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Here we explain the purpose and function of steam traps or Hoffman traps on steam radiators.
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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 article series 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.
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.
Shown at left (adapted from ITT's Steam Book ) 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.
Is the Steam Trap Working?
ITT reports 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. 
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.
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 ) 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 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.
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.
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:
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
Questions & answers or comments about radiator steam trap function, installation, repair or replacement: Hoffman steam traps and similar devices .
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.