Steam ingress in return pipe--puzzling

Saldanah · December 14

I’m in a coop in Lower Manhattan with a two-pipe steam heating system. After addressing boiler pressure and cooldown calibration issues (as discussed in a previous post), most of the banging from the supply lines has stopped. However, one return pipe still bangs for about a minute and then clicks loudly for five minutes every hour.

The sequence starts with the pipe cold, followed by banging, then condensate gushing for five minutes, after which the pipe heats up and starts clicking. A few minutes later, the pipe becomes as hot as the supply lines, starting from the bottom and heating upwards. This suggests steam might be entering the return line from below.

The riser is between two radiators on the second floor, and the pipes turn just below this floor, traveling back to the boiler room. I’m puzzled by the upward direction of steam in the return line. If the steam were coming down, it could point to a bad trap in an upstairs radiator. Could the steam be entering from below due to a bad trap or poor venting?

Complicating matters, all pipes below this floor are concealed within the ceiling of a retail tenant and wrapped in asbestos, making access difficult. We’ve checked the vents in the boiler room but haven’t identified a bad trap in the asbestos-wrapped section.

Is there a workaround, such as further reducing steam pressure or adapting the riser in the apartment? If not, we’re considering insulating and boxing the pipe to minimize noise. Suggestions on how best to insulate or resolve the issue are highly appreciated.

Thank you!

Charlie from wmass · December 14

You need to do a trap survey. It may be the cross over trap that is failed open or has been gutted in the past

Jamie Hall · December 14

It does indeed suggest that steam may be entering that riser from below. Now the question is… how does it get there.

It could be a bad trap, and perhaps not on a radiator. I wonder if there are crossover traps on this system?

It could also be a location where is supposed to be a water seal between a steam main and a dry return. It could take the form of a drip from the main and a drip from the dry return going into a wet return — but it could also be a plain vanilla loop seal (a pipe down, over, and back up) which simply isn't deep enough to hold against steam pressure. There are pipe configurations which can behave in either of those two ways even though they aren't really intended to.

Can you correlate boiler pressure with when the problem starts to happen?

Saldanah · December 16

Thank you. I will ask our building super (who is very handy) to vary the pressure to see if we can learn anything. It was recently lowered from about 2.5 to 2 psi. We have 14 floors. We also lowered the cooldown period between cycles since another unit experienced banging when the supply lines cooled for too long. Should we expect to see the situation improve with lower pressure? And what are the downsides of lowering the pressure—longer boiler cycle?

The challenge is we are not able to easily access the full length of the pipes below the second floor since they are buried in the ceiling of the first floor.

Wonder if there is a workaround.

Charlie from wmass · December 16

The traps are likely at the heat emitters. The setting at 2.5 is that with a subtractive or additive differential? Left or right style?

Saldanah · December 16

Thank you. My super told me the differential is 2 and he lowered the main to just below 2. We are waiting for the next cycle. Attaching a few photos in case you have other suggestions

Saldanah · December 16

Charlie from wmass · December 16

The differential has to be smaller than the cut out pressure. If not the boiler cannot reset without it going into a vacuum. In other words the differential should be a pound and a half and the cut out pressure should be barely under two

Saldanah · December 16

Doesn't the differential start at 2 in the picture? How would it go down any further? On the right side level seems to go below 2?

Charlie from wmass · December 16

That is a subtractive differential. It's the left column is lower than the right you get a negative number. You roughly guess where 1.5 would be.

Saldanah · December 16

IMG_7989.jpg

What our super did was to change the pressure setting as seen in the attached image. He also changed the system temperature requirement from 155 to 150. The hope is these setting will cause the steam to rise/move slowly when it does get into the return line, thus reducing the banging. It is not a particularly cold day in New York today so it is hard to tell for sure if that helped. Any other feedback is welcome. Thank you.

Charlie from wmass · December 16

Lower pressure makes faster steam not slower steam. Lower pressure is less likely to over power weak traps and reduces flash over steam when condensate enters returns.

Saldanah · December 16

I thought the cut-out at 1.5 psi would prevent excessive pressure buildup and ensure steam moves more gently through the system. And the differential of -0.5 psi should make the boiler turn back on at 1.0 psi. The smaller pressure range would keep the system running in a gentler cycle, minimizing rapid pressure changes and allowing steam to rise more gradually.

