Vaillant VUW 242E hot water problem

[chrismcc]

I have a Vaillant VUW 242E (or it might be a 282, not sure), about 7 years old. The heating works fine but I can only draw hot water from any hot tap at a very slow rate to keep the burners lit. If I use the shower or turn the taps on full the boiler cycles on and off with the water going hot then cold. Any ideas to what the problem could be and possible solutions greatly appreciated.

 

[DP]

Initial thought is the system was not chemically cleansed/ power flushed then to be treated when the boiler was fitted. The muck that was in the installation is now resident in the hot water heat exchanger. You could clean the system (not just drain it as muck remains in the system) and replace/ reverse flush the secondary heat exchanger

 

[kevplumb]

wouldn't that work the other way round
dp ?

 

[ChrisR]

Wot, you mean PD? eh?

The secondary heat exchanger isn't getting the energy out of the primary quick enough. The boiler doesn't modulate so it has to go Hi Lo on the burner pressure.

I guess it's possible that the pump is slow or the secondary side of the h/e has a lot of limescale in it which would act as an insulator, but I haven't seen that.

Is the tap water VERY hard? London is sort of middling and limescale isn't a major problem here.
If you're in a hard w area and you've left the temp knob on max it COULD have deposited scale, I remember they say don't do it, for that reason, on that model.

You really need to probe around with a contact thermometer to diagnose, I think - unless it's the pump.

 

[chrismcc]

The water in my area (South coast near Portsmouth) is quite hard and I often have to descale the shower. The hot water temp is set quite high at the moment because of this problem and the boiler has never been descaled since it was fitted. Has been serviced though. Do you think descaling the DHW heat exchanger would help to alleviate the problem?

 

[bogstandard]

Without a doubt cleaning the heat exchanger would be a very positive move particularly in the Portsmouth area which has a high chalk level. The exchanger should be of the sardine can type much easier to service than the previous used exchangers ( large bakalite exterior jobie)

 

[chrismcc]

Thanks for that information. I think that the heat exchanger is fairly straightforward to remove/refit but is there a particular chemical I should use and how long do I leave it for? Also, will I need new seals? On the same subject I have just noticed that whenever I run the hot water I get a leak from the diverter valve above a small microswitch on the right hand side. The water stains show a lot of scale. Not sure what to do about it.