Hoffman steam trap (C) Daniel Friedman Guide to Steam Traps on Steam Heating Systems
Explanation & identification of steam traps: Hoffman trap, mechanical traps, ball & float traps, inverted bucket traps

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Steam trap types, applications, & locations:

Here we explain the purpose and function of steam traps use on steam piping systems at the radiator, in the steam piping system, and on the steam mains.

If your steam heat radiator is not getting hot the problem could be the steam trap.

This article series answers most questions about all types of heating systems and gives important inspection, safety, and repair advice. Here we include a discussion of Hoffman traps on steam radiators and on steam piping, thermostatic steam traps, ball & float steam traps, and mechanical steam traps: inverted bucket type steam traps.

We include identification photographs of different types of steam traps and we list steam trap sources & manufacturers where more technical specifications and literature are available. Our page top photo shows an inverted bucket steam trap incorporating a cooling coil connected to the steam piping system in a private home.

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Steam Traps: types, locations, identification, troubleshooting, replacement

Sketch of radiator with steam trap and hoffman supply valve[Click to enlarge any image]

Article Contents:

As we introduced at STEAM HEATING SYSTEMS, steam traps such as the Hoffman-style thermostatic-type steam trap shown at the top of this page are installed on residential steam heating systems (usually at the bottom of the radiator at the opposite end from the steam input side) 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).

If you don't know what kind of heat your building uses, see our introduction at BOILERS, HEATING.

If your heating system is not working properly, see NO HEAT - BOILER or NO HEAT - FURNACE.

Our photo (page top) shows a typical Hoffman steam trap at the radiator on a two-pipe steam heating system.

Functions of a steam trap or Hoffman steam trap

Steam trap on the steam main (C) Daniel FriedmanOur photo (left) illustrates a steam trap located on the steam piping system in a residential building.

The functions of the steam trap, also often referred to as the Hoffman Trap in the field, include:

Steam traps may be found on steam heating systems both on radiators (at the radiator bottom opposite end from the entering inlet valve), and on older steam heating systems a steam trap may also be found on some steam piping where the trap handles condensate produced inside the steam riser piping.

How the Hoffman Trap or Steam Trap Works

Hoffman steam trap (C) Daniel Friedman Steam trap schematic

Shown at above left is a steam trap at a steam radiator outlet and at above right is a thermostatic steam trap schematic (adapted from ITT's Steam Book [2]). In this thermostatic steam trap design, used at radiators and steam rising in the two pipe steam heating system piping enters the radiators, usually through a Hoffman-type supply valve near the top of the radiator, and a mixture of steam and condensate is separated by the steam trap at the radiator outlet.

Air escapes: he incoming steam pushes air ahead of itself and out through the steam trap at the radiator bottom..

Steam is trapped: as steam begins to pass out through the steam trap it warms the thermal element inside the trap.

A "bellows" [(c) in the sketch above] inside the steam trap is actually a flexible sealed container filled with alcohol and water - a mixture that boils at a lower temperature than the temperature of the steam itself. Thus as the bellows boils (warms up) it expands, closing the steam trap.

The steam trap stays closed until sufficient condensate has been produced inside the steam radiator to enter and cool the steam trap. The condensate then causes the bellows to cool, shrink, open the steam trap.

Condensate escapes: As the incoming steam cools inside the radiator, returning to its water state as condensate, condensate falls to the bottom of the radiator and also needs to exit through the steam trap. Exiting condensate follows return piping back to the boiler.

Ball Float Steam Traps

A ball float type trap sense the difference in density between steam and condensate. This type of steam trap is very responsive to conditions in the steam system and provided the ball float trap includes an automatic air vent, the trap will discharge condensate quickly "as soon as it is formed" independent of changes in steam pressure within the system.

These are the only suitable steam trap on a system where steam locking occurs, and these traps in their modern version are resistant to water hammer in the steam system.

