I have reasonably low water pressure and my combi boiler's pressure is fine and the central heating works ok - unfortunately the DHW is putting too much energy into the hot water. The flow issues have been checked and everything is open with the boiler's servo valve toward the cold end but there's no change. Any ideas for reducing the energy going into the DHW?
The model number of the boiler is:
Vaillant - VCW GB 221 H
The boiler should adjust the burner pressure based on water temperature and modulated down
You may find it is a thermistor fault or the gas valve is not modulating properly
Has anyone checked this boiler?
No one has checked the boiler.
I understand that the thermistor leads to a cut out rather than to modulate on this model? If I'm mistaken, that's plausible
Do you have the GC no. and I will dig out a diagram?
GC number is 47 04414
Is it a Turbomax?
Or a Combi Compact?
Page 33 says the hot water should modulate, this is why it is saying to run the hot water fast
That refers to the CH thermostat. The DHW is adjusted with a water pressure selector but with low water pressure, I think it's overheating - see p.69.
Also, this is the boiler:
Looking at the drawings on page 68;
If the CH overheated it would put the pilot out
If the main heat exchanger overheated then it would 'pop' the stat and you would have to manually reset it once it had coolled
Yes, the CH does not overheat. The DHW however gets very hot, then cuts out when the cold is put on (and the flow rate into the DHW is reduced)
It behaves like this whether or not the CH is on.
The hot water has a thermostat and a NTC thermistor (an early type) but it should still make your boiler modulate?
Have you had the thermistor swapped too?
I should add that after turning the cold water down, the DHW then heats up again - is that normal behaviour given the functional setup of the boiler?
I haven't carried out any works on the boiler yet. I am keen to solve the problem myself however!
If you can watch the burner and it does not modulate then I would start with the sensors
Could you provide more direction - ideally with reference to the instruction booklet?
Ok, page 35 shows the cover open and the viewing window
Can you look through the viewing window when the hot water is running and see if the flames reduce in size whilst holding the hot pipe underneath and see if the two happen at the same time?
The whole boiler turns off, and then turns back on again.
I've ordered a voltage tester, I'll have a go myself and see where we get to. Thanks so far.