Steam radiator hammer isolated to one radiator - what's next?

southpawboston · May 2016

I have a home with one-pipe steam heat and a boiler supplying steam to 8 radiators on two floors. Most radiators and piping date to about 100 years ago. The Burnham boiler is about 27 years old. For the first four years we had silent heat, just the pleasant whisper of the air vents until the radiators warmed up. But for the last year, one radiator started hammering until heated. It's the newest radiator in the house, a small in-wall semi-flush mount radiator in the bathroom. It's probably about the same age as the boiler. I've pinpointed the hammering to the side of the radiator the steam enters. The hammering is not in the piping leading up to it. I swapped out the vent for a varivalve vent (despite the old one working fine) and played around with different venting rates. The new vent didn't solve the hammering, only the timing of it (with the vent fully open, the radiator reached full heat very quickly and the hammering stopped quickly; with the vent closed, the radiator heated slowly and the hammer was prolonged-- this was as expected). The pitch of the radiator has not changed as it's built into a wall, so no recent floor sagging that could have changed its angle.

I have not done anything to the water in the boiler. There does not appear to be an oily film on top when looking through the glass column, although it turns rusty brown when stirring up by draining and refilling with fresh water which my plumber told me to do monthly. A few hours after a thorough draining and refill, the water appears clear.

One thing I have noticed since the hammering started a year ago is that the boiler drain valve is suddenly draining very slowly, essentially a trickle. It used to drain a lot faster. Could this indicate a clog of rust scale at the bottom of the boiler, or a vent problem (since to drain the water it needs to suck air into the radiators)? All the vents seem to work fine and all the house radiators get hot in a reasonable time (20-30min)-- that has not changed since we've had the house.

Any ideas? I actually really love my steam heat and it's just this one source of hammering that has me stumped and annoyed, especially since it was silent for years.

southpawboston · May 2016

A few other details I left out:
- all inlet valves are fully open
- Return vents in the two branch mains can be heard "breathing" so I suppose they're working
- I never see water bouncing or spatting in the glass column
- water does not have auto-fill, so it gets a little low after about a month in winter with constant cycling. Never fill beyond "max" level (which is a hand drawn line, ostensibly, by the person who installed the boiler)

BobC · May 2016

The boiler should be flushed out to get rid of any crud and you may have to replace the boiler drain valve.

It's not inconceivable that the house has settled a bit and that in wall radiator (Sunrad?) has gone out of slope, put a level on it and make sure there is a little slope to allow water to drain out. Also varivents are very aggressive, do you have any other vents you can try? My Surad has a #5 Maid O Mist air valve on it and is completely silent except for a slight ticking from expansion from time to time.

Post a picture of the radiator.

Bob

Fred · May 2016

You should address that slow boiler drain before it completely clogs shut. The next time you drain the boiler, either clean that drain valve and boiler mud leg out and/or replace the drain valve after cleaning the boiler. That is not the source of your hammer at that radiator though. As has been said, double check the pitch of that radiator and make sure it is sloped back towards the steam supply pipe. Also, check any horizontal pipe runs to that radiator. They may have lost their pitch back towards the steam main. If there is a horizontal run between the floor and a ceiling that you can't see or get to, try to raise both ends of that radiator a little, to give you some pitch in the pipe and then repitch the radiator.
A third item that you should check, again seperate from that radiator, is to make sure you have enough venting on the mains/returns so as to put most of the burden of air evacuation on those larger main vents and allow the radiator vents to just remove air from the supplies that feed each radiator and the radiator itself.

GreenGene · May 2016

BobC said:
The boiler should be flushed out to get rid of any crud and you may have to replace the boiler drain valve.

It's not inconceivable that the house has settled a bit and that in wall radiator (Sunrad?) has gone out of slope, put a level on it and make sure there is a little slope to allow water to drain out. Also varivents are very aggressive, do you have any other vents you can try? My Surad has a #5 Maid O Mist air valve on it and is completely silent except for a slight ticking from expansion from time to time.

Post a picture of the radiator.

Bob

AGREED!!, check the pitch of that radiator, hammer is water turning to steam BOOM so you're not draining back properly.

I had a house where they did the floors over and it came out pitched wrong and raised, on 2nd floor couldn't get to pipes or anything, stuck I had an epiphany............I carefully and I mean carefully marked the front legs and routed the floor so that the legs sat down at the pipe connection and you couldn't tell, it just looked like the legs were on the floor.

southpawboston · May 2016

This is a photo of the identical radiator (but not my radiator) in question:

It would be extraordinarily hard to re-pitch it since it's recessed into the wall and tiled around it, but I'll take a level to it and see where it's at currently.

I'll unthread the boiler drain valve and give it a cleaning.

Jamie Hall · May 2016

Are you sure he valve on that radiator is fully open? Could it have failed in such a way as to partly block the inlet pipe? It wouldn't be the first time...

southpawboston · May 2016

I'll double check the valve tonight, maybe even pull the bonnet and check inside.

Steam radiator hammer isolated to one radiator - what's next?

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