Gas Work Question

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

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 12th Apr 2011

Southsea Plumbing, Heating, Electrical & Gas Services

Member since 8 Nov 2010

could be your d/h/heat exchanger or thermista

Answered 13th Apr 2011

JP Boilers, Kitchens and Bathrooms

Member since 4 Feb 2011

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

Answered 12th Apr 2011

tm property services

Member since 9 Mar 2011

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

Answered 12th Apr 2011

DMW Heating & Gas Services

Member since 22 Jun 2008

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 12th Apr 2011

Gavin Clarke Contracting Services Ltd.

Member since 5 Oct 2010

probably the diaphragm....brian

Answered 12th Apr 2011

E&M Plumbing & Heating

Member since 26 Feb 2009

hats of to 20years plate to plate gone fit new

Answered 20th Apr 2011

roy morgan plumbing&heating

Member since 29 Jan 2011

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

Answered 23rd May 2011

AM heating

Member since 13 May 2009


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 12th Apr 2011


Member since 22 Jun 2009

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


Member since 8 Oct 2009

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

Gas Force Direct

Member since 27 Apr 2015

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

CT Heating & Plumbing Ltd

Member since 30 Apr 2018

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