PIPES, PUMPS, STEAM we explain that
a leaky condensate line such as the one shown in our photo at above left can be lead to

You can check the water level in your steam boiler quite easily by using the

The Hartford Loop is also discussed

Steam Boiler Sight Glass

Steam boiler sight glass showing required water level (C) Daniel FriedmanThe 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.

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 to check and set a safe water level in the boiler or to check that the automatic water feed valve is working.

Even if your steam boiler has an automatic water feeder , 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.

Some folks refer to the sight glass as a sight gauge - it means the same thing: a vertical tube, usually of glass, that shows the current level of water in the steam boiler.

Details about using the sight glass on a steam boiler to check or set water level are found

If you need to clean or replace the sight glass on a boiler

Also see STEAM BOILER FLOODING REPAIR where we explain the causes of water too high in the sight glass or in the steam boiler.

Photo Guide to the Steam Boiler Pressure Control Safety Switch

Steam boiler pressure control switch

The pressure control switch on a steam heating boiler is designed to shut the heating system down should unsafe high pressures develop.

As we emphasize at our description of pressure gauges on a steam boiler, residential steam heating systems are almost always designed to operate at very low pressures, perhaps around .5 psi - that' s 1/2 of one psi.

See STEAM BOILER PRESSURE for details about the operating pressures of steam boilers.

For details and pressure readings of the steam pressure control switch see STEAM PRESSURE CONTROL

Photo Guide to Identifying & Operating Steam Boiler Low Water Cutoff Valves & the Blowoff Flush Valve

Steam heat low water cutoffHere's a photo of a Low Water CutOff safety valve (LWCO) on a steam boiler. The LWOC incorporates an internal float connected to an electrical switch. If water in the steam boiler falls to an unsafe low-level, the electric switch opens to shut down the heating system.

This valve on a steam heating boiler needs to be flushed clean weekly (more or less) during the heating season using what heating techs refer to as the "blowdown valve". The yellow handle you see in the photo is used for that purpose.

The blowdown valve is opened briefly to allow sludge and grubby water from the LWCO valve interior to flush out into a bucket for disposal. On some LWCO valves the blowdown valve is spring-loaded so that it closes when released. The yellow-handled ball valve shown in our photo is a manual flush valve.

While the LWCO blowdown valve is open and water is flushing out of the boiler, if an automatic water feed valve is installed it will automatically provide make-up water for the boiler to keep water level at the proper level.

If the steam boiler does not have an automatic water feeder, after you use the blowdown valve to flush the LWCO control you will need to use
the SIGHT GLASS, STEAM BOILER and a manual water feed valve to set proper boiler water level.

Typically we open and close this valve several times, for just a few seconds at a time, until water flushing out of the LWCO valve runs clean.

Watch out: we don't like to introduce a large volume of very cold water from the street into a very hot steam boiler. While it's never happened to me I've heard reports of boiler cracks or damage from this thermal shock. Waiting until the boiler is at a cool state, or avoiding unnecessarily large volumes of flush water should keep you out of trouble. Also check the installation and maintenance instructions for your boiler to see what the manufacturer recommends.

Details about low water cutoff valves on steam boilers & how to use the blowdown valve to regularly flush the LWCO can be read

Automatic and manual water feeders for steam boilers detailed

Water feed valves for hydronic boilers are discussed separately

Steam Boiler Automatic Water Feeders

Steam heat automatic water feeder Steam heat lwco

Above are photographs of a couple of different but still modern automatic water feeder valves on a steam boiler.

Here we show the "manual feed" button which can be found on the automatic water feeder in the photo at above left.

Water feeder button on steam heat

Details about water feeder valves can be found

Automatic and manual water feeders for steam boilers are detailed


Temperature / Pressure Relief Valves on Steam Boilers

Steam boiler temperature/pressure relief valve label (C) Daniel Friedman 1999Low pressure steam boilers still require a pressure/temperature safety relief valve.

A pressure relief valve is a spring-loaded device that will open to spill excess pressure (and temperature) in the form of water, steam, or a mix of the two, at a pre-determined pressure in order to protect the heating appliance from damage, or worse, a dangerous BLEVE explosion.

