two valve radiator on one pipe steam system - why?

Logik

I have a one-pipe system from 1908. All radiators have one pipe EXCEPT a long low 30 segment radiator in the dining room. That radiator has a 1.5" pipe and valve at each end. The vent is on an end, right above one of the two inlet valves.
Why?
My _guess_ is that the valve near the vent should normally be closed, but opened periodically to drain condensate? Any other explanations?
Thanks!

ethicalpaul

It can't be for that reason, or you'd have to drain it multiple times every day during the heating season. Condensate forms every heating cycle.

Can you tell the slope of the radiator? Maybe it gave someone trouble with condensate draining/interfering with the incoming steam, so they sloped it toward the non-supply end and ran a drip pipe to prevent it from interfering with the incoming steam.

Where does the other valve's pipe go? How does it reattach to the system?

If the radiator is sloped toward the supply side, then I have no idea.

Logik

Thanks for the quick reply. The radiator seems to be approximately level. It has a slight slope down toward the middle from each end - no doubt slight sagging with time. This is by far the biggest radiator in the house - near 7 feet long, 25 inches high. The steam valves at each end both appear to be original. Both connect to the same supply line in the basement ceiling. This is the first radiator on the loop.
My new-and-improved guess: The two valves provide 3 heat levels: left valve only open: high heat, steam travels along the full length of the radiator toward the vent at the far right end. Medium heat: both valves open. Steam is admitted at both ends, but the steam being admitted at the right end soon closes the vent and reduces steam acceptance. Meanwhile both ends of the radiator have heated (but not the middle). Finally, Low heat: right valve only open. Only the right end of the radiator gets hot.
Plausible?

JUGHNE

Does it ever have water hammer or any noise?

Logik

As I remember it gurgles a bit when steam is full up. This winter I will check it, using an IR thermometer, under the 3 possible states of the inlet valves: left only open, right only, and both open. I'll compare which sections heat.

jumper

Rad gets hot faster with two supplies? Maybe the rad is too big for only one? Condensate can drain from low side while air is expelled through other valve?

Steamhead

Maybe they were assuming the condensate would return to the main more effectively through the second pipe. I ran into a similar situation years ago:

https://heatinghelp.com/systems-help-center/steam-piping-gone-wrong/

A loop seal on the return connection to the steam main would do the same thing.

Logik

Update: I measured the ODs of the 2 supply lines to this long radiator. The right is smaller (a little over 1.25" OD); the left is a little over 1.5" OD. The radiator sections join only at the bottom. Both steam supply lines enter low on the end sections. Vent is near top of rightmost section.
So I am still partial to my hypothesis that the two valves give rough control over how much steam the radiator will accept per boiler cycle: left open, right closed: high. Left and right open: medium. Right only open: low, right end of radiator heats just a bit before incoming steam closes the vent.

gfrbrookline

I am going to go out on a limb and say they installed the 2nd valve to drain the condensate on such a large/long radiator so it doesn't have to use the smallish diameter pipe to do double duty. That small a pipe could get overwhelmed by the amount of condensate that radiator can produce. It should be sloped towards the smaller pipe and the sag in the middle should be shored up. My theory works if the smaller pipe is connected to a return or the system when installed didn't produce enough steam to fully heat the radiator and the second feed was added to do the job. If it is a secondary feed the radiator should be highest in the middle so both sides can drain.

Logik

The problem with having the supply valve at the right end of the radiator open is that it quickly closes the vent. Then only a couple sections at the ends heat. So it seems the right valve was not intended to be open when demand for heat is high. The radiator accepts the most steam when the left side valve (1 1/4" ID pipe) is open and the right valve (1" pipe) is closed. Unfortunately it is impossible to tell what size vent was original - replaced long ago.

Steamhead

@Logik , read my article again. I'll bet that's the same issue you're having. Find out where the pipe leading from the right-hand valve goes and get back to us.

dopey27177

Back in the day a system called a 2 pipe air vent system (subject to the rules of one pipe steam} was a Cadillac heating system. It was designed before steam traps were perfected.

The very large radiator is piped as the two pipe air vent system design. The outlet valve is where the air vent is. Both valves must be fully open or fully closed. {make sure the valves have good washers in them or you will have a flooding or banging problem}.

Can't tell you why it was installed this way without knowing the length of runs and pitch of the steam supply pipe and the return.

Most probably the steam supply pipe could not be pitched to allow the condensate to return without banging. Remember the steam supply pipe to the radiator is counter flow.

This system is explained in my book Steam the perfect fluid for heating and some of the problems.

Jake

jumper

I enjoyed the book. Don't understand why air vents were installed on radiators?

dopey27177 said:

Back in the day a system called a 2 pipe air vent system (subject to the rules of one pipe steam} was a Cadillac heating system. It was designed before steam traps were perfected.

The very large radiator is piped as the two pipe air vent system design. The outlet valve is where the air vent is. Both valves must be fully open or fully closed. {make sure the valves have good washers in them or you will have a flooding or banging problem}.

Can't tell you why it was installed this way without knowing the length of runs and pitch of the steam supply pipe and the return.

Most probably the steam supply pipe could not be pitched to allow the condensate to return without banging. Remember the steam supply pipe to the radiator is counter flow.

This system is explained in my book Steam the perfect fluid for heating and some of the problems.

Jake

dopey27177

Air vents were installed on radiators to cause a pressure drop. That pressure drop allowed steam to flow to each radiator. Additionally, Vent valves open to allow air into the radiator to prevent the radiator from going into a vacuum, The air entering the radiator also allowed the condensate to drain from the radiator and flow back to the boiler.