Jamie Hall · December 16

The way I read your pressuretrol you have it set to cutout at about 1 psig — with a 2 psig differential. That ain't gonna work. Set the cutout indicator to 2 (the right hand scale) and the differential to 1 (about half way from 2 to the bottom of the left hand scale) and that might work…

The way you have it set now if it does cut out on pressure, it is very unlikely to cut back in and you will be left wondering why…

The pressyretrol has nothing to do with how fast pressure rises in the system when the boiler fires. All it does is turn off the boiler when the pressure gets to the cutout valve. How fast the pressure rises is determined by how big the boiler is in relation to the size of the rest of the system.

The system temperature requirement has nothing to do with the steam pressure.

ethicalpaul · December 16

Your pressuretrol is a subtractive type. That means the indicator on the right is the cut OUT pressure, then you subtract the indicator on the left to get the cut IN pressure.

You can forget about steam moving more gently, it's not relevant to anything.

When the boiler is making steam, it is always making it at the same rate. When radiators are still filling up, the steam probably moves fastest through the pipes, but don't worry about that. The "speed" of steam is basically an unimportant number, especially when it comes to setting your pressure device.

The Pressuretrol does not set the pressure, or the speed of anything.

All it does is turn off the boiler when the pressure gets to a certain point, then allow it to come back on when the pressure drops to a certain point.

The reason you want your pressure control to cut out at basically the lowest possible setting is simply because your system doesn't need any significant pressure to successfully push the steam to every radiator.

So if your burner is still burning when the pressure is anything over let's say 4-8 ounces of pressure per square inch, it is burning fuel for no reason.

Caveat: Yes, OK, it's not for no reason, because by it's true that by building pressure in a system you are increasing the energy in that system and the fuel isn't completely wasted because after all, that heat energy is going into the building envelope. But it will lead to overshooting the thermostat. Lots of multi-unit and/or large buildings (if my understanding is correct) run their boilers on a timer where depending on the outside temperature, they just run, regardless of how much heat is actually needed in the building. They are just guessing that if it's x degrees outside, the boiler has to be running for y minutes out of ever hour, something like that. It's a horrible system. In those systems, the pressure control is really doing a lot of heavy lifting. In our homes, not so much in normal circumstances. The thermostat gets satisfied before the pressure gets too crazy, even when the pressure device is faulty.

Complete digression: It's my belief that the only reason Dan even spoke about the speed of steam in his book was to dissuade people's strongly held (erroneous) beliefs at the time that higher pressure did a better job of pushing the steam where it needed to go. This is complete conjecture on my part.

Charlie from wmass · December 16

@ethicalpaul you almost got it. The steam does change speed as Dan describes based on system pressure. It's travel speed is effected by the density of the gas. Also higher pressure allows the condensate to be above 212⁰F so when it enters the low pressure of the return it flashes to steam. This can cause water hammer in the return piping.

ethicalpaul · December 16

Yes, I agree that when pressure starts to build, the "speed" of the steam slows to a standstill. Condensing of steam into water no longer creates a vacuum, so much as it just reduces the pressure locally, and the pressure wave travels around balancing the pressure in the system.

But I think it's a distracting and unimportant metric. Who cares how fast steam is traveling really (outside of some problem like missing or insufficient main venting for example)?

I like your point about condensate flashing to steam at the end of a heating cycle. I'm thinking about the end of a heating cycle because for sure in 1-pipe systems, the pressure in the return is nearly the same as the pressure in the main (excepting the pressure from the weight of the water). I guess in 2-pipe systems the return could be at about atmosphere even when the main is at some higher pressure.

Charlie from wmass · December 16

Leveling radiators is my least favorite red herring btw. Off topic but got to say it.

Saldanah · December 16

To be clear, the current settings in the dial are shown in my last post (not the one before that). This certainly has quietened things down in the return line that was clicking loudly for several minutes in each cycle.

I am repeating the current settings with this message.

Thank you.

Jamie Hall · December 16

https://forum.heatinghelp.com/discussion/comment/1827252#Comment_1827252

Oh come now. It's such fun to repeat it! I agree almost completely — though I can think of one possible exception: a very long, dead level one pipe baseboard with a very fast vent on one pipe steam. Might build up enough condensate to make one feeble clank… But… even then…

Steam ingress in return pipe--puzzling

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