"Ray" wrote in message
news:rL6gj.1104$EN6.1069@trndny07...
Sorry to be such a pest, but I have another question about our aging steam
radiator system in our six-unit apartment building. Last night, an hour or
so after the weekly flushing of the systerm, we heard a very loud knock in
the pipes. As if someone had hit the pipe with a sledgehammer.

Is this dangerous? What should I do? Was the knock, which we heard only
once, probably tied in with the flushing?

Banging steam pipes come from water in the pipes being shot by the flowing
steam. Dangerous? Usually not on a low presure system, certainly on a high
pressure system. The water can be condensate laying in pipes that are not
properly pitched, or it can be "carry over" where the steam picks up water
from the top of the water and carries it into the pipes and slams it into
the fittings at elbows and such. Filling too high can cause that also. I've
seen 12" steam pipe bounce and shake violently from water hammer. It can
also break open or fitting fly off. Try to avoid it.

You may want to consider how you perform the maintenance. Instead of a
weekly flushing, a daily blowdown is peferable. There is no sensible reason
to do a flushing as you are just adding fresh water and all the oxygen it
contains. You want to flush the minerals that condense out by doing a
blowdown of the bottom of the water chamber. If you have a lot of minerals,
a softener would be a big help. Most industrial boilers also have water
treatment to reduce oxygen and scaling of the tubes. Residential cast iron
boilers usually don't do that.

If it continues, get the advice of a professional that knows steam and how
the system works. Get your water tested.