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Discuss Fluctuating hot water after new rad. in the Plumbing Forum | Plumbing Advice area at

Just after some advice from someone that knows more than i do please .

Boiler is a viessmann Combi boiler less than three years old.

I recently replaced an old radiator and moved the location, I have cut and capped the old pipe work and added new. I froze the pipes to cut but one of the plugs failed leading to an unintentional draining of the system (the boiler was switched off at the time). The new rad is working fine with no leaks and the central heating is pressured and working fine.

Since then however the hot water does not stay hot for very long, it gets hot buts goes cold within 30 seconds before getting hot again and so on.

Are the two things related or an odd coincidence?.

Any help would be very much appreciated.


It could be anything really, you need someone to check it out.

Diverter valve, transducer, inlet filter, blocked heat exchanger, thermister to name a few.


Gas Engineer
Is it the same at all hot taps?
You could have disturbed some muck that had now blocked a boiler part or it could just be a coincidence.
The joys of plumbing.

king of pipes

Gas Engineer
Advent Win
Think you have disturbed some muck in the system and is now blocking the plate heat exchanger GSI needed to clean the boiler out I'm afraid . Kop
Thanks everyone, boiler is still under warranty so I’ll get someone to have a look, if it’s muck from the system then it’ll cost me but sounds like it’s needed 👍

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