Help with problem re: Cold water from basin hot tap on combi system

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Hi All, I'm not Gas Safe so don't touch boilers, (as one shouldn't when not Gas Safe), however, a customer called me to ask if I could fix a problem thats just started to happen with their combi system in a small flat with 5 radiators.
When they turn on the hot tap at the bathroom basin it runs cold until they turn on the tap on the kitchen sink then the water from the basin hot tap comes out hot.
No problem with the kitchen sink hot tap, this gives hot water every time.
They have a service contract with that energy supplier from north of the border and southern (not mentioning any names). They called out the service engineer but all they suggested was the system needed a powerflush (at £500 cost, probably 'cos they get a bonus on extra products sold), but the Central Heating side of the system is working fine so don't see how that will cure the problem. It's not a problem for me to solve but thought I'd ask you guys who know so that I can give the customer some advice for when the service engineer comes out again to blind them with science and suggest a powerflush as a solution. I'd be grateful for any help/advice.

 

[Puddle]

well you do not say make of boiler,it could be the diagram in the combi is worn,or maybe ,if they have a shower mixer ,the non return valve on the hot side has failed,therefore when hot tap on basin is turned on,cold is also being drawn from the shower,to check ,if surface mixer,run shower till hot ,turn off,then turn basin hot tap on,see if hot side of shower goes cold

As you say not really likely to require a power flush but I will do it for £ 498.30

 

[tamz]

Did you actually ask her how many rads she had?
Just joshing.
Might be a flow switch problem. What boiler is it?
How much are they turning the basin tap on?

Disappointing to see SSE (i'll name them) are adopting big gas attitude with everything being down to a powerflush as they started as a decent company.

 

[tamz]

I will do it for £ 498.30

Is that plus vat

 

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Thanks for the rapid replies guys. I don't know the make of boiler but will ask, also I will ask if they have a shower mixer. Thanks also for the bargain price for a powerflush. I'll advise the customer to take you up on that......