Vailant VCW240 pilot cutting out after long dhw draw off

[mark]

I have a Vailant vcw240, approx 14 yrs old, still doing excellent service, but recently the pilot light has been going out.

I had originally thought that it was blowing out due to a draught, but yesterday I was watching it when a lot of hot water was being drawn off. Just after the tap was turned off the boiler was making boiling noises, and I opened the door to see the temperature gauge rising through 70 degrees up to 95ish. It didn't stay there and immediately cooled to around 50 degrees. This all happened in about 15 seconds. When the temperature was at its peak there was a click and the pilot light went out. I am fairly certain that an internal temperature cutout is switching off the gas.

I have tried watching it several times since, switching the dhw on and off, and the temperature never rises above 50 when I'm watching it, but around 1/3 of the times I draw water off for a shower, the pilot light goes out immediately afterwards.

The system is in good condition, and gets flushed and dosed with corrosion inhibitor every year. The pressure is set to about 1.5bar.

Any help would be appreciated as my wife generally has a shower after me every evening and she is getting fed up with the water being cold.

Thanks,
Mark

 

[fatoftheland]

Suspect the hot water heat exchanger is scaled. More heat is needed to heatthe scale and the water. When the water stops flowing there will be alot of latent heat in the exchanger due to the scale and so pops the over temperature stat. Flushing and inhibitor only protect the heating side.

 

[mark]

What you're saying makes sense. The DHW heat exchanger does fur up badly - I keep a spare and swap them over every autumn then flush out the one Ive removed and let it soak in vinegar to clean out the deposits. I diod change it only a couple months ago though so I wouldn't expect it to have clogged up again so quickly.
Cheers,
Mark.

 

[mrgas]

i might be way off but if you suspect that a thermal cut device is cutting off the hot water supply would that suggest that DHW thermastat is no longer sensing the correct temprature and keeps tripping the high limit? like i said just a thought i could be talking nonsense.

 

[fatoftheland]

The opposite sometimes applies. Imagine the flame heating the heat exchanger as the cold water passes through the waterways. In normal circumstances the heat from the flame is transferred through the heat exchanger (basicly a tube with fins attached) straight to the cold water.
Now imagine the same scenario with scale inside the heat exchanger. The flame heats the exchanger, the heat slowly heats the scale, the scale gets hot and passes the heat to the water. When the tap is turned off the flow switch cuts the flame and that should be it.
But the scale now has a lot of heat built up in it. The tap is closed so the heat cant transfer to the water. The heat radiates and conducts from the scale, back to the heat exchanger and the temperature is higher than the water set point. Pop goes the over limit stat.
I have seen on large industrial calorifiers (hot water tanks) the boiler is turned off because the boiler thermostat is cut off at 80degrees C whilst the scaled up Calorifier has a stored temp of 30 degrees C.
In areas of hard water (south and South east) this is a very common occurence. In some areas combi boilers dont last more than two years!