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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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.

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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.

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 gaugesOn 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 © D Friedman at Steam condensate return pump system © D Friedman at

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


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