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Rav 12-21-2011 05:36 PM

Banging pipes, already tried bleeding
I have a hot-water boiler and radiator system. House was built in 1931, so have "black" pipes running along the ceiling in the basement, then on up to the two floors above to the radiators. The pipes have been banging for the last year or so, and it seems to be getting more frequent (once every few minutes or so). I have already bled the radiators, and that hasn't done any good. Only one radiator had any air, and not very much. But the banging continues. I can't tell where it's coming from. I've checked the hangers in the basement, and nothing seems loose. It may be a coincidence, but it almost seems as if this started after we had some new A/C duct work put in in March 2010, but it doesn't sound like the sound it makes if I bang on the duct work (a hollow sound, not a pipe banging sound). Any idea what could be causing this and what I should look for? If this could have anything to do with the expansion tank, I know how to drain it, though that was last done 1.5 years ago. Thanks.

yuri 12-21-2011 06:02 PM

sounds are hard to diagnose over the net w/o hearing them. if you can record them with a video camera and make a U tube vid and post it here maybe we can help. shoot the boiler and exp tank and circ pump and pipes so we can see your setup.

biggles 12-21-2011 07:26 PM

your bleeding with the system off circualtor and the boilers auto water feed doing the pressurizing from the bleeds? empty the EX tank

how 12-21-2011 07:35 PM

The exp tank is the possible problem.
Turn up the thermostat and watch the boiler tridicator guage that measures temp & pressure. If the pressure starts to jump around or rise above 28 PSI then the expansion tank should be looked at.
If it is an old horizontal ceiling tank then turn of it's water feed and open up the tank's drain valve to drain off a couple of gals. Close off the drain valve and open back up the water feed valve to the tank and your done.

If it's a newer stubby tank by the boiler, then try tapping the outside of the two halves of the tank. One half should sound hollow while the other half that is filled with water will sound solid. If the whole tank sounds solid then it's time to change it.
Banging can occur from hangers or anything that binds and releases the heat expanding pipes. From a pump that isn't circulating the water well enough causing the boiler to overheat. From excess air that's there from trying to bleed the rads while the pump is operating.(pump should be off as Biggles says to bleed the air).
Telling us what your operating pressure and temp might help!

Rav 12-22-2011 08:55 AM

The pressure doesn't jump around and doesn't rise above around 17 PSI. I do have an old horizontal tank mounted in the ceiling in the basement. This morning I drained it (the first bucketful was pretty dirty). Since it hadn't been done in years, I also ran the boiler drain valve until it ran clear (it wasn't at first). I also tested the pressure relief valve, which worked fine. I also bled the radiators while the system was off (no air). After I turned the system back on and let it run for at least 1/2 hour, the pressure was 17, temp 140 (hi-limit switch is set at 150). A new circulating pump was installed in '97 (Bell & Gossett “Little Red Booster Pump” LR20BF). At least for now, in the last half hour I've only heard a banging sound once, so perhaps emptying the expansion tank helped. I'll keep you informed. Thanks to everyone for their help.

Rav 12-22-2011 10:06 AM

I THINK (hope) I have found the problem and resolved it. I heard the banging noise and it seemed to be coming from one particular radiator on the first floor. I pushed on bottom of the radiator a little and heard the clank. There wasn't anything obvious loose there, so I went to the basement where the black pipe goes up and attaches to that particular radiator. I shook the pipe just a little and sure enough, there was a clank. I found that one of the floor joists had a small semi-circular area cut out of it to make room for the pipe to pass by, and with just a little shake the pipe was banging against the joist. My wife upstairs said it sounded like the clanking noise we've been hearing. So I inserted a little fiberglass insulation between the pipe and the joist in order to dampen the vibration. I probably won't know for sure for a day or so if this fixes the entire problem, but I'm hopeful.

biggles 12-22-2011 11:26 AM

that Ask This Old House show had a segment on banging pipes with a hot water system like yours...he went right for the bleed(no air)and tank(drained it) but found out the run was so tight coming out of the floor and the first turn the elbos were banging he shortened them up and all was quiet...any metal expands with hot water passing thru it and does move even the slightest..hey:thumbsup: at least you go to drain the EX tank repeat that one in Feb.might consider adding a nipple and valve so you don't have to shut the system down....draining it...if it's not set up already..

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