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Plumberpro, Plumber
Category: Plumbing
Satisfied Customers: 634
Experience:  Heating and plumbing answers. 20 years experience
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Biasi M90E 24S. Domestic Hot Water, thermostat full on,

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Biasi M90E 24S.

Domestic Hot Water, thermostat full on, boiler fires up, water at tap gets warm (not hot) then boiler shuts down. Pressure measured as low. Control board replaced, then diverter valve and motor. No change.

Plumberpro : Hi . I need a little more info , but pretty sure of the fault .

What other info do you require?


Serial N019580804 GB


G.C. No.: 47-970-17

Plumberpro : Hi . That's fine
Plumberpro : Hi . It sounds like the plate heat exchanger is blocked , either with sludge or scale . This make the water heat up too quick and boiler shuts off quick on thermostats . Replace plate heat exchanger

Does that explain the low gas pressure when HW selected?

Plumberpro : Yes , the boiler is unable to move to high gas rate

Thank you for the further explanation. One more question. Could it possibly be caused by dirt in the pump? You suggest that water would heat up too quickly if heat exchanger blocked but water at tap never gets above warm.

Plumberpro : I am assuming the boiler runs ok on heating only !

Good question It being summer the heating hasn't been used for weeks. It was switched on briefly for a check of that very thing but only until heat was felt at the nearest radiator. That check, of course, does not prove that CH is working properly.

Plumberpro : This fault is a tricky one for boiler engineers . The first time I saw it it was confusing but I I can say for sure it is the hot water plate heat exchanger that needs replacing . . If you want to run the central heating to check it ., you can come back to me in 10 mins or so

I'm afraid that isn't possible as the boiler is in my daughter's flat, some distance from here. OK, and finally I hope, if it is the heat exchanger that is u/s would the temperature gauge on the boiler shoot up before the burners switch off?

Plumberpro : Yes , because that is the problem . The water can't move quickly enough through the plate heat exchanger , so it overheats almost instantly and then the overheat stat shuts it down .

Thanks for the info. It gives more things to examine tomorrow morning.

Plumberpro : Seriously . I have worked for british gas and myself and seen this so many times . Just get someone to replace(not clean it out ) and it will be fine .
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