Hot Water Banging

petermatheson

I recently moved into a 1915 single-family house in Minneapolis. The house has an unfinished basement, a first floor w/ 2 radiators, and a second floor w/ 2 radiators. There is a small (probably <4' tall) and seemingly inaccessible attic. 1200 sf excluding basement. I believe this is a two-pipe system, each radiator has a supply and return, both near the floor. There is a metal expansion / compression tank hung from the basement joists near the boiler, and I'm not talented enough to tell if it sounds full/empty by tapping, although the guy who serviced it said it was completely full and he had to drain a lot of water to get it to 1/3 full. All copper pipe in the basement, likely installed when upgrading to a gas boiler. Older pipe throughout upper floors/walls.

One of the radiators on the first floor bangs fairly loud every 5 mins for the 45 mins it takes the boiler to appease our thermostat, every 8 hours.

We recently had our 2004 gas boiler serviced (new pressure gauge, cleaned/dusted, etc.) and I tried bleeding the radiators with the special little key starting with the first floor, and then the second. There was very little air in any radiator, with water coming almost immediately. I'm not sure what would cause banging/water hammer in a HW system if it isn't air trapped in the radiators, unless I should be more aggressive when bleeding them.

The banging isn't terribly annoying, mostly just curious what the cause is. I've seen other similar threads/posts on this site, but still can't figure it out. I plan to reach out to the boiler service guy after the holidays with this same q.

Thanks for any advice!

neilc

post a picture of the radiator,
I'm guessing it's expansion / contraction with each call for heat,
folks have used plastic milk jug as slip sheets under feet of rads, or where Pipe pokes thru floors, or ???
post a pic of what's making noise

Jamie Hall

OK. On the compression tank. It will only get full of water if somebody messed up the way in which it gets air from your system -- which is very likely. Newer systems often have a fitting on them to remove air from the system. You don't want that with a compression tank, and not all people installing boilers have figured that out. Can you post a picture showing the connections from your system to the tank?

On the banging -- it is as likely to be expansion noises as anything else. Try slipping something slippery -- such as a square of plastic from a milk jug -- under the feet of the radiator. That can help. Another possibility is a pipe rubbing against the floor or something. See what you can find that might be that.

petermatheson

petermatheson

(The only short radiator in the house)

petermatheson

neilc

ok, well, again, little squares of plastic milk jug under the rad legs will help them move and slip a bit, instead of possibly catching and releasing.
You do seem to have a bit of clearance where pipe comes thru the floor, but if something is binding there also, insert more plastic as a slip bushing,
Then there's those covers, they look nicely done, but,
and maybe it's the 1 dimensional aspect of the pictures,
but they look built tight to the rads, and may add (many) more locations where expansion can catch, and release, that's a lot of milk jugs now,
in one of those shots, it looks like the joinery may be pushing apart, hummm,,,,,