Glow Worm Ultimate 30c boiler not reaching temperature

13 Dec 2018
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United Kingdom
hope there is someone out there who knows how this issue is resolved or what could be causing it.

We have a Glow Worm Ultimate 30c central heating boiler installed 4 years ago - this problem has been on-going since installation with various 'solutions' suggested - none of which have worked. The system has been flushed, there is a Magna thing for debris, new pipework has been installed (as it was suggested flow was the issue), new radiators. The original installer referred it to Glow worm as he had no more solutions to suggest.

Basically the boiler will not stay at the temperature it is set at for heating (75deg currently) - it will go up to it temporarily (ie about 3 seconds ) then drops back down to 50deg - it varies between 36deg and 50deg whilst trying to heat consequently the house very rarely gets up to temperature..

Could it be an internal wiring issue to the internal boiler stat - what other mechanisms are there internal to the boiler that could be causing the issue? The only other thing that hasn't been changed is the room stat - could a fault on this be causing it?

Before this boiler was installed the other boiler worked fine on central heating and was only changed because of an issue with the hot water. Any help much appreciated before the next engineer visit on Monday.
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How big is the system ? big rads long pipe runs ? and what id d0 set to ?
Yes big 10 rads over 3 floors - boiler on middle floor - but everyone says boiler should cope with it - what is d0?
Can you press the minus button ( - ) on the display when the heating is running and post what the s code is ?
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There's no S code or anything comes up - it just changes the temperature


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You have press and hold the button for more than 3 seconds.
Hi D0 - was 20 have changed to 25 as per factory settings, D40 - 49 and D41 48 - (I think) - should these be higher? It says you can't modify them
Hi it's 5.3 (the 3 is flashing) - but is that S.53? Waiting cycle lack of water in the installation / temperature rise between heating flow and return is too high
Ignore that - eyes going to wrong code - just the temp diff bit If triangle T >30 - forcing to Pmin
Help still required - to summarise above - house is 3-stories - old Victorian - boiler is on middle floor - 10 rads - many big ones - everyone says boiler should cope. The D0 code was set to 20 - changed it to 25. The D40 and D41 codes are set to approx 49deg - is this the problem? Won't let me change them. The Status of the Appliance code shows 5.3 (assume this is S.53) (Waiting cycle:temperature differential between heating flow and return is too high. If DeltaT >30 forcing to Pmin.) What does this mean?
sounds like you have a circulation problem the D40 and D41 are temperature readings not settings
The boiler is suffering from insufficient flow, without seeing the boiler I cant be 100% sure but would hazard a guess at pump being undersized, I would fit a pump switched by room stat to improve flow.
The boiler is monitoring the temperature across the main heat exchanger...if that temperature difference becomes too large then it will drop into an S53 status whilst temperatures close...however it sounds like it is always jumping into S53 "mode".That could be due to the heating power setting being too high for the system and/or insufficient water flow through the radiator system.

For now it might be worth you lowering D0 to say 15 and see if that improves things. That may not be sufficient KW power to bring the system fully up to temp but the running temperature may get higher. Look at D40 and D41 whilst it's running...this will tell you the temperature drop across the heat exchanger..15 to 20 degrees is ideal.

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