Why does steam rise from manholes in the U.S.?

I just read the “answer” to the question “Why does steam rise from manholes in the U.S.?”. The reply leaves one with the impression that there are huge clouds of freewheeling steam roiling through the storm sewer systems of large American cities. Yet, the reply also mentions that the steam is contained in insulated pipes. The real question is “How does the steam escape from the pipes to manifest itself through the manhole covers?”. So, what is it? Free-range steam? Fugitive steam? Something else entirely?

The mailbag item being referenced is:

Why does steam rise from manholes in the U.S.? (24-May-2000)

You’re really on to something here. The Mailbag answer is very dissatisfying. Even if there is steam coursing around New York City in pipes, that doesn’t explain how it escapes through the manholes. And didn’t the original inquiry reference Denver?

My own theory based on living in many colder climes: Many manholes lead to the sanitary sewer system whose contents (use your imagination) are typically warmer and moister than the open city air, particularly in winter. Hence “steam” through the same mechanism that makes your breath visible when it’s cold outside.

Three separate threads on steam from manholes… Oh well, I’m of the opinion that we can never have enough active threads about sewers…

Anyway McCaff and/or Manhattan wrote:


As for what people put down there that causes steam to rise . . . why, they put steam, of course.


Which may be technically true, however fails to answer the question of why the steam is rolling out… for reasons that have been pointed out.
Then erniecooper wrote:

I think this is the best answer. I’ve lived in several northern towns that had sewer systems, but no steam to pump into them… guess what? Steam still came out of the sewers. Also, I used to do a bit of spelunking. We preferred non comercial, uncharted caves. The best way to find them was on dry winter days. The cave entrances could be found by the steam rolling out.

In other words, when warm moist air meets cool dry air, you get steam.

Interesting that Mr. Brooks, from England never saw steam come out of the sewers in Europe. I did see this once in Scotland, but it was because the manhole cover was off. Most of the manhole covers I saw in the UK were a pretty tight seal - in the US, we usually have the little holes for the lifting tools. This is where you usually see the steam. Also, while you rarely see steam from manholes in the UK, it’s not uncommon to see evidence of the heat differential in the sewer air. Manhole covers rarely have snow on them - it melts.

The idea of temperature differential occurred to me too. But I don’t think it is a comprehensive explanation. I live in a fairly large mid-Western city that does use centrally generated steam for heating buildings in the downtown area. I have noticed steam rolling out of manhole covers when the ambient temperature was in the 60s, seemingly too warm to be caused by the proposed mechanism.

My bet is on some type of pressure relief system that vents excess steam when transient usage drops. But this is me just talking through my hat, I know absolutely nothing about distributed steam heating.

Yikes! when good tags go bad… hopefully my last post was decipherable…
Lampare wrote:

Well, I’ve never seen this myself, but I’ll take your word for it and ammend my previous response to say “air temperature differentials AND loose steam piped in”.

So, let me get this straight, erniecooper… You’re saying that sewer fumes would be allowed out into the street in prodigious quantities in the middle of a densely-populated area? Sheesh, remind me to bring my nose plugs when I visit [ul]your[/ul] city… The storm sewers would certainly be exposed to the outside air, but I don’t see that they’d be that warm. Open sanitary sewers would just be a health risk. It’s due to the steam heating system, and there’s pressure releases periodically on the pipes so they don’t burst. Further evidence for this:
The steam rising from manholes is warmer than body temperature, which you would expect to be the upper limit on sewer temperatures
The steam only appears downtown, where steam heating is used, and not anywhere else, even though the sewers run everywhere
There’s too much steam produced to just be due to a temperature differential
The steam comes out with some pressure, rather than just drifting out.
All in all, I think that the column as written was just fine, aside from neglecting to mention the pressure releases.

Actually, I used to work for Con Edision, so here’s the SD. The Con Edison steam pipes are thick cast iron, surrounded by lots of insulation. These pipes do not leak, or if they do there’s a huge steam geyser due to the high pressure of the steam.

