Steam vent on a radiator (C) Daniel Friedman Automatic Steam Radiator Vent Controls for Regulating 1 Pipe Steam Heat

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Automatic steam radiator vents:

This article describes the use, adjustment, diagnosis & repair of automatic, thermostatically controlled steam heating radiator air vents or steam vents to control or balance steam heat in buildings. This article focuses on automatic vents for one pipe steam heating systems.

This article series answers most questions about all types of steam heating systems and gives important inspection, safety, and repair advice.

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.

Automatic or Thermostatically Controlled Steam Vents

Steam vent on a radiator (C) Daniel Friedman

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.

Our photo (left) shows what looks like a Hoffman 1Afloat-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]

Article Contents

One-Pipe Steam Heat

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.

Role of the steam vent on one pipe steam heat systems

Unlike radiator valves that are closed manually or automatically to control the entry of hot water or steam into a heating radiator, steam vents are located at the opposite end of the radiator from the valve and are used to vent air out of a steam radiator so that steam can rise into and heat the unit. When steam reaches and heats the steam vent, the vent is designed to close and stop venting air (or steam), and the radiator will heat.

The steam vent opens, allowing air inside the radiator to be pushed out by rising steam that enters the radiator at the radiator bottom where a radiator valve is present and open.

The "hissing" sound you hear from the steam vent is air being pushed out of the radiator, and is normal. When the steam radiator and steam vent have become warm or hot, the steam vent closes (and is quiet).

Two Pipe Steam Heat Vent Differences

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.

Watch out: two pipe steam heat systems generally do not use the same steam vent models nor are vents at the same locations as steam vents found on one pipe steam heat system discussed in this article.

Two pipe steam vents and the venting differences between one pipe and two pipe steam systems are discussed at STEAM VENT TYPES, SELECTION where we add detail about venting requirements for 2-pipe steam systems and the requirement for steam traps and vacuum valves.

1 Pipe System Steam Vents: Choosing & Installing Adjustable or Thermostatically Controlled Steam Vents to Control One Pipe Steam Heat

Types & Properties of Steam Radiator Vents

Adjustable steam vent helps regulate heat output (C) Daniel FriedmanBy controlling the rate at which a steam radiator gets hot, adjustable steam vents are a key instrument for balancing steam heat in a building. Each of the following steam vent types is discussed in the article below.

What about 2 pipe steam system venting?

Separately at STEAM VENT TYPES, SELECTION we explain the different venting requirements for 2 pipe steam systems and we describe the types of steam vents by where they are used in the system: at radiators, on steam mains, on convector heaters etc. for both 1 pipe and 2 pipe systems.

Watch out: although adjustable steam vents can control the rate at which a steam radiator gets hot, these adjustable air vents are not identical in function and application nor is heat control using adjustable steam vents identical with that provided using thermostatically controlled radiator valves.

Thermostatically-controlled no-float steam vents

Hoffman specialty steam vent (C) Hoffman Specialty

Watch out: Some "thermostatically operated" steam vents include user-adjustable controls that permit the steam vent to operate as an individual room thermostat. But watch out: other "thermostatically operated" steam vents may be units that are not adjustable for controlling room temperature. These "thermostatically controlled" vents are describing how the vent works internally, not its function as an occupant-adjustable room temperature control.

Hoffman Specialty heating products offers thermostatically operated steam vents (such as the Hoffman Special Steam Vent Model 3 (Part No. 401419) no internal float, image at left) that can automatically regulate heat from individual steam radiators.

These vents are used for Air Line or Paul Systems. The Hoffman Specialty Thermostatic Temperature Regulators (Series 1140 & 1141) are designed for commercial and institutional HVAC systems. These devices permit a set temperature ranging from 40°F. through 220 °F in increments of 40 °F. (no fine tuning).

Watch out: these steam regulators fail "open" (meaning heat fully on), so Hoffman warns that an alarm or cut-off must be installed where overheated water (or steam) could cause harm. Failure to follow this warning could cause serious burns, personal injury, or death.