However the steam boiler relief valve operates at different temperatures and pressures than found on a hydronic (hot water heating) boiler.

Residential steam boilers typically operate at below 1 psi, and on residential steam systems the pressure/relief valve is usually set to open at 15 psi.

These are much lower operating pressures than on hydronic heating systems.

Conversely, the operating temperature on steam systems in the boiler reach boiling - 212F or 100C - much hotter than hydronic heating systems.

If your boiler pressure/relief valve is leaking
see RELIEF VALVE LEAK for the steps in problem diagnosis & repair.

Our steam boiler TP relief valve photo at above left shows typical operating parameters for these safety controls.

Steam boiler temperature/pressure relief valve label (C) Daniel Friedman 1999

This particular valve, a Watts No. 315-M1 is a 3/4-inch diameter valve set to open at 15 PSIG and has an energy release rate of 375 LBS/Hr.

Watch out: The steam heating boiler TP valve shown at left is leaking and lacks its discharge tube - this is an unsafe installation. Not only are we worried that leakage may damage the boiler or its controls, a leaking TP valve may eventually clog itself with mineral debris and crud that block the valve, preventing it from opening as it should in the event of unsafe temperatures or pressures in the heating appliance.

Steam boiler TP relief valve photo contributions needed - . CONTACT us.

Also see

Steam Traps on Residential Steam Heating Systems

Hoffman steam trap (C) Daniel Friedman

Definition & function of the Hoffman trap:

Steam traps such as the Hoffman-style steam trap shown at left are installed on residential steam heating systems, usually at the bottom of the radiator at the opposite end from the steam input side.

The steam trap is installed in order to allow air and condensate out of the radiator while at the same time, stopping the escape of steam (or slowing it) until the steam can condense to water (thus transferring its heat to the radiator itself).

Details about steam traps: we explain the function, identification, & troubleshooting of steam traps

Steam Vents on Residential Heating Systems

Steam vent on a radiator

Details about the inspection, repair, replacement, function, and identification of steam vents on one pipe and two pipe steam heating systems are


Also see RADIATORS for more extensive information about steam radiator troubleshooting such as how to diagnose and fix a cold radiator.

Condensate Return Pumps & Condensate Systems on Residential Steam Heating Systems

Steam condensate return pump system (C) D Friedman Steam condensate return pump system (C) D Friedman

Our photo at above left illustrates a steam condensate return pump system in a home

Our second steam condensate return system (above right) shows the reservoir and condensate pump motor (gray pump and new piping on the right side of the condensate reservoir at the left in that picture). This system also includes chemical injection into the steam condensate.

Details about steam condensate piping, return pumps, and leaks are found

Steam Boiler Pressure Gauge and Normal Steam Pressure Range

Steam pressure gauge on a steam boilerPressure and Temperature gauge on steam heating boilers: Residential steam heating systems are almost always designed to operate at very low pressures, perhaps around .5 psi - that' s 1/2 of one psi.

You should see similar settings on the pressure gauge (at left in our photograph) and on the steam pressure control switch (the gray box at right in our photo) on your boiler.

Details about normal steam heat system pressures are

Details about pressure gauges on steam heating equipment can be read

If your heating system uses forced or gravity circulated hot water rather than steam, controls and gauges are different:

or see PRESSURE GAUGE, BOILER for more details about pressure gauges on hydronic (hot water) heating boilers. These are not steam systems.


Continue reading at LOW WATER CUTOFF VALVE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see COLD STEAM HEAT RADIATORS - diagnose & repair


Suggested citation for this web page

STEAM HEATING SYSTEMS & CONTROLS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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Frequently Asked Questions (FAQs)

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Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.

Question: steam pipe configuration question at the boiler

what's the term they call the 2 steam pipes that form an H above a steam boiler. in other words the pipes from both sides of the burner connected together?THANKS,Walter - 9/25/11

Reply: steam pipe header

Walter: sorry we couldn't give an instant reply; sometimes the volume of reader questions is considerable and takes some time to get thorough.

