Vokera Linea 24 - Hot water problems. Please help!!

Hello. My heating works fine. My problem is my hot water.
When you run a tap the boiler lights, goes into high flame, temp rises to approx 35 degrees then the flame goes out & doesn't modulate (modulation works fine on heating). Temp drops as the water still pumps, fan is also still operating (which rules out pressure switches i think), then the boiler fires up again, temp rises and flame cuts out again at the same temp. A combi engineer has looked at the boiler and gave up!

He has replaced all thermistors, i have stripped and cleaned the diverter valve. I have also replaced the PCB with a spare, it has had a new main heat exchanger fitted (fault occured previous to this).

Fault totally baffles me as everything works ok on heating and doesn't this rule out most parts apart from the thermistors & PCB??

Please help as on the verge of spending a lot of money on a new boiler as i refuse to spend any more cash unless i can pin down a definate cause.

Any comments/help will be welcome.

Boiler (most probably) has been fitted to an existing system without cleaning the installation to remove sediment. This sediment is now in the HW HE.

Heat generated in the combustion chamber is not being utilised by the HW HE. Temperature rises too rapidly for ther control circuit to modulate. Modulation stage is bypassed to shut the burner till primary water cools sufficiently.

Very common problem. Mainly due to boilers banged in without taking proper steps.

Another common cause of your problem is the venturi in the fan system.
You need to remove the collar above the fan where the air switch pipes are connected and clean out the venturi with a fine wire and an air blow.
The venturi is mounted within the collar and looks like a tiny jet engine.
Make sure you mark the air pipes so that you reconnect the right way round.

Hello again.

Many thanks for your responses. I will try out your suggestions at the weekend. Fingers crossed!!

Only other question to DP's reply is what can you do about the sediment. Can you get additives to put into the system to remove it?? Or do you need to flush the system out somehow??

Cheers!!

An easy way to diagnose blocked plate heat ex
run hot tap whil you hold the black water isolating valve (on rhs of boiler which is also the filling valve) when boiler shuts off mains gas immediately turn yhe valve from 12:00 position to 09:00 position & check display pcb temp.It changes from measuring dhw temp to blr temp so if it is around 90 deg then your plate heat is blocked
Your plumber should have done this to start with.

Excellent advice!

I've tried the suggestion of shutting off the black valve and the temperature shoots up to 98 degrees!!

Question is now 'What can i do about it?' Do i have to replace the heat exchanger, or can I remove it & flush it out?? Is there an additive that can be added to remove the sediment / blockages??

Many thanks again.

You can try cleaning it by removing & leaving in a bucket of water & a chemical like deposit remover for a few hours (make sure you put it back correct way & replace the 4 o rings