For more conventional steam vents, take a look at the Hoffman Specialty series 2000 and the Hoffman Model 3 Steam Air Line Valve (Part No. 401419) for an example.

A Hoffman Vent, Model 3, is also thermostatically controlled, operates on temperature only, and does not close against water. Hoffman also produces conventional steam radiator vents such as their traditional Model 1A air valve, and also the Hoffman Model 74 float operated steam unit heater air valve. But these vents do not include a readily-accessible temperature adjustment. For steam heating convectors, different steam vent models are required due to the different operating pressures and vent locations that may be present.

Watch out: when replacing a steam vent be sure that you buy the proper vent for your heating radiator or convector type, or that the steam vent specifications of the new steam vent match the old one. Hoffman and other control manufacturers provide selection guidelines for steam vents as well as their other controls.

Float-type steam vents

As we introduced above, float type steam vents use a water+alcohol-filled float inside the steam vent heats, rises, and closes the vent as the radiator heats.

Vent-Rite No 51 adjustable steam radiator vent being disassembled (C) Daniel Friedman

The float also rises to prevent water from spitting out of the vent should condensate rise inside the radiator or vent. Rust or sediment can clog the float vent opening and interfere with its proper operation.

Excessive steam pressure can also prevent the vent from operating properly.

At STEAM RADIATOR VENT CLEAN & TEST we show the disassembly and inspection of the float-type steam vent shown just above.

Adjustable Steam Radiator Vents Permit Adjusting Radiator Vent & Heat-Up Rates

Adjustable steam radiator vent: the Vari Vent (C) Daniel FriedmanAlso as we introduced above, manually adjustable steam vents operate similarly to the float-type steam vents described above, but add the feature of adjustable air vent opening size so that the air venting rate can be better matched to the radiator size. (Larger steam radiators need a larger air vent opening.)

Shown at left: the adjustable VariVent steam radiator vent from VariValve.

Examples of manually adjustable vent-rate steam vents and their steam venting rates include:


  1. Steam radiator vent rates and system pressure: If your steam system pressures are lower than the 1" W.C. test data above, (as likely the case) the steam radiator vent rates will be slower, but the vent rate relationships among these valves probably will be the same.
  2. Main line air vents are also used on steam systems to quickly purge air from the main steam pipes ahead of the radiators. Hoffman, Gorton, and Heat-Timer also provide main line steam vents and automatic or adjustable steam vents for the main lines in steam heating systems.

Additional steam vents are listed at REFERENCES below.

Especially in a single pipe steam heating system, the venting rate of radiators affects how the heating system works, how quickly heat rises in individual rooms, and the ability to balance the delivery of heat among different building areas.

Adjustable Thermostatically-operated steam vents

Thermostatically controled adjustable steam radiator ventThermostatically-operated steam vents (shown here) include an adjustable room temperature thermostat that allows the occupant to set the desired room temperature.

Setting this control actually adjusts the rate at which the steam vent permits air to escape from the individual radiator, similar to the adjustable steam vents described above, but in this case once the thermostat is set, the automatically regulating steam vent is responding to the room temperature setting.

Some adjustable automatic air vents for steam radiators include:

Thanks to reader Paul Ruud for discussing improved steam heat controls and thermostatically operated steam radiator valves and air vents.

Watch out: If the radiator continually makes noises (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.

Watch out: to be sure your steam heating system and its controls are properly adjusted excessive steam pressure can be dangerous.


Continue reading at STEAM RADIATOR VENT REPAIR where we discuss both cleaning clogged steam radiator vents and parts replacement for these devices, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see STEAM VENT TYPES, SELECTION for a separate guide to all types of steam vents used on steam heating systems.

Or see STEAM RADIATOR VENT CLEAN & TEST for simple cleaning and testing procedures for steam vents - avoiding disassembly.

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


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