Condensate that enters the trap will cause a ball float to rise inside the trap, lifting the valve off its seat and releasing condensate. In this design the ball float steam trap valve is always flooded so neither steam nor air will pass through it.

Although they are generally larger and more robust, ball float steam trap mechanical operation is not unlike that of some radiator STEAM VENTS except that the latter are operated by temperature rather than the level of condensate within the trap.

Early ball float steam traps were vented using a manually operated cock at the top of the body. Modern traps use a thermostatic air vent.

Spirax Sarco notes that previously if a steam system was suffering from water hammer the ball float stem trap could become damaged but comments that modern ball float steam traps are robust and can endure water hammer.

Inverted Bucket Steam Traps

Inverted bucket steam trap schematic adapted from Armstrong Industries (C) Armstrong Inspectapedia 2014Bucket type steam traps like those shown in our steam trap identification table above and listed in our manufacturers list below are a robust mechanical steam trap that reduces banging pipes and clanging steam radiators by resisting water hammer.

Bucket type steam traps are resistant to water hammer problems that occur in some steam systems and unlike the ball float steam traps described just above, bucket traps tolerate high steam pressures (not found on residential steam heat systems) and also can be used on superheated steam systems.

The image at left adapted from Armstrong Industries illustrates the components and operation of an inverted bucket steam trap. - Armstrong Industries (2014) & Daniel Friedman

How Bucket Steam Traps Work:

The name of this steam trap describes the bucket steam trap operation. Inside of this steam trap an inverted bucket-container - formed like an upside-down bucket - is attached to a mechanical lever that is in turn connected to a valve controlling condensate outlet from the device.

There is a small air vent in the top of the bucket.

As steam enters the bucket trap at its bottom the bucket floats "up" in the condensate-filled trap, lifting the lever that closes the condensate outlet valve on the trap. Condensate inside the trap flows down (and air moves up) inside the trap.

As the condensate level rises (and as air and some stem escapes out of the interior of the bucket through the air vent) condensate eventually accumulates enough inside the inverted bucket to allow the bucket to drop, opening the condensate valve outlet from the trap.

This mechanism has in effect "trapped" condensate, separating it from the steam and allowing it to flow out of the trap.

As Armstrong International explains the operation of their bucket steam traps and the effects of wear as follows:

Armstrong inverted bucket steam traps open and close based on the difference in density between condensate and steam—the inverted bucket principle. They open and close gently, minimizing wear.

This simple fact means that inverted buckets are subject to less wear than some other types of traps. In fact, as an Armstrong inverted bucket trap wears, its tight seal actually improves . The ball valve and seat of the Armstrong trap provide essentially line contact—resulting in a tight seal because the entire closing force is concentrated on one narrow seating ring.

An Armstrong inverted bucket trap continues to operate efficiently with use. Gradual wear slightly increases the diameter of the seat and alters the shape and diameter of the ball valve. But, as this occurs, a tight seal is still preserved—the ball merely seats itself deeper. - Armstrong International (2014)

The bucket trap does have its own limits and vulnerabilities: it can be damaged by freezing, it cannot vent air at high rates, it needs to be protected from fluctuating steam pressures and from water loss. If the trap runs dry the result will be wasted steam.

Main Drip Traps on Steam Piping Mains

Main steam drip trap schematic adapted from Watts Water Technologies (C) InspectAPedia 2014Watts Water Technologies (2104) and other steam trap experts point out that a main drip trap should be used for every 100 to 150 feet of straight steam piping run.

Main steam piping drip traps should be installed where the steam piping changes elevation (at the lower end of that change of course), in front of expansion loops, and at steam risers

Watts points out that because the condensate quantity (load) at these main drip trap locations is small, usually the main drip steam trap will not be larger than 2/4" (20mm).

The schematic below is adapted from Watts Water Technologies. Watts (and other manufacturers) will provide a chart of recommended steam trap products depending on the psi operating range of the steam system.