I am guessing you are referring to the steam header or manifold piping that allows steam to enter more than one steam supply line or riser.

A second piping configuration that will be along side, not above, the boiler is the Hartford loop - a loop of piping sort of like an upside down trap that prevents water from siphoning out of the boiler should there be a leak in the condensate return line.

Question: steam boiler parts identification

Wow, what a great web site. I am trying to identify a vessel on the side of an old National steam boiler/furnace. It is about 12-18" tall, about 8" diameter and is attached to a manifold in the side that is about 12" off the floor. It is heavily rusted. The boiler is an old coal fired converted to oil. Can I send you photos? Thanks, Stephen or strentsch 10/4/11


sure Mr. Strentsch,

use the CONTACT link found at page top, left, or bottom to see our email and send along some sharp photos and we'll be glad to take a look

Question: My steam boiler seems to be taking more water than usual

I blow the system down regularly and then make up the water in the sight glass. However I seem to be using significantly more water then years past whereas I'd top the sight glass every couple of weeks in the past compared to every other day now. What do you think the problem is? - Tom C 10.27.11


Tom C: a common reason for increased water consumption in a steam heating system is one or more radiator steam vents that are not closing properly. Check also for a leak in the condensate return line.

Question: I need to prepare for a steam boiler license and want book recommendations

I’m happy to have found this website. I’m getting back into heating and cooling and need to prepare for higher license. Question: Is there a DVD collection you recommended as an additional studying tool? I see that Harold J. Frost, Frederick M. Steingress, Daryl R. Walker’s collection is close to 600.00 which is very steep for my pocket. Would you happen to know of a website or cheap place I can buy such DVD or a CD? If not these guys than another author is ok. Thanks for the feedback. Kym 11/23/11


Kym if you are looking specifically for information on steam heating I'd pick up any of Dan Holihan's books on steam heat. He's the best.


The HomeServ tech just left after installing a new auto water feeder, and the PSI gauge on my steam heater is off the chart, over 30 PSI. I shut it down and called HomeServ and they said leave it on and they'll come in the morning. How do I lower the PSI myself so I can leave it on tonight and my little kids (two with strep) don't freeze? Thanks! 2/22/2012


Watch out: If the PSI gauge is showing abnormal pressure and the gauge itself is working properly the system is unsafe and should be shut down. Call someone else and fire that tech - from your description the system sounds unsafe

Question: water drips out of the air valve at the bottom of the tank



Mike, an air valve stem is replaceable, I would tr that first. Check also for abnormal system pressure ( an unsafe condition). But if the internal bladder of the tank has burst - another explanation for finding water at the air pressure adjustment valve, then most likely you need to replace the tank. You'll know this condition because the tank will become waterlogged, heavy, and ultimately the boiler TP valve will drip (another unsafe condition).

Question: radiator banging noise repair procedure

I have a one pipe steam system. My dining room radiator air vent was just replaced because the old one was leaking profusely (damaged my floors). The porch radiator air vent was also replaced because of too much steam condensation in the room, but now the open/close valve is leaking at the base. The radiator upstairs in middle bedroom is banging and clanking. The pipe that runs along the basement ceiling (finished basement) also bangs when the heat is coming on. My plumber cannot really give me an answer. Any suggestions would be appreciated. Thanks p Rita 11/21/12



If you search InspectApedia for


you can read about the diagnosis & cure of this problem

About the leaking valve - unfortunately that means you need another service call.

Question: relief valve is leaking

cleaned on a vav [?] system is leaking close the cilanod [solenoid ? -Ed] valve need to be replaced

- Anon 11/23/12


See RELIEF VALVE LEAKS for procedures to diagnose and fix leaky TP relief valves.

Question: trouble controlling level of steam heat - can I close off the radiator

My building uses steam heaters. I have no control over the heat in my unit because the thermostat is controlled by my landlord/owner who lives above me. However I have a small 1 bedroom with two heating units so even with all of the windows open, it can still get very hot. Is it okay for me to completely close the "open/close" black knob valves on my units or will that cause problems for the whole system? My landlord had previously told me not to touch the valves... Emily 12/10/12


Yes if the radiator control knob is not jammed or frozen you can try closing it partly or fully; radiator control knobs are intended to be a homeowner-resident-operator control. But if the knob is not easy to turn, don't force it as you might end up with a leak.