Back in the day there several types of steam systems some were 3 pipe, the third pipe was for air removal only and had to terminate at a master vent contraption.

The two pipe vent system had some limitations in buildings above two stories.

Go back to the chapter on two pipe air vent systems and reread.

Additionally these systems worked best with steam pressures under one pound of steam.

One system shows one return where the first and second floor used only one return. If the steam pressure was to high steam could flow into a radiator though the return pipe and cause banging or no heat.

The second system shows each radiator having its own return piped into the wet return. Because each radiator was separated from each other a higher steam pressure could be used.

Jake

ethicalpaul

I bought and like your book, and I like all your posts, Jake, but I have to ask you about this:

> The air entering the radiator also allowed the condensate to drain from the radiator and flow back to the boiler.

The condensation will flow out of the radiator regardless of the pressure or vacuum in the radiator, right?

It's not like a straw where if you cover the top with your thumb, the water won't run out of it--the pipes and steam valves are too large for that effect to work.

The volume of condensation from one radiator just runs or trickles down the pipe, no lack of air pressure will hold it in the radiator.

dopey27177

if the radiator goes into vacuum because the vent valve won't open you have the proverbial straw. The condensate will not drain and if the radiator is hot enough the condensate will flash into steam. If flashing does not occur water will be retained in the radiator. When steam begins to flow into the radiator banging can occur, in the case of a two pipe vent system condensate flows to the discharge side of the radiator and out through the return side piping provided the return riser does not have steam in it higher than the steam in the supply riser. Every thing is based on a pressure differential in the heating element and or the supply or return piping.


Jake

Logik

I checked the piping to the radiator and there is nothing unusual. No sign of any steam traps; both pipes serving the radiator rise from the main line that runs just below the floor and parallel to the radiator. The long 31 segment radiator is above the boiler room and is the first radiator on the south main loop (there is a second loop on the north side of the house).

The house is on the site of this 1891 building, and was one of four built from its pieces when it was dismantled in 1907:

Steamhead

Wow. That was quite a place. Where is this located?

Is it possible that 2-pipe radiator came from the original building, and whoever installed it didn't know how to hook it up?

Harry_6

https://forum.heatinghelp.com/discussion/161167/odd-radiator-piping#latest

I don't know if this pertains, but I have on a couple of occasions encountered one-pipe radiators with a smaller pipe (say, 3/8" or 1/2") connected at the bottom of the opposite end (air vent end) from the typical supply pipe, and sometimes connected directly into the side of the supply (1-1/4") pipe. After much head scratching I came to believe that this was a way of being able to throttle the radiator's heat supply without the inherent problem of a restricted valve opening causing conflict between supply steam in, and return condensate out. If one throttles the supply valve, the condensate just exits via the other end's pipe. This would make sense on a large radiator if you didn't want to overheat the room. Just a thought.

jumper

I like Harry_6 theory. There is a JJ Wilson patent somewhere on this site. Similar idea of throttling supply without impeding drainage.

luketheplumber

I sometimes re rope original windows in my neighborhood, one of my good costumers has a beautiful 2 story american foursquare house built in the mid 1920s. It had been built with mostly salvaged parts from an old mansion called Somerset Villa that was dismantled in 1924. (here is a fantastic page on the place http://www.opendurham.org/buildings/somerset-villa ). The house I worked on had some of the doors, trim, fireplace tile and one of the most beautiful radiators that I have ever seen. the house has a two pipe steam system that unfortunately had the boiler quit and leak about 6 years ago 😢. the home owner couldn't find anyone who would replace the boiler, so he replaced his straight AC with a heat pump 🤢 and he hates it. The home owner has told me that he might try to get the system working again sometime in the future hopefully.

luketheplumber

I also found an article on the old 1891 building for any of you who are curious like @Steamhead .
http://zenithcity.com/archive/lost-architecture/hardy-hall-aka-craggencraft/

Logik

Harry's is an interesting idea (throttling) BUT won't that only work if steam doesn't enter from the pipe at the vent end? There's nothing to stop that, and it will shut the vent off and prevent the radiator from accepting more steam. I will have to do experiments to check its behavior when heating season begins.

That will begin soon enough: the building is in Duluth MN. I imagine the radiator in question is from the 1891 school building pictured above (they re-used as many chunks as they could); but I believe they had a reason for piping this lone radiator the way they did, and I think the most promising general theory is that doing so allowed for more control of the heat output from the radiator, one-way-or-another. It's the details that are puzzling.
More boiler history: the original boiler in the current (1907) house burned coal. The boiler stood about 8 feet from the door to the coal room. The bottom 12" or so of that door was cut off and a feeder carried coal from the coalroom to boiler. Another door opens from the boiler room to the ash room.
That coal-burning boiler was replaced with a 300,000 BTU American Standard gas boiler in 1965, when a natural gas pipeline arrived. The 1965 AS boiler still does its job, with a Flare automatic vent damper added in 1985 (9" flue). The sight glass crumbled around 2010. Apparently the glass gets eaten away by the ebbing water?

Jamie Hall

Two pipe air vent systems really aren't a mystery -- just a little odd, as they dropped out of favour pretty much as soon as the trap was invented. The radiator itself functions exactly as a one pipe radiator would -- the vent lets the air out in the usual way, which lets the steam in in the usual way. Which condenses... in the usual way. Only difference is that the steam can come in from both ends, which is a little weird. Most of them were pitched so the condensate was supposed to go out one end and most of the steam come in the other. They sometimes worked that way...