Vaillant Combi Boiler - HW Issue

[speshman]

Hi all

I have a 14yr old Vaillant combi boiler in my house that is becoming increasingly annoying with regards to hot water.

It does always supply hot water but the boiler 'pulses' on and off whilst supplying leading to a greatly varying temp coming out of the taps from lukewarm to moderatly hot. It never seems to get hot hot if that makes sense. If you were to run a full bath on the hot tap alone you can just jump right in.

No difference with the CH on or off and it doesnt appear to be diverter as with the CH off there is no heat going down the CH pipework.

There appear to be 2 methods of adustment on the hot water side of things. One is right under the boiler that appears to be a tap to shut off the cold feed and then there is a large knob protruding at the bottom front of the boiler which seems to have no effect whatsoever.

The boiler is serviced annually and the 2 so called gas engineers that have visited to find the problem have failed and both suggested almost randomly swapping parts out till it gets cured, an approach which my wallet objects to!

So.. I've found a thermister on the HW circuit that I assume is used to measure the temp of the water but have no idea what the values on there should be if I were to measure it... anyone shed any light?

Also could it be a flow rate issue? anyway of checking?

Basically and idea's at all will be most welcome and i'm 100% confident as a time served spark at sorting anything that does not involve messing with the gas

 

[speshman]

oops forgot to say..

if you pop the front