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Andrew Smith
Andrew Smith, HVAC Technician
Category: HVAC
Satisfied Customers: 3586
Experience:  Heating and Controls engineer
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Hot water too hot then cold

Customer Question

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

Submitted: 2 years ago.
Category: HVAC
Expert:  Andrew Smith replied 2 years ago.

Andrew Smith :

The boiler should adjust the burner pressure based on water temperature and modulated down

Andrew Smith :

You may find it is a thermistor fault or the gas valve is not modulating properly

Andrew Smith :

Has anyone checked this boiler?

JACUSTOMER-ltytsz0l- :

No one has checked the boiler.

JACUSTOMER-ltytsz0l- :

I understand that the thermistor leads to a cut out rather than to modulate on this model? If I'm mistaken, that's plausible

Andrew Smith :

Do you have the GC no. and I will dig out a diagram?

JACUSTOMER-ltytsz0l- :

GC number is 47 04414

Andrew Smith :

Is it a Turbomax?

Andrew Smith :

Or a Combi Compact?

JACUSTOMER-ltytsz0l- :

A CombiCompact

Andrew Smith :

http://www.acwilgar.co.uk/Boiler-Manual-PDF/Vaillant/Combi%20Compact%20VCW%20GB%20242EH%20282EH%20242EB%20282EB.pdf

Andrew Smith :

Page 33 says the hot water should modulate, this is why it is saying to run the hot water fast

JACUSTOMER-ltytsz0l- :

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.

JACUSTOMER-ltytsz0l- :

Also, this is the boiler:

JACUSTOMER-ltytsz0l- :

https://www.heatingspares247.com/download/Boiler_Manuals/Vaillant/VCW-GB-221-H.pdf

Andrew Smith :

Nice find!

Andrew Smith :

Looking at the drawings on page 68;

Andrew Smith :

If the CH overheated it would put the pilot out

Andrew Smith :

If the main heat exchanger overheated then it would 'pop' the stat and you would have to manually reset it once it had coolled

JACUSTOMER-ltytsz0l- :

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)

JACUSTOMER-ltytsz0l- :

It behaves like this whether or not the CH is on.

Andrew Smith :

The hot water has a thermostat and a NTC thermistor (an early type) but it should still make your boiler modulate?

Andrew Smith :

Have you had the thermistor swapped too?

JACUSTOMER-ltytsz0l- :

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?

JACUSTOMER-ltytsz0l- :

I haven't carried out any works on the boiler yet. I am keen to solve the problem myself however!

Andrew Smith :

If you can watch the burner and it does not modulate then I would start with the sensors

JACUSTOMER-ltytsz0l- :

Could you provide more direction - ideally with reference to the instruction booklet?

Andrew Smith :

Ok, page 35 shows the cover open and the viewing window

Andrew Smith :

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?

JACUSTOMER-ltytsz0l- :

The whole boiler turns off, and then turns back on again.

Andrew Smith : The boiler is not modulating then.
Andrew Smith : you will need someone to check voltage at the gas valve to see if it goes up and down but the flames do not? This would point to a problem with the gas valve
JACUSTOMER-ltytsz0l- :

I've ordered a voltage tester, I'll have a go myself and see where we get to. Thanks so far.

Expert:  Andrew Smith replied 2 years ago.
No problem

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