One pipe steam heat radiator with steam vent (C) Daniel Friedman at InspectApedia.comGuide to Steam Vents on Steam Heating Radiators & Steam Pipes
Steam vents & how they work; how to diagnose and fix steam vent problems: won't open, won't close, hissing, leaks, etc.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Steam vents on heating systems:

Here we provide an inspection and repair guide to the vents found on radiators and piping used with Steam Heating Systems.

We explain one pipe and two pipe steam heating vents and how they work, and the article includes diagnostic and repair advice for hissing sounds at steam radiators, steam vents that don't close when they should, steam vents that don't open when they should, noisy steam vents, and steam vent leaking or spitting.

We describe how to identify, service, or repair steam radiator air vents. How to regulate heat using air vents on steam radiators & troubleshooting steam heating system radiator vents & controls. How do we know if the steam vent is working on a radiator?

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.

Guide to Steam Vents on Residential Heating Systems

Steam vent on a radiatorArticle Contents:

Our photo (above left) shows what looks like a Hofman 1A float-type radiator steam vent. This is a residential low-pressure (1.5 psig) steam vent. Similar models include the Hoffman Model 40 (6 psig) and model 70A (11 psig).

[Click to enlarge any image]

Steam Vent Safety Warning

Watch out: Never remove a steam vent nor any other fitting or plug from a steam radiator if the heating system is hot and/or in use. Both the hot radiators and steam itself can cause serious burns. If you are not trained to do so, do not attempt to adjust or remove steam vents or control valves on steam heating radiators.

Even something as simple as removing a non-functioning radiator steam vent to try to allow air to escape and thus allow the radiator to heat up can be very dangerous.

At first it seems easy: remove the steam vent and rising steam in the system will push air out of the radiator. But suddenly things turn ugly.

When steam begins to enter the radiator it finds and streams out of the opening from which the steam vent was removed.

At that point it becomes essentially impossible to re-install the steam vent because you're reaching into a plume of boiling hot steam!

See COLD STEAM HEAT RADIATORS if you need to fix a cold radiator using safer and more-effective means. .

In 2017 the New York Times reported on the steam heat scalding deaths of two girls in the Bronx. The article gave no details about exactly what went wrong but the article states, "Ibanez, 2 and Scylee, 1, were scalded to death on Dec. 7 by radiator steam in a city-funded apartment for the homeless in the Bronx.

A valve on the radiator in their bedroom had come off; ..." - Neuman, William, "Parents Sue in Deaths of 2 Girls From Scalding", The New York Times, 16 Dec. 2017, p.17.

OPINION: we don't know what actually happened in this tragic apartment in the Bronx, but as residential steam systems operate at a low pressure of under 1 psi, it's not likely that a steam vent "came off" of a steam radiator on its own.

Take care not to bump, hammer, bend, whack or remove a steam vent if you are not trained about how and when to do so safely.

How Steam Vents Work on Steam Heating Systems

At the start of a heating cycle as steam rises in the building steam pipes and begins to enter the heating radiator, air in the cool radiator is pushed out through this valve.

As the radiator gets warm, then hot due to the rising steam, a mechanical thermostatically operated valve inside of the air vent, also called a steam vent, closes the vent so that steam does not continue to escape.

Automatic or Thermostatically Controlled Steam Vents

When steam is first rising in a one pipe or two pipe steam heating system, the steam heating radiator will be cool as will be the steam vent.

One pipe steam heat radiator with steam vent (C) Daniel Friedman at

One-Pipe Steam Heat Radiators use Steam Vents

On one-pipe steam heat systems the radiator valve is normally fully open (for the radiator to operate properly) or fully shut (no heat); you may find that some one-pipe steam heating system radiators will not work properly if the valve is left "in between" these positions.

Steam rises and enters each heating radiator through a single pipe, pushing air out of the radiator through its vent.

Condensate returns to the boiler through the same pipe, passing out through a special passage in the radiator control valve.

