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Guide to steam heat radiator piping connections: this article describes the piping connections for steam heating radiators. We explain the difference between one pipe and two pipe steam heating systems.
We illustrate upfeed steam pipes, downfeed steam pipes, and we make clear how to figure out what type of steam heat is installed in a building - insofar as the steam heat distribution is concerned. We explain how steam rises or enters radiators and how condensate in a steam radiator gets back to the steam boiler.
We also describe antique vacuum / vapor steam heat systems. Our page top sketch, adapted from ITT's The Steam Book, shows piping connections for a one pipe steam heating radiator.
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Question: where do steam pipes enter steam radiators: top, bottom, or both?
I had a question about the article
I’m looking at the sub-section “Types of Radiator Valves: Hot Water vs. Steam” and here is a copy of the 1st paragraph; the writing becomes nonsensical and near the end and it may turn out that some of the information is [was - we've fixed it] incorrect: “In our photo at left you can see not only the radiator control valve, but lots more information:
[Click to enlarge any image]
We can conclude that this is a hot water heating system, not a steam heat system because first, the valve is mounted at the top of the radiator (water, not steam - steam enters at a radiator bottom but sometimes so does not water; the reverse is never true).”
Photo at above-left illustrates a two pipe steam heating radiator installed at Vassar College.
The control shown on the inlet end at the upper right of the radiator is a No. 8 / 3000-2 Piston Operator from Johnson Service Company, Milwaukee, now Johnson Controls. 
I was trying to make sense of what was written and discovered another article (which also has nonsensical syntax- it must be hard to describe these hot water and steam heating systems) which seems to indicate that in fact steam DOES often enter a radiator at the top.
At this point I am just lost. We don’t have many residential systems with radiator systems (steam or hot water) so I am trying to edumacate myself but don’t have anything in front of me to compare what I think I understand.
I am totally reliant on clear, logical, linear and detailed examples which your site usually has in abundance (thank you by the way.) Here is the link to that other article (refer to the 5th question/answer pairing): oiltechtalk.com/pages/cast_iron.htm
Kind Regards, -Doug
Up-feed - Down-feed, One-pipe - Two-Pipe Steam Piping & Radiators - What's the Difference?
Doug, the other page (not at InspectApedia) that you gave provides information from Dan Holihan  - Dan is probably the most-expert fellow alive when it comes to steam heating systems. Dan's text to which you refer includes these two statements:
Mr. Holihan is a steam heat expert - I absolutely defer to Dan, but I'm not sure that the word usually in the Dan H's second statement can be taken to mean always. 
Most two-pipe steam heat radiators will indeed show up with the steam entering the radiator at one end at the radiator top (photo at left), and the condensate return will exit at the bottom of the radiator at its opposite end.
We do on occasion find a two pipe steam system with both steam in and condensate out connections at the radiator bottom, and for sure, all one pipe steam radiators are fed with a pipe connection to the bottom of the radiator.
All one pipe steam heat radiators are fed from the radiator's bottom at one end - as we explain and illustrate later in this article.
At above left is a top-fed two pipe steam radiator at Google Headquarters in New York City.
Steam to Water Radiator Conversions
At left is a bottom-feed (two pipe) hot water (not steam) heating radiator. What's interesting about the radiator at above right is that it was converted to hot water heat from prior two-pipe steam heat.
Notice that crazy location of the air bleed valve on the left side of the radiator?
That's where a steam vent used to mount. You'll have a heck of a time getting all of the air out of this radiator now that it's converted to hot water - at least not through that bleeder.
This was originally a two pipe top-fed steam radiator when it was first installed. A steam vent worked correctly in the location we show and where now an air bleeder (for hot water heat) has been installed.
This won't work.
On a hot water heating radiator we need the air bleed to be at the top of the device - where the air will be.
In clarifying what we are talking about here I will include some sketches from ITT's now antique Steam Book. 
Up-Steam Pipe Feed Direction: Down-feed vs Up-Feed: Two Pipe Steam Heat Radiator Connections
Downfeed Steam Risers Entering at Radiator Top
Yes of course Dan is right that there absolutely are also "downfeed steam risers" on some two pipe steam systems (sketch at left).
