Slow Bang in 2-Pipe Air Vent Parallel Gravity Return System

Dave Stockhoff

I have searched the FAQs here thoroughly and don't find anything that quite relates to my issue. First of all, my system, I just found (thanks Dan!) is a 2-pipe steam system that lacks steam traps but has air vents. (It's a lot like the system Dan describes in his story about the elephant in Manhattan, very simple, with condensate running down the outflow pipes to the steam pipes---pipes parallel to and connected to them---so it's almost like a one-pipe system with 2 pipes.)

The system as a whole works great so far this season (just bought the house) and the return pipes are partly new copper and in great shape; the boiler is 5 years old. But one radiator in the bedroom (out of all the rooms it had to be that one) bangs gently as it heats/radiates. It's definitely not a pounding or a hammering, because it bangs only once per minute or so. Therefore, if my issue is a trapped air/water problem it's a very slight one. It's also extremely consistent.

The offending radiator is one of a pair that are very close together on opposite sides of a half-wall, so it's impossible to tell which one is banging. Both appear to warm normally across the radiator from supply to drain.

(1) Is the noise caused by a bad vent, a bad supply/outflow valve, bad tilting (none of the radiators appear tilted at all), expansion, dirt, or something else? I can test the vents by swapping them, although I don't yet know how. Or is it a pipe, not the radiator?

(2) Should I use the supply or drain valves to control heat, or both? Or should they all be fully open or closed?

(3) How can I tell if the supply/drain valves work or not? If they don't turn, should they be replaced?

Thanks!

Steamhead (in transit)

Sure it's not

the sound of expanding iron?

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Dave Stockhoff

Probably

That's exactly what I thought it sounded like, but why would one radiator clank so consistently and yet alone among all the radiators in my house?

I did some thinking after I posted and wondered whether maybe a long pipe was changing shape and rubbing against another pipe or something hard that prevented it from expanding consistently---i.e., something that made it jump periodically. So I looked.

Sure enough, the supply pipe to the floor above runs right past this radiator. It's a somewhat congested spot. And this pipe is firmly lodged against the plank supporting the radiator as it comes through the floor.

I can't test this now because it's not cold enough to have the heat on, but I've jammed a thick pillow between this pipe and the wall. If the sound is muffled or stops, I've found my cause.