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.
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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.
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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.
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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.
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.
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
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.
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
You can check the water level in your steam boiler quite easily by using the
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.
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
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. You should see similar settings on the pressure gauge (at below right) and on the steam pressure control switch (the gray box at right in our photo) on your boiler.
Because the operating temperature, pressure, and form of heat distribution are different from hot water heat (typically 20-30 psi of hot water), steam boilers (typically less than 0.5 psi steam) use use controls that monitor steam pressure operated controls like the Honeywell steam pressure control shown at left.
Additional controls on steam heat that affect pressure include(shown later in this article) and other steam piping controls.
And because all steam heating systems consume some water during every heating cycle, steam boilers carefully monitor the water level in the boiler and require that water be added either manually (now rare, but found on some older steam systems) or automatically
Reader Question: the gray box on my steam boiler is set under 2 psi but the boiler pressure is 7-8 psi. Is this normal and safe?
Since your steam boiler pressure control valve is set under 2 psi and the boiler pressure is way over that, and assuming that your steam boiler gauge is working and not "stuck" at an abnormal reading , the problem may be the pressure control valve itself.
We discuss how to set heating system pressures for both hydronic boilers and steam boilers over
Also see RELIEF VALVE, TP VALVE, STEAM BOILER.
Reader follow-up: changing out the steam pressure control valve did the trick
got the tech. he took off the gray box on my newly installed williamson boiler (2 weeks ago) and replaced it with the one from my older rotted out 12 year old burnham! both gray boxes are honeywell and seem to be identical. works ok now. go figure!
Photo Guide to Identifying & Operating Steam Boiler Low Water Cutoff Valves & the Blowoff Flush Valve
Here'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
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
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.
Details about water feeder valves can be found
Automatic and manual water feeders for steam boilers are detailed
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
Our steam boiler TP relief valve photo at above left shows typical operating parameters for these safety controls.
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.
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
Details about the inspection, repair, replacement, function, and identification of steam vents on one pipe and two pipe steam heating systems are
and at STEAM VENTS, AUTOMATIC.
Also see RADIATORS for more extensive information about steam radiator troubleshooting such as how to diagnose and fix a cold radiator.
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
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 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 the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: proper setting for a Honeywell PA-404-A 1099 Presstrol on a steam boiler
DOEs ANYONE KNOW THE PROPER SETTING FOR A HONEYWELL PA-404-A 1009 PRESTROL CONTROL FOR A WEIL-MCLANE STEAM BOILER - Rick 9/11/11
Rick the exact pressure needed varies by installation but in a low pressure residential steam system the pressure range is usually between 0.2 and 0.4 psi.
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.
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 email@example.com or strentsch 10/4/11
sure Mr. Strentsch,
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
I HAVE AN OIL BURNER HOT WATER SYSTEM MY EXPANSION IS DRIPPING WATER OUT THE AIR VALVE STEM AT THE BOTTOM OF THE TANK SHOULD I REPAIR OR REPLACE - Mike S 10/12/12
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
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: too-high steam pressure traced to a blocked sensor on steam pressure control
Pressure on gas steam boiler was rising far too high, so high that the pressure relief valve released twice when it hit 15 psi (as it was supposed to do). Good buddy who really knows his stuff checked everything from pressure valve to pressuretrol. Nothing . He then checked "pigtail" and found blockage .Problem solved!! - Ken 11/24/12
Thanks for this note it may help other readers
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: 28 psi and water coming out of my return on my American Standard boiler
II have a pressure of 28 psi and water is coming out of my return how can i lower the water pressure from my steam American standard boiler - Ronald 1/13/2013
Reply: 28 psi on residential steam heat is very unusual, probably unsafe - shut the system off
Watch out: If your heater is a residential steam heating system it is operating at an abnormally high temperature and is unsafe. Turn the system off immediately and call your heating service company.
If your heater were a residential hot water heating boiler (hydronic heat) its pressure might be close to the upper end of normal operating pressure - and the leak at the valve could be due to a valve problem or to any of quite a few other causes. See RELIEF VALVE LEAKS for procedures to diagnose and fix leaky TP relief valves. See the links just below for information on how the steam heating system pressure is controlled and set.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
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