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Posted by HeatPro on January 05, 2008 at 12:54:21:
In Reply to: worcester bosh 240 posted by Dave on January 05, 2008 at 12:43:16:
I'd guess that the expansion tank has failed, so the unit loses some heat system water through the relief valve. When domestic hot water is drawn, the burner is on to heat the water so the system water also heats and expands to make the system pressure rise. When the burner goes off, the water cools and shrinks again to drop the pressure lower because some water has left.
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