Comparing Mechanical Steam Traps: Ball Float & Inverted Bucket Traps on the Steam Main

Two types of mechanical steam traps are found on the mains of commercial and some residential steam systems: Ball Float steam traps and Inverted Bucket steam traps. These explanations are adapted from Spirax Sarco's excellent free online tutorial found at the citation we provide below. Quoting from Spirax Sarco, a producer of all types of steam traps,

Mechanical steam traps rely on the difference in density between steam and condensate in order to operate. They can continuously pass large volumes of condensate and are suitable for a wide range of process applications. Types include ball float and inverted bucket steam traps. - Spirax-Sarco (2014)

The photographs of mechanical steam traps shown below can aid in recognition of these features on steam heat systems.

Table of Brands & Models of Steam Traps with Identifying Photographs

Mechanical Steam Trap Brand & Model

Steam Trap Photo

[Click to enlarge
any image]

Armstrong International 813 inverted bucket steam trap Armstrong international inverted bucket steam trap - bucket trap (C)

3/4" NPT, 2" NPT

Operating Pressures to 125

Horizontal steam piping

Armstrong inverted bucket steam traps handle differential pressures according to model series:
Series 200 & 800: 10 - 125 psi
Series 300: 125 - 600 psi
Series 400: 600-1000 psi

The products vary in capacity measured in pounds per hour from 300 to 25,000 or measured in kg/hr fro 150 to 10,000 kg/hr

Armstrong International #1811 steam trap Armstrong international #1811 high pressure inverted bucket steam trap - bucket trap (C)

3/4" NPT

Stainless steel

Operating Pressures to 125 psi

Clark Reliance 119 inverted bucket steam trap Hoffman bucket trap steam trap #404300 1/2-inch inverted bucket Series B1 for steam mains (C)

3/4" NPT

20 psi

Inverted bucket steam trap

Hoffman Series B1 inverted bucket trap for steam mains Hoffman bucket trap steam trap #404300 1/2-inch inverted bucket Series B1 for steam mains (C)

Hoffman 404300 B1015A-2 (at left)

Hoffman Specialty B3125A-5 B3125A4 404427 Bucket Trap operates at up to 250 psig.

1/2" NPT, 3/4" NPT, other

Inverted Bucket Steam Trap

Used on steam mains, laundry steam piping & other. These traps are also found painted other colors such as a gray-green.
ITT Hoffman Specialty, Indianapolis IN, USA. Trap data tags typically indicate model, size (NPT), maximum pressure differential, a maximum allowable pressure (e.g. 250 psig at 425°F, and a date code.

ITT B3030A steam trap Hoffman bucket trap steam trap #404300 1/2-inch inverted bucket Series B1 for steam mains (C)

1" NPT

30 psi

Juker SA8 inverted bucket steam trap Juker SA8 inverted bucket steam trap (C) InspectApedia

Juker P/N SA 8 SA-8

1/2" NPT

Top fitting 1/4" NPT

Pennant steam trap Spence Engineering steam trap 84-CSF9 (C) InspectApedia  
Spence Engineering 84-CSF9 steam trap Spence Engineering steam trap 84-CSF9 (C) InspectApedia

Inverted bucket steam tarp

250 psi

1 1/4" NPT

Spence Nicholson 80s steam trap Steam Trap, Nichols 80s inverted bucket steam trap (C) InspectApedia 3/4" NPT
United 850 steam trap United 850 steam trap (C) InspectApedia

Model 850 1/2" steam trap

50-200 psi

Other similar steam trap models: Model 849 1/2", Model 849A/B 3/4" steam trap

United Brass Works, Inc,

Watts Thermostatic Radiator Steam Trap Watts thermostatic steam trap used at radiators (C) InspectApedia

These traps are recommended for use at the outlet of steam radiators and steam convector heaters. When the radiator is cool the trap opens to allow discharge of air (allowing steam into the radiator) and cooled condensate that may be in the radiator bottom

When the radiator and thus the condensate water are hot the valve closes.

