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Posted by HeatPro on November 02, 2010 at 19:30:46:
In Reply to: No backflow valve posted by Dan on November 02, 2010 at 16:14:16:
It is usually code to have one when the cold water feed line is hard-piped to the boiler. If the boiler is hose-filled with a disconnected hose after fill, there's no need, as water won't backflow to the cold feed line then.
They might still be not there for boilers installed before the code change. It's a good idea to add one to keep the water system from boiler water, especially if antifreeze is added.
http://www.livepast60.com/heatpro/purgeairtree/documents/790D47AEEB4B6A2B51BF2E7764EE0FAB22E6E0B3.html