Hot water demand causing combi to trip..

[Dirntknow]

I've just changed the secondary heat exchanger for a plate heat exchanger on my Worcester 280. I also changed a u/s expansion vessel as the bladder had perforated. Initial testing proved good (see previous topic!) but just found a bizzare fault when drawing hot water.
Previous topic: //www.diynot.com/forums/viewtopic.php?t=217827
Central heating seems to work fine and the pressure is regulated alot better now the new expansion vessel is in place (charged to 0.7bar before fitting). The issue arises when hot water is called for with CH off. The burner kicks in and within a minute the pressure increases like a greyhound on steroids. I knock the hot water switch off and use the PRV to stop the pressure breaking the needle (literally!)
I suspect an airlock and/or scaled primary heat exchanger but didn't have this problem before replacing parts...any suggestions welcome!!

 

[powell30]

Diverter blocked or locked in central heat position. Airlocked (doubt it if heating ok)

 

[combispecialist]

when u turn the hw off does the burner instantly go off?????

 

[Agile]

I am a little confused by this nposting.

As I recall the secondary heat exchanger is a bottle type and NOT a plate.

Do I take it you have somehow mofdified the connections to fit a plate HE ? If so then why one might ask.

Seriously increasing pressure usually indicated no flow through the primary HE.

Did this "fault" occur when you fitted the plate?

If so could you have made some kind of connection error?

Tony

 

[Dirntknow]

The original was a white bottle type but the replacement is a plate heat exchanger. Apparently this is a more efficient upgrade. The kit comes with plate HE and associated pipework required to fit this mod. The connections are correct, the thing virtually falls into place and all looks good to the diagram.
To Combispecialist, i'm not sure about the burner. I ran a hot tap, and watched the pressure closely. As soon as it rose rapidly, I switched the hot water switch off and dropped thr pressure straight away with the PRV. It all happens very quickly and as mentioned, like there's no flow thru the primary heat exchanger..
To Powell30, i'll have a look at the diverter valve today. I'm not sure why the heating would be good but have a major problem with hot water though? If it stuck midway would it prevent flow back to the HE and give this fault?

 

[Dirntknow]

Diverter valve is ok but noticed the expansion vessel is completely empty...is that good or bad? I'd have thought there'd be some water in there!!