Guide to Steam Traps on Steam Heating Systems

Guide to Hoffman traps or Steam traps on steam heat radiators - schematic & photos of steam traps
- Functions of a steam trap or Hoffman trap - what's the difference between a thermostatic steam trap and a Thermostat & Float type steam trap?
- How the Hoffman Trap or Steam Trap Works
Troubleshooting Steam Traps that are not working properly
- Float & Thermostat type Steam Trap Troubles
- Thermostatic Steam Trap Troubles
Dirt & Debris in the Steam Trap - how steam traps at the boiler are cleaned
Tests for steam trap operating temperatures - crayon
Changing out an Old Steam Boiler May Mean Changing the Steam Traps
Questions & Answers about radiator steam trap function, installation, repair or replacement: Hoffman steam traps and similar devices
References

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Here we explain the purpose and function of steam traps or Hoffman traps on steam radiators.

Hoffman traps or Steam traps on steam heat radiators

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 your steam heat radiator is not getting hot the problem could be the steam trap. This website answers most questions about all types of heating systems and gives important inspection, safety, and repair advice. 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 steam trap on a two-pipe steam heating system.

Functions of a steam trap or Hoffman trap

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

Permit air to pass out of the radiator as steam is entering (yellow in our sketch) notice that unlike a one-pipe steam system, some two-pipe steam radiators do not use nor have attached Steam Vents.

Air enters steam radiators through the steam vents when the system is off and cooled-down; air is also pushed up into steam heat radiators through the steam piping as the steam heat system is warming up, leading steam into the radiators.

If air cannot escape from a steam radiator, the radiator will not heat up.
Permit water (steam condensate - blue in our sketch) to pass out of the radiator
Reduce steam heat system corrosion: because carbon dioxide CO2 enters in the boiler feed water and passes through the system piping as a gas, if we do not vent the CO2 gas the water and steam condensate in the system will become acidic and thus corrosive. Steam condensate is high in dissolved CO2. Venting air and condensate out of the radiator through the steam trap avoids accumulating acidic condensate.

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

Shown at left (adapted from ITT's Steam Book [2]) is a thermostatic steam trap. 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.

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.

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

A 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.

Hot Float & Thermostat type steam trap: check for a failed thermostat - it's not opening or closing the valve in the upper part of the trap. On an older trap the valve pin and valve seat (see the valve seats at both green and red areas of the sketch - could be worn and need replacement. If the valve seat is worn the valve may be unable to close - hence the trap will be too hot.
Cold Float & Thermostat type steam trap: a clogged trap that can't open will not drain condensate and will prevent heat (and steam) from entering the trap body (or the radiator); check for a clogged trap strainer - clean or replace it. check for a blockage in the condensate drain piping downstream from the trap. Check the trap itself for debris clogging (look at the assembly above the green color in our (adapted from ITT adapted from Hoffman) sketch above.
Slow heating of the radiator or uneven (inconsistent) heating of the radiator could be caused by a wrong sized Float & Thermostat trap (unlikely) or a failed thermostat that is not allowing air to escape.

Thermostatic Steam Trap Troubles

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.

Hot steam trap: the steam trap discharges continually - it never closes, probably because of dirt on the valve seat or because the trap bellows has failed. You can detect this by sound and by noting that the trap is always hot (it is passing steam) and the radiator is not heating properly. Repair: clean the trap; replace a failed trap bellows assembly.
Cold steam trap: the trap is not opening, so it is not discharging condensate and the radiator is not heating. Typically this is caused by a failed bellows assembly - that failure can leave the trap stuck open (hot steam trap) or closed (cold steam trap). ITT noted that water hammer in the system can damage the bellows assembly. [2]

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]

The boiler is turned off.
The valves to every radiator are closed.
A pound of boiler cleaning compound (such as MEX™) is added to the boiler through the relief valve by dissolving the cleaner in hot water.
The boiler is turned on and heated to 5 psi with the water feed valve "open".
The plumbing valve (usually a ball valve) at the Hartford loop is opened to drain waste and debris from the system into buckets (or drained with a sufficiently large hose into the sewer) until the water runs clear.
The boiler is turned off.
The drain valve is closed.
The radiator valves are returned to open.
The boiler is turned back on and checked for proper operation and for no leaks.

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

Because 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.

