Combi boiler hot water fault

Old 10-04-05, 02:51 PM
fire boy
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Combi boiler hot water fault

Noob to the forum so a big hello to all

Now I wonder if anyone can help me.

I live in an ex council property which has a valiant combi boiler (not sure on the model).

The central heating works fine with no probs but when I run the hot tap (any room) the boiler fires up for a few seconds and then cuts out.

With the hot water running the boiler seems very noisey(water cavitating?)
along with a hissing sound which gets louder until the boiler cuts out.

Does anyone have any ideas?

Kind regards Brian
Old 10-14-05, 02:09 PM
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Could be a few things and depends on the model. It could be a worn diaphragm but if it always fires up first time before cutting out then will either be looking at a faullty dhw thermister or scale in the domestic hot water heat exchanger. I would probably say its the latter and this can be cured sometimes by cleaning out with some descaler (sentinel X400) or sometimes just water and a lot of shaking. You would need to isolate cold water to the boiler and drain the ch side of the boiler and some new washers if you are to attempt this.

Hope this helps.
Old 10-14-05, 02:27 PM
fire boy
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Thanks gasmandan

I removed the heat exchanger and flushed through with de-scaler and refitted
and hey presto!!!!

I'm now thinking that I should flush the system to stop any futher problems and refill with inhibitor.

Can U recommend some products to clean the system?

I take it that U add the cleaner and run for a few days and drain the system and start with freash this the done thing?

Ta for any advise
Old 10-14-05, 02:38 PM
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Yeah sure,

That would be a good idea. I generally use sentinel or fernox products and find them quite good. All directions for use are usually on the back of the tub!

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