Question: hissing boiler

I have 2 yr. Boiler. It hisses like venting steam off and on loudly. Seems like when heat comes on venting starts until off. -Mr. Ed 12/28/12


Steam boilers include steam valves that hiss; we need more details.

Question: leaky relief valve

I am having a pressure problem and it looks like one of my relief valve is leaking or letting the pressure out would this keep my boiler from running. It seems to be shutting off and I need to set the pressure all the time. is it as simple as changing the valve and resetting the Pressure? Rick 1/3/2013


See RELIEF VALVE LEAKS for procedures to diagnose and fix leaky TP relief valves.

Question: steam boiler stopped working

(Mar 23, 2013) home able said:
Oil-fired steam boiler was functioning properly upto two weeks ago, now doesn't fully run to designated temperature on thermostat (digital). It will stop a degree or 2 short of desired setting even though thermostat says boiler is running. Why?


Home able,

I'm not sure what's happening with your system. IN general the gauges are not lab grade precise. It's possible that your gauge or its sensor port are debris clogged. Try tapping gently on the gauge to see if the needle moves. Also watch the gauge to see its reading throughout the operating cycle.


(june 19, 2014) anonymous said:
I have hot water baseboard system boiler in my crawlspace (.Basement) im thinking about replacing the boiler with a horizontal warm air furnace adams furnace. I keep getting freezeups and they are very difficult to repair. The furnace adams make is supposed to be 95 percent efficient. Plus the fact there are no pipes to freeze is the warm air furnace the way to go? Thanks paul



You're commenting about hot water and hot air heat on a steam heat article - so this may be a bit confusing to other readers, but in general, conversion to another heating method is fine if you can bear the expense; each heating type has its pros and cons so IMO there is not a single "right" answer. Keep in mind that when you change from steam heat or hot water heat to forced warm air heat, your costs are much greater than just the heater itself as you have to install a heat delivery system : air ducts and registers. That can be convenient in some building designs and quite costly in others.

Question: pressuretrol settings for second floor in an office buildings

(Oct 16, 2014) Antonio said:
I just installed a 299 MBH gas fired steam boiler and connected to an existing system serving the second floor of an office building, approx floor area is 2,600 sq ft.
Existing EDR calculated as approx 900 sq. ft. The boiler started up successfully yesterday and I have been testing it ever since. Everything seems to be working fine. My question is what would be the right pressuretrol settings: I am using 1 psi cut-in and 1 psi differential and everything seems all right, but I may be missing something, maybe the boiler could work more efficiently with a lower or higher cut in pressure.

(Dec 15, 2014) Kevin M said:
I have a gas-fired steam boiler that has worked flawlessly since its installation six years ago. Suddenly it is overfilling. I let out several extra gallons a day to reduce the level in teh sight glass. Shoudl I replace the automatic feed? It is attached to the hot water heater, unlike previous installation I have had in othe rhouses where it is connected to teh cold water line. I sit possible that excess pressure from teh expansion in teh hot water supply line is forcing water past the automatioc feed? What could cuase this condition?



Over-filling of a heating boiler is indeed usually a problem with the automatic water feeder, more rarely leaks into the boiler from a tankless coil.

Question: bothered by banging pipes

(Feb 15, 2015) Bevery Streeter said:
I live in a 16 unit apt complex. What make the heat clan and banging?


See the banging pipe diagnostic and repair suggestions at BANGING HEATING PIPES RADIATORS

Question: steam boiler won't keep running

(Apr 9, 2015) george said:
steam boiler heater runs for about 15 minutes then it cuts out


George I'm stuck on this one with no onsite data; could be related to heating of components, clogging of a filter, I dunno. It's time for an onsite heating service tech.

DO NOT keep pushing a re-set button. That's unsafe.


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