Watts Water Technologies produces a wide range of steam traps including

  • Watts Thermostatic Radiator steam traps
  • Float & Thermostatic steam traps
  • Watts Inverted Bucket steam traps
  • Watts Open Float & Thermostatic steam traps
  • Watts thermodynamic steam traps
Watt float type steam trap Watts float trap steam trap (C) InspectAPedia Watts Water Technologies

This air vent on this steam trap is open when the system is cool - at the start of a heating cycle - allowing large volumes of air to discharge into the condensate return piping.

The float in this trap is closed and stays closed from start-up and while steam is in the trap body.

When hot condensate accumulates in the trap body the float lifts, closing the valve head and the trap. This product is intended to continuously discharge condensate in modulating conditions and is used on heat exchangers, air handling coils, and steam MAIN DRIP stations.

Watts thermodynamic steam trap Watts thermodynamic steam trap (C) InspectApedia Watts Water Technogies (2014)

Thermostatic steam traps such as this Watts unit are used as high pressure steam main distribution line traps in the 75-600 psi range (and so won't appear on common low pressure residential steam heating systems).

At the start of a heating cycle the trap is open to discharge into the condensate line. As hot condensate "flashes to steam" as it passes through the trap body the valve closes and stays closed until the flash steam condenses.

Watts open float thermostat steam trap Watts open float type steam trap (C) InspectApedia Watts Water Technologies

Watts open float steam traps are used on low and medium pressure steam systems on the MAIN DRIP trap and also on heat exchangers, air handler coils, and steam process applications.

Watts explains the valve operation in four steps:

  1. Condensate fills the trap until it overflows into float. When the weight of the condensate overcomes the buoyancy of the float, the float begins to drop independent of the float valve head
  2. The float continues to drop until the collar at the bottom of the valve stem engages the internal stop. At this impact point, the float falls to the bottom of the trap snapping the valve open.
  3. Condensate travels up the discharge tube, through the orifice and out the outlet port. The float will remain at the bottom with valve fully open so long as there is sufficient condensate entering the trap
  4. As the discharge drains the float, buoyancy returns and the float begins to rise. The valve head is snapped closed into the valve seat by the velocity of the discharging condensate. - Watts Water Technologies (2014)
Watts inverted bucket steam trap Watts inverted bucket steam trap (C) InspectApedia Watts Water Technogies 2014

This inverted bucket trap used on steam systems must be manually primed at start up to create a water seal around the inverted bucket that allows this trap to operate.

At start of a heating cycle the trap is open allowing air and condensate to enter the steam trap body. Air is discharged through a vent at the top of the device (see INVERTED BUCKET STEAM TRAP OPERATION).

When steam enters the body of this inverted bucket steam trap the buoyancy of steam lifts the bucked, closing the valve in the trap head, closing the air vent.

When condensate accumulates in the bucket trap the bucket falls, opening the air valve and also allowing condensate to drain into the condensate return line.

These bucket traps are used as MAIN DRIP traps at pressures up to 250 psi and also on some steam systems where minimal air venting is acceptable.

Watson McDaniel Watson W1031 series bucket steam trap Watson inverted bucket stam trap Series W1031 (C) InspectApedia

Watson @1031-13-150 W103113150 Bucket Steam Trap, also appears painted blue as in model shown below

3/4" NPT

150 PSI


Watson McDaniel inverted bucket steam trap Series 1038 Watson steam trap No. 1038 (C) InspectApedia Inverted bucket steam trap

Note: varying models of most of these steam traps accommodate a wide range of pipe sizes and fittings in both female and male NPT threads. Prices also vary widely depending on application, brand, model, and also used, refurbished, or new.

The steam traps shown in these identification photos are only representative examples. You will want to consult the manufacturer, the manufacturer's parts catalog, or your heating equipment supplier to be sure of selecting the proper steam trap for your application. See STEAM TRAP MANUFACTURERS for a list of suppliers and steam trap supply contact information.

Troubleshooting Steam Traps that are not working properly

Is the Steam Trap Working?

