Pressure high on baxi combi boiler

I have a problem with a Baxi 105e combi boiler where the pressure is constantly rising and blowing out of the pressure relief valve.
The filler pipe is disconnected so no chance of filling from there. However if the cold water input (for domestic hot water) is turned off the pressure rise stops.
I suspect that there must be a hole in the DHW heat exchange unit - but before I go swapping it out is anything else likely to cause the problem? And what would cause the heat exchange unit to develop holes after only 18 months.
Many thanks Peter
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On Wed, 18 Feb 2004 22:07:36 +0000, graham wrote:

A manufacturing fault. Very soft water. Acid flux residues not being cleaned out from the primary side.
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Ed Sirett - Property maintainer and registered gas fitter.
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