For details about steam heat piping & steam pipe radiator connections & definitions of different types of steam heat


Two Pipe Steam Heat - may not use steam vents

In a two-pipe steam heat system, steam rises through the supply side of the radiator (where the radiator valve is found) and may push air and later condensate out through the return or condensate drain side of the radiator.

A two pipe steam system may use a steam trap at the outlet end of the radiator and no air vents or steam vents may be present on the radiators.

Curved two pipe steam radiator with steam trap at Ellis Island NY (C) Daniel Friedman at

Above my photo shows a lovely curved two pipe steam radiator installed at Ellis Island, New York.

A thermoststatic control valve at the radiator's right end (white knob) controls the steam input into the radiator while condensate returns through the Hoffman trap at the radiator's left end. There is no steam vent on this radiator.

Details are at STEAM TRAPS and


Diagnosing Hissing Sounds at Steam Radiators

Steam radiator vent (C) Daniel FriedmanIt is normal to hear a hissing sound from the steam vent as air escapes from the radiator at the start of a heating cycle.

The hissing sound should stop and the valve close when the radiator gets hot.

Steam Vents that Do Not Shut Off When They Should

If steam continues to escape from the valve even after the radiator is hot, the valve is defective and should be replaced since continuous loss of steam increases the water consumption by the heating system.

Other causes of steam radiators to fail to get hot include a radiator valve that is shut or a steam radiator valve that has become clogged with rust and debris (more likely on a one-pipe steam heating system where returning condensate through the valve can become blocked),

or a one pipe steam radiator that has become tipped the wrong way so that it becomes clogged with condensate.

Steam Vents that Do Not Open When They Should

If a steam vent stops working and fails to open, rising steam cannot enter the radiator and it will be slow to heat or may not heat at all.

If your one pipe steam radiator won't get hot, the air vent may be clogged or it may just be worn out and need replacement.

Or as ITT states it:

If your steam radiator won't get hot

As ITT/Hoffman and all other steam vent manufacturers will tell you:

A cold upper part of the radiator may indicate the vent is plugged with scale or is malfunctioning.

See COLD STEAM HEAT RADIATORS for diagnostic and repair suggestions.

Also see this

ITT Hoffman Specialty STEAM VENTS for RADIATORS MODEL 1A 1B AIR VALVE INSTALLATION INSTRUCTIONS [PDF[ (2007), ITT 8200 N. Austin Ave. Morton Grove, IL 60053 tel: 847-966-3700 fax: 847-966-9052


A cold upper part of the radiator may indicate the vent is plugged with scale or is malfunctioning. Vapor or slight moisture from the vent is normal. Steam continuously passing through the vent indicates a malfunction. Replace the vent as needed or every 10 years. - Op. Cit.

Steam vents that won't close properly:

Steam vent on steam piping (C) Daniel FriedmanIf a steam vent stops working properly and fails to close, the vent will hiss and release steam (and sometimes produce condensate or water) continually all during the heating cycle, wasting heat, increasing heating cost, and in some cases creating a moisture problem or even a burn risk at the radiator.

Steam vents on individual radiators or main steam vents may not be opening when they should. If an individual steam radiator vent fails to open, that radiator won't get hot even though its supply valve is open.

But if a main steam vent (master steam vent) is not opening, the whole heating system may continue to work, but we expect to see two problems:

Main steam vents: Our photo (left) shows an automatic air vent found on steam piping in a basement - not all steam vents are located right on the radiators. This is a line vent or main vent, and it too needs to work. (Most likely this is a 2 pipe steam system]

This vent may have been added to help speed rising of steam in a building by purging air from a section of steam piping.

If no air escapes from this valve during the heating cycle it may be that the valve is jammed, is not releasing air, and the steam radiator may not ever warm up.

The valve may need to be replaced if it is corroded, leaky, doesn't vent, or if the steam system appears not to be venting properly.

We also discuss this valve at STEAM VENT TYPES, SELECTION.

Noisy Steam Vents or Steam Radiators

It is normal for steam vents to make a hissing noise (also described as a whistling noise) at the start of a heating cycle while air is being vented out of the steam piping or steam radiator as it is pushed by rising steam.

