2 Pipe system - water hammer on one radiator

heus33

I've got an old home (1880) and we have a steam boiler in the basement (replaced in 99). Its a 2-pipe system with the old stand up cast iron radiators.



The 2nd floor bathroom has some terrible water hammer. It only happens during start up (about 5 minutes after the heat kicks on - when the steam first reaches that area). Its a loud banging sound - VERY loud. Its definitely on the supply line, before the radiator (somewhere in the pipe). Once the radiator heats up the banging goes away (About 2 minutes in duration) and wont bang again until it completely cools and another heat cycle is started. Meaning once its hot, I can continue to run heat cycles and it wont bang.



I've replaced the thermostatic trap and blew the return line out with nitrogen to clear any obstructions (that was before I realized it was on the supply side).



I dont know anything about steam and the couple of 'experts' I've had over have just thrown ideas at it without any success.



Any other ideas out there?



The piping for this radiator takes two turns. One from the boiler to the common wall in the basement. I've verified that I have good pitch on this pipe. It then heads straight up through a common wall to the 2nd floor, then takes another turn under the floor in the bathroom to the radiator. I suspect this is where the water hammer is happening. I've pitched the radiator on the supply side up as much as I can using spacers but the hammer persists.



Any other ideas? All of the piping in the basement has encapsulated asbestos around it. Happy to take pics if that helps.



Thanks!



Tom

VA_Bear

When you replaced the trap...

and blew out the return line, did you notice a significant amount of water come out of the radiator? I'm trying to determine if the water may be standing in the radiator or in the supply piping. One pipe or two pipe system / radiator? Gravity return or condensate pump? If there is a steam control valve on the inlet of the radiator is it fully open?

Heus33v2

2 Pipe system - water hammer on one radiator

Sorry for the late reply - I couldnt get back into the site and had to register under another name.



No water came out of the radiator when we blew out the return line. I dont think there is any water that is staying in the radiator - Its likely in the supply line that is under the floor.



Its a two-pipe system with Gravity return.



Yes, there is a steam control valve on the inlet and yes, it is completely open. I've tried at fully closed, half open, etc etc, all results in the same water hammer. It occurs before the supply reaches that radiator. I've narrowed it down to the section under the floor.



When the steam travels up from the boiler it heads into a common wall - no noticeable hammer at that stage, then makes a turn to the upstairs bedroom - one turns left and the other turns right - I can hear hammer on both lines.



Any other ideas? Its been like this since we moved in a year ago. And I'm sure it was like this when the previous owners sold the house (although not disclosed).

JStar

Hammer

I'm going to guess that the bathroom was remodeled at some point. That's usually when bad things happen to good steam systems. Sounds like there is some improper piping in the floor.

VA_Bear

Have to agree jStar...

I would start with pipe size, slope and any garbage buildup which could hold water in the supply and return lines. This sounds like the system is trapping water in the supply on cold start and the flow / heat eventually drives the water out of the supply. No water, no hammer... If you close the radiator inlet valve completely, does the hammer still occur and does the radiator stay completely cold?

VA_Bear

Have to agree jStar...

I would start with pipe size, slope and any garbage buildup which could hold water in the supply and return lines. This sounds like the system is trapping water in the supply on cold start and the flow / heat eventually drives the water out of the supply. No water, no hammer... If you close the radiator inlet valve completely, does the hammer still occur and does the radiator stay completely cold?