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My combi boiler is only heating water to lukewarm. the central heating seems okay though. what could be wrong?

Boiler make is Vokera. It's approximately 20 years old.

12 Answers from MyBuilder Gas Engineers

Best Answer

Firstly, your boiler still chugging away after 20 years is good !

Your problem could lie with a thermostatic sensor, which is cutting out the flame before the water has reached the correct teperature.

Or more likely the heat exchanger is scaled and the heat transfer from the fame is not being able to penetrate through properly.

You don't say whether or not there has also beem a reduction in flow (volume) of the hot water ? If the outlet pressure has dropped significantly, this is also a classic sign of a blocked or scaled up heat ecxhanger.

Kind regards,

Adrian Hagger


Answered 13th Apr 2011

could be your d/h/heat exchanger or thermista


Answered 13th Apr 2011

sounds like the thermostat for the water is faulty regards Terry.


Answered 13th Apr 2011

It look like divert valve faulty by limescale.
Replacement new one by gas safe registered.


Answered 13th Apr 2011

It sounds like your HW plate heat exchanger could be scaled up. More info on the boiler would be helpful to diagnose. Is it a combi or a system boiler.



Answered 13th Apr 2011

probably the diaphragm....brian


Answered 13th Apr 2011

hats of to 20years plate to plate gone fit new


Answered 20th Apr 2011

new boiler lol..........


Answered 23rd May 2011


It could be a number of things including blocked plate heat exchanger, passing diverter valve, h/w temperature sensor amongst other things.
It would be best to contact a gas safe registered engineer to have a look for you.
Good luck,



Answered 13th Apr 2011

it could be the boiler's needs to be serviced and checked such as the heat exchanger , heat plate , expansion vessel there number of things that might be wrong so the stating point is to check the gas rate, working pressure and full service
regards hunar


Answered 13th Apr 2011

This boiler is a nightmare..I had exactly the same issue; changed plate to plate, diverter, actuator, ntcs, pump, flow switch and pcb board...problem still existed...central heating runs to as you run the hot water tap hence its lukewarm.

Only option am left with is checking the main heat exchanger but it's not logical to me as why it's diverting



Answered 1st Jan 2018

Themista at first instance as central heating seems to be working ok as you mentioned, then If in hard water area and the boiler has been going for 20 years, it’s probably never been flushed and cleaned, it’s worth to do a power flush when you want to change the boiler in future.


Answered 10th Jun 2018

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