These "downfeed steam risers" (sounds like an oxymoron) push steam into the radiator from a valve entering at the top
Sketch at left - ITT 
Downfeed steam piping that enters at a radiator top will always be two-pipe steam heat systems and the condensate return will always be take off of the bottom of the radiator. . Shown in the ITT sketch are Hoffman Traps. A steam trap, check valve or other device is required on two pipe steam systems to keep steam from entering the condensate return line.
In very early steam systems radiator sections were connected (to permit steam flow) only across the radiator bottom. Steam entering a steam radiator at its top would have been forced down through that radiator section and through bottom nipple connections into the remaining radiator sections.
Holohan points out that two-pipe steam designs became common in the U.S. around 1905 and made use of hot water radiators whose sections are connected to permit [more rapid] heat flow through nipples at both top and bottom of each radiator section.
Up-feed Steam Risers Entering at the Radiator Top
There are also upfeed steam pipe systems that may feed into the steam radiator at its top - through a Hoffman Supply Valve (sketch at left - ITT).
These systems will also always be two-pipe steam heat systems and the condensate return will always be take off of the bottom of the radiator.
Certainly some of the two pipe steam heat radiators I have found in homes are bottom-fed steam radiators, have both entering and exiting pipes at the radiator bottom.
The sketch at left shows that two-pipe radiator connections may be taken from either an upfeed or downfeed steam supply line - with just the information in the sketch we don't know which way steam is flowing in this system.
Holohan points out that because steam moves across the top of the radiator and condensate drips down along the sides of each radiator section, heat from a two pipe steam radiator system will be more "even" as the radiator is heating up.
In our OPINION, once either radiator has become hot their heat output will be about the same regardless of one pipe vs two pipe design.
One Pipe Steam Radiator Piping Connections
At left is a one pipe steam heat radiator.
You can just make out the steam vent on the right side of the radiator, and at the radiator's lower left you can see the steam pipe and radiator valve - the steam enters and the condensate returns through the piping and valve at the radiator lower left end.
This is the most common one-pipe feed steam heat radiator piping we see in private homes.
An up-feed steam connection brings steam into the radiator through the Hoffman valve and that same valve allows condensate to return through the valve and into the steam line where it returns in the same steam pipe, flowing back to the steam boiler.
There are upfeed steam risers that flow steam into radiators at the radiator bottom in both one pipe and two pipe steam systems.
A one pipe steam system would not work if its steam entered at the radiator top - the radiator would fill up with condensate and would stop heating.
Wet Return vs Dry Return Steam Piping on One Line Steam Heat Systems
The number of steam radiator piping arrangements is quite large, I'm not showing all of the 20+ configurations commonly listed, though I can do so if there is a need.
Wet Return Line Steam Piping
The one pipe steam heat radiator and piping shown at left is connected to an overhead steam supply main and uses a wet return - condensate flows in the return line. 
Dry Return Line Steam Piping
At left is a one pipe steam radiator connected with a dry return line.
Indeed there is a wide range of both hydronic and steam radiators. A reasonable approach would be to
Vacuum / Vapor Steam Systems
Some older steam systems, less likely to be encountered now were a vapor/vacuum design, now considered obsolete with oil and gas-fired heating equipment, but you may still encounter piping in older buildings built before or at the time when vapor/vacuum steam systems were being installed.
Vacuum steam heating systems are all two-pipe systems, but include a mechanical vacuum pump at the end of the condensate return piping system.
Vapor / Vacuum steam systems could be designed as either a one-pipe or two-pipe steam heating system and look much like modern steam heating systems but a vapor/vacuum steam system used special air vents to eliminate air from the heating pipes and system.
Vapor steam systems (just to add to the confusion) run at very low pressure but never at a vacuum. These systems used an oversized steam supply pipe to provide nearly-constant flow of steam vapor.
Continue reading at STEAM VENTS or select a topic from the More Reading links shown below.
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