How can I test my DHW Heat exchanger?

[RogB]

I have a Vaillant VCW 25 Combi Boiler. The CH is working but when the hot tap is turned on, the burners fire-up and remain burning until the demand for hot water is terminated but the water out of the tap is only 'luke warm'. The diaphragm has been changed by a plumber but it had no effect. He now suspects the DHW heat exchanger, but can't guarantee it. Is there any test I can do to prove that it is the heat exchanger? Hot water going into Heat exchanger about 70 deg, and dropping to about 60. The cold water is going in at about 6 deg and I am getting about 25deg out.

 

[corgiman]

Whats the temperature like when you only turn the tap off half way?

 

[RogB]

If I reduce the flow of the hot water the Boiler will eventually go out, but the temperature does not increase. It looks like the circulating water from the main heat exchanger has reached its temperature and turned the burner off

 

[ChrisR]

Can't picture a 25!
Is it a tall black hex or a flat stainless steel plate type?
Sounds like a partially sludged hex, but
With the hw tap on full, time thgas usage at the meter, for one rev of the small dial if cu ft, or measure for 2 minutes if metric. We'll do the sums!

Could just also be a faulty temp sensor. Check resistance for that.

 

[Agile]

Whilst you have measured the temps which is good you also need to measure the DHW flow rate!

Than we can get an idea of whats happening!

Tony

 

[bengasman]

Blocked secondary heatexchanger typically results in fluctuating temperature.
If the tapwater is consistently low, even at low flow I would look at the thermistor, diverter or occasionally the pcb. pcb is quite rare but possible

 

[DP]

Guys, this is our old friend the VCW25 (Sine's bigger brother). Changing the diaphragm on this boiler will not have cured the problem. Diaphragm is tell the boiler you are looking for hot water but it plays no part in heating water foor the hot tap.

When boiler is delivering depid water, do the central heating pipes heat up?

 

[RogB]

The DHW flow rate is approx 10L per min. The CH pipes do not get hot when hot water is being demanded, so this this prove that the divert valve is ok? How about the pump?

 

[RogB]

The NTC was also replaced as part of the diagnostics, but made no difference.

 

[Agile]

Then the problem is that the secondary HE is not transferring the heat to the DHW much.

Cleaning or replacement time!

If you reduce flow rate to 3 li/min then what are the temperatures measured?

Tony

 

[RogB]

Thanks to everyone for their interest in helping me.

With regard to the Gas usage, I did the test as you suggested. I have a metric meter and in a 2 minute period the meter went from 13.0 to 22.2

 

[RogB]

I haven't got a thermometer available to me today, but I 'feel' no major difference. However when I reduce the flow rate too much the burner goes out. When the hot water is turned back on I get 'acceptable' hot water for a few seconds.

 

[bengasman]

Thanks to everyone for their interest in helping me.

With regard to the Gas usage, I did the test as you suggested. I have a metric meter and in a 2 minute period the meter went from 13.0 to 22.2

that would be either 3 kw or 30.
3 would be hardly noticable, and 30 would give you a nice hot shower.

 

[Agile]

If its 30 which is about right then I would have expected the flow to be higher than 70C.

Its possible that the primary HE is also quite scaled up.

Tony

 

[RogB]

The boiler rating plate gives it as nominal input 35.5KW and nominal output as 26.6KW. As I have only recently moved in I have little history on the boiler but it was producing very hot water over the summer months. I appreciate that the cold water will be colder now but the difference is not proportionate.