Bucket traps on steam systems

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Frequently Asked Questions (FAQs) about Steam Traps

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[2] The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
[3] Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
[4] The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
[5] Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
[6] "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
[7] "Steam Boiler Operations" (Program 4, booklet & video), Weil-McLain H.E.A.T. (Hydronics Education and Training), Blain St., Michigan City Indiana 46360 Tel: 219-879-6561
[8] Tempil Division, Big Three Industries, Inc., South Plainfield N J 07080
Audels Oil Burner Guide, Installing, Servicing, Repairing, Frank D. Graham, Theo. Audel & Co., New York 1946, 1947, 1955 (out of print, copies occasionally available from antique book dealers and on EBay)
The Steam Book, a Primer for the Non-Engineer Installer, Wallace Eannace Associates, Inc. 1984, reprinted by ITT Fluid Handling Division. Original articles appeared November 1982 - October 1983, "The Problem Solver - Steam without Tears", by Wallace Eannace Assoc., edited and supplemented in The Steam Book for use by non-technical support people to help identify steam heating problems. If you can find a copy of this book we recommend purchasing it for its clear and concise explanation of key steam heating concepts.
Barnes & Jones Corporation, 91 Pacella Park Drive Randolph, MA 02368, Tel: 781-963-8000calibrated steam traps and vents, Email: bnesbitt@barnesandjones.com. Vent-Rite steam vents for radiator control.
Energy Savers: Heat Distribution Systems, Steam Radiators & Hot Water Radiators, U.S. Department of Energy, web search 12/27/2010, original source: http://www.energysavers.gov/your_home
/space_heating_cooling/index.cfm/mytopic=12580
Gorton Heating Corporation, 546 South Avenue East, Cranford, New Jersey 07016, Tel: 908-276-1323, email: info@gorton-valves.com, fast venting valves for steam heating systems.
"Rv-4 One-Pipe Steam Radiator Valve", available from Armstrong International, 816 Maple Street, Three Rivers, MI 49093 USA, Tel: (269) 273-1415, Armstrong has offices in Beijing, China, Belgium, India and Mexico. Web search 12/27/2010, original source:
http://www.armstronginternational.com/thermostatic-valves-rv-4-one-pipe.
"Special Steam Vents", Hoffman Specialty heating products, web search 12/27/2010, original source:
http://www.hoffmanspecialty.com/pdf/hs900/HS900-3474.pdf
"Steam and Water Vents, Selection Guidelines - Steam Vents", Hoffman Specialty, web search 12/27/2010, original source:
http://www.hoffmanspecialty.com/pdf/hs900/HS900-stvents.pdf
Hoffman 1A adjustable steam vents, available from Hoffman Specialty, ITT, web search 12/27/2010, original source:
http://www.drillspot.com/products/51570/Hoffman_1A_Adjustable_Automatic_Air_Vent_For_Steam_Radiators
Hoffman Convector Steam Vents, available from Hoffman Specialty, ITT, web search 12/28/2010, original source:
http://www.hoffmanspecialty.com/pdf/hs900/HS900-conv109.pdf
Report 95-14, PB96-198163 Thermostatic Radiator Valve (TRV) Demonstration Project, contact NYSERDA. 17 Columbia Circle, Albany, NY 12203-6399 Toll-Free: 1-866-NYSERDA, Tel:(518)465-6252, Ext. 241. Web Search 12/27/2010, original source: http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/96/960509.html
VariValve® Quick-Vent from Heat Timer, adjustable radiator and main line vent valves for one pipe steam systems, web search 12/27/2010, original source: http://www.heat-timer.com/enFiles/ProductDocument/literature/VariV056082C.pdf
"Care & Feeding of Air Vents", Dan Holohan, Old House Journal Online, November 2004
Steam Boiler Inspection, Diagnosis, Repair books at the InspectAPedia bookstore - Amazon.com
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update, (see next item in this list). ITT Fluid Technology, 1133 Westchester Avenue
White Plains, NY 10604, tel +1 914 304 1700 fax +1 914 696 2950 www.ittfluidbusiness.com
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
The Heat Efficiency of Steam Boilers, Bryan Donkin
Steam Boiler Operation, Principles & Practice, James J. Jackson

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Hoffman traps or Steam traps on steam heat radiators

Functions of a steam trap or Hoffman trap

How the Hoffman Trap or Steam Trap Works

Troubleshooting Steam Traps that are not working properly

Is the Steam Trap Working?

Tests for Steam Traps

Float & Thermostat type Steam Trap Troubles

Thermostatic Steam Trap Troubles

Tests for Steam Traps

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

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

Bucket traps on steam systems

Frequently Asked Questions (FAQs) about Steam Traps

Ask a Question or Search InspectApedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Guide to Steam Traps on Steam Heating Systems