DIY Doctor

pressure build up on combi

Postby bakerevans » Mon Jan 21, 2008 7:23 am

Ive got a worsester bosch 280cdi thats about 7 yrs old,the pressure keeps going up,its up to 3 bar now so ive switched the boiler off,any ideas why its gone so high?
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Postby roger196 » Mon Jan 21, 2008 3:48 pm

Have you left the flexible filling pipe connected, after re-pressuising the system. If so, the most likely cause is a leaking valve in this pipe.
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Postby rosebery » Mon Jan 21, 2008 4:45 pm

Three possibilities:

Filling loop valve not properly shut.

Pressure vessel problem.

Pressure release valve stuck.

Immediate fix is to let some water out of the system either via drain cock if you have one or by opening bleed valve on a rad and catching the output in a bucket. That will reduce the pressure and do it soon.

After that check the filling loop valve first. Your system should operate at iro 1.5 bar. Mains water pressure is iro 2.5 - 3 bar. So if the valve is open, even just a bit, then the system pressure will rise. If it's static at around 3 bar that sort of says this might just be all you need to do since the pressure inside the system and mains are now at equilibrium.

Hope that helps.

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Postby htg engineer » Mon Jan 21, 2008 6:12 pm

When does the pressure rise ?
If it rises when heating is off, possible water to water heat-ex has corroded. This will allow the heating and hot water to mix.

Check the filling loop is closed if an external one is fitted, not sure if this model has an internal filling loop, Do you have to use a white plastic key to re-pressurise ?

If the pressure only rises when the heating is on, then it's probably the expansion vessel needs re-pressurising or replacing.

If the pressure is set to 1.0bar when cold, then it will rise when the heating is on but should remain under 2.0bar.

I know you said it's at 3.0 bar now, what if you left the system running, does it continue to rise ? does is discharge via the pressure relief valve ?
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