ITT reports[2] that steam traps have about a three-year life expectancy, and that on an older steam system chances are the steam traps are not working.

An external visual inspection doesn't tell if the steam trap is functional or not, but in addition to the "tests" we discuss below, observation that radiators are noisy, banging, pounding, or not heating, or too hot, can all point to a steam trap problem - if a steam trap is installed.

Steam traps may be found at the bottom of a steam radiator on its outlet end, or on steam piping in the condensate return system where they are serving the piping itself.

Tests for Steam Traps

Figuring out if a steam trap is working properly - that is, opening and closing at the proper temperatures - has been described as complicated enough that books have been written on the problem. But ITT recommends a simple practical approach that can make a rough test. [2]

Rough steam trap test:

A special temperature-sensitive colored crayon is used to test steam traps. The user makes a crayon mark on a steam pipe, radiator, or right on a steam trap. The crayon melts if the temperature of the tested surface is hotter than it should be. Tempilstiks are sold by Tempil Division and probably other distributors for this purpose.[8]

Float & Thermostat type Steam Trap Troubles

Steam float and thermostat trapA float-and-thermostat steam trap, if present, is more likely to be found on newer, modern steam heating systems in which the boiler and piping heat up rapidly.

The trap is operated (opened or closed) by a float assembly that will drain condensate through the trap without depending on temperature or heat-up time.

Our sketch (left, adapted from ITT's Steam Book [2]) illustrates a float and steam trap. That round ball is the float ball. You can see the red thermostat at the top of the image, and the green color indicates where condensate can flow out of the trap.

You can see that condensate can exit the trap either through the thermostat port or through the interior of the trap body, depending on the float position. A common float and thermostat steam trap found on residential buildings is the Hoffman 53-FT.

Thermostatic Steam Trap Troubles

Steam trap schematic
Thermostatic steam traps (left) include only a temperature sensing thermostat - no float assembly. The steam trap shown at the top of this page is a thermostatic type trap - as you can recognize from its size and body shape.

Thermostatic steam traps are devices that operate more slowly than "float & thermostat steam traps" described above, and they are more likely to be found on older, slow-heating steam systems.

Tests for Steam Traps

Rough steam trap test: colored crayon method

A special temperature-sensitive colored crayon is used to test steam traps. The user makes a crayon mark on a steam pipe, radiator, or right on a steam trap. The crayon melts if the temperature of the tested surface is hotter than it should be. Tempilstiks are sold by Tempil Division and probably other distributors for this purpose.

Dirt & Debris in the Steam Trap - how steam traps at the boiler are cleaned:

When a boiler is first installed or has just been replaced, the steam trap (and other steam components) should be cleaned to flush out debris stirred by the mechanical activity on pipes and equipment during installation or repair.

If you see a plumbing valve between the Hartford Loop and the system piping and a tee and cap on the piping on the boiler side, these fittings were probably installed to permit the Hartford Loop to be opened and cleaned without sending crud into the steam boiler itself.

ITT provided this procedure [adapted and paraphrased] for cleaning the steam trap once the fittings above are installed:[2]

Changing out an Old Steam Boiler May Mean Changing the Steam Traps

Steam float and thermostat trapBecause newer steam boilers heat faster, hold less water, and push steam faster than the older steam boilers, thermostatic steam traps like the Hoffman trap are not likely to be found.

Instead the installer places float and thermostat traps.

The F&T trap as these devices are called in the trade, achieve the same function as the traditional steam trap, but the float switch will open to permit condensate to drain regardless of the (presumably high) temperature inside the trap.

Hot condensate will drain easily and more rapidly out of an F&T trap on a modern steam system.

Hoffman also manufactured float and thermostatic traps such as the Hoffman 53-FT used on steam risers and at the end of main condensate drips.

Mechanical & Thermostatic Steam Trap Suppliers

Spirax Sparco Steam and Condensate Loop Book ISBN 978-0-9559691-5-4 at







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