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There's water hammer near the end of the heating cycle

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Published
July 10, 2009 Subscribe to RSS feed

The Hartford Loop isn't piped properly.
Make sure the connection between the return and the equalizer is far enough below the boiler's water line. If steam can work it's way down the equalizer and into the wet return, it will hammer, and usually at the end of the cycle.
Check, too, for a long nipple on the Hartford Loop. Long nipples create water hammer as the condensate returns. Replace the long nipple with either a close nipple or a wye fitting.

The condensate- or boiler-feed pump discharges into a Hartford Loop.
The Hartford Loop does a good job of protecting the boiler in a gravity-return system. Should a return line spring a leak, water can flow from the wet returns, but because of the Loop, it can't flow from the boiler.
On a gravity-return system, the wet return connects to the boiler's equalizer at a point about two inches below the lowest operating point. This varies from manufacturer to manufacturer so you should always check their installation instructions.
When you have a condensate pump, you no longer have a gravity-return system. Should a return spring a leak, the boiler water can't back out of the boiler because of the condensate pump's check valve. Should the check valve fail, boiler water will back into the condensate pump. The pump will turn on and pump the water back into the boiler. Should the check valve and condensate pump fail simultaneously, water will back into the pump's receiver and rise up the vent piping. Since this piping is usually several feet higher than the boiler's water level, the water still can't get out.
If there's an overflow pipe in the vent line, however, the condensate can back out of the boiler, should both the pump and check valve fail. In this case, a Hartford Loop would help on a pumped return system. But other than that, the Loop may cause problems. Water under pressure from the pump can splash up into the boiler header and create water hammer. If this is your problem, relocate the pump's discharge line to the bottom of the boiler's equalizer.