However, New York City water pipes leak like crazy (maybe 20% of the water is lost), and the sewer system is much, much worse. The water from these systems soaks through the steam pipe insulation, hits the pipe and vaporizes. This is what comes out of the manholes, at least in middle and lower Manhattan, which is the only area served by the steam system.

WARNING. It is recommended that the following paragraph not be read by hypochoncriacs (or those with dependents).

The steam system was put in 30 or 40 (or 90) years ago, when asbestos was thought to be harmless. Almost all the steam pipes are encased in it. Since the asbestos is embedded in adhesive, and is underground, it doesn’t get into the air, unless the insulation is disturbed. Of course, no one knows how much asbestos is disturbed when water soaks through, is vaporized by the hot steam pipe, expands back through the insulation and then comes out of the manholes.

One more thing that’s killing us.

I can no longer just remain in the dark on this one…
The mystery of street steam must be revealed from behind the mist and see the light of day.

It is relatively simple to question the “official” excuse for street steam when one realizes that steam still oozes from manholes when it is 90 degrees outside. Forget that nonsense about steam being used for cooling…

The truth is that street steam is the result of alien activity beneath our streets. There are massive “hives” of alien activity beneath the streets of various cities around the U.S., and these alien “facilities” generate the so-called “steam”, and vent it on the unsuspecting public. Go ahead and step into one of these “steam” clouds and take a whiff-- doesn’t quite smell like spaghetti now does it?

It has a peculiar funk to it…which is the result of the alien processes which produce it (it is not made from plain water…duh…). God only knows what evil toxins are being showered upon unsuspecting minions. This can explain a great many things about big city folks (like New Yorkers), and can certainly explain a lot about New Jersey…

Chronos wrote:

The air inside would most likely be at the ambient Earth temperature of about 62 degrees. Plenty warm enough to cause a steam effect in below freezing outside air.

The same applies to a vented steam scenario. I’m sure we’re only talking about storm sewers here.

As has already been pointed out, this is not true. I’ve seen steam from manholes in small towns where there could be no steam being piped in.

I’ve never seen the steam coming out with pressure. I’ve only ever seen it with a slight drift where the steam dissipates within about two feet of the manhole. This fact alone suggests that we’re not dealing with superheated steam by this point. If pressurized steam were being forced out of the manhole the steam would rise possibly dozens of feet into the air the way it does from factories.

I’m not an expert on this subject, but I think a couple of reasons asbestos was (and is) so useful is the fact that it is not very soluble in water and has a high vaporization point. I don’t think leaking steam would present much of a contamination hazard unless it was leaking REALLY fast, as in an explosion. In fact, I believe it is common practice to soak down asbestos during amelioration projects in order to minimize airborne particles. It is the inhalation of suspended asbestos fibers that presents the health risk. I don’t think waterlogged asbestos would travel very far, or remain suspended for very long, even in a cloud of steam.

Joeyblades, I don’t think that we’re talking about the same thing here… What I’m referring to are the plumes of hot vapor, a few meters high, that one sees downtown at all times of the year. Under some conditions, sure, it’d be possible to get a fog from storm sewers (and where are you from, that the ground temperature in winter is 62?), but I don’t think that’s what’s happening here. As for steam venting being a health risk, how do you figure? The only health risk I can think of would be burns, and it’s cooled off too much for that by the time it reaches street level. Surely you’re not suggesting that it’d be unsanitary?

JoeyBlades writes,

I have. Like Chronos, I was interpreting the original question as referring to the geysers of vapor coming from manholes (although the ones I’ve seen are more prevalent in the winter). Now, I doubt there’s a whole lot of pressure differential across the manhole, but it’s enough to send a plume ~20 feet in the air and fill the street with enough vapor that, on a still morning, visibility is completely obscured.