Watch out: If the radiator continually makes noises (hissing, whistling or wheezing) at the steam vent, there is a problem that needs to be fixed: a bad steam vent, steam piping problem, steam pressure set too high, or boiler oversized for the heating distribution system.

First, make sure that the noise complaint is coming from the steam vent and not from water hammer or clanking steam piping.

Next: mineral deposits or dirt in a steam vent can prevent it from opening or closing properly. While it is often possible to disassemble and clean (soak in vinegar) a mineral-clogged steam vent, other internal vent failures can also occur, so considering the low cost of replacement steam vents, it usually makes sense to simply replace the steam vent entirely.

At RADIATOR STEAM VENTS, 1-PIPE SYSTEMS we list a variety of replacement steam vents.

A rattling sound at a steam vent may be heard at any radiator or at a main steam riser where a main line vent is installed. If your steam vent is rattling an internal part has become loose. Replace the vent.

Watch out: Steam condensate return problems can cause banging clanging steam heating pipes & radiators.

As we discuss

at BANGING HEATING PIPES RADIATORS, if your heating boiler does not have an automatic water feeder and you've been putting makeup water into the boiler manually, a blocked condensate line and low water in the boiler will eventually lead to total loss of heat when the low water cutoff switch.

See LOW WATER CUTOFF CONTROLS , a key boiler safety device, simply shuts down the boiler.

Also see HEATING SYSTEM NOISE DIAGNOSIS - where we list heating system noise types, sources, problem indicators, cures

What to Do About Steam Vents that Leak or "Spit" Water

If water is "spitting" out of your steam radiator's air vent it's most likely that debris, sediment, mineral deposits, or rust flakes have partly clogged the air vent. Steam experts such as Dan Holohan suggest that you may be able to clean a spitting or partly-clogged steam vent by disassembling and boiling the vent in vinegar (to dissolve minerals).

Watch out: don't remove a steam vent when the heating system is in use or hot. You can get badly burned. But some specialty steam vents (air valves) such as the Hoffman Model 508 (Part No. 401475) can be disassembled and cleaned without removing from the line.

If a lot of water is really leaking out of a steam radiator's steam vent, the radiator may also have become waterlogged (according to some writers). We don't think this is a common problem, since most steam vents are mounted at mid radiator height or higher - the radiator would have to be really full of condensate for a lot of water to be coming out of the steam vent - unlikely.

If a steam radiator contains condensate (water from condensed steam) that fills the radiator to a level above the steam vent, the steam vent cannot function, air won't be pushed out of the radiator by rising steam, and the radiator won't heat.

If your steam vent is mostly venting steam but spits a few drops of water, the vent may not be working properly and may need replacement, but it is not as likely to be a sign that the steam system has become waterlogged.

Watch out: are you sure your system is using steam heat? Hot water radiators often include a manual air-bleeder valve that must be opened to let air out of the hot water (not steam) radiator or the radiator won't heat. On a hot water (hydronic) heating system if you leave the air bleeder valve open after air has stopped coming out, indeed water will come squirting out of the valve - indefinitely. Don't do this.

See AIR BLEEDER VALVES for details.

Automatic and thermostatically operated steam vents used to provide room temperature control are discussed in detail also at RADIATOR VALVES & HEAT CONTROLS.

Also see RADIATOR STEAM VENTS, 1-PIPE SYSTEMS where we explain automatic steam vents and heat regulation using air vents on steam radiators

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

If your heating system is not working properly, s ee NO HEAT - BOILER

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: If your building uses forced hot water heat see BOILER CONTROLS & SWITCHES. Some of those controls also appear on steam heating equipment.


Continue reading at STEAM RADIATOR VENT REPAIR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see STEAM VENT FAQs - questions and answers posted originally on this page

Or see STEAM VENT TYPES, SELECTION where we explain more about the different venting requirements for 1 pipe vs. 2 pipe steam systems, we describe the extra controls and vents needed at two-pipe steam systems.

Or see these

Steam Vent Articles

Suggested citation for this web page

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


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