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There's water hammer in the middle of the heating cycle

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Published
July 10, 2009 Subscribe to RSS feed

There's not enough vertical space between the boiler water line and the end of the main.
We commonly call this space the "A" Dimension. You need 28 inches of "A" Dimension on a gravity-return, one-pipe steam system. Returning condensate stacks in that space and builds pressure. That pressure combines with the "leftover" steam pressure at the end of the main to put the condensate back into the boiler. If the job doesn't have enough "A" Dimension, water will back into the main and cause water hammer in the middle of the cycle.
This often happens when an installer removes a boiler from a pit and replaces it with a new boiler that's not in the pit. The higher water line of new boiler shortens the "A" Dimension and causes the water hammer.
Either lower the boiler, or use a condensate- or boiler-feed pump.

The gravity-return lines are clogged.
Clogged returns won't pass condensate. The condensate backs up into the return during the middle of the cycle and creates water hammer at the ends of the mains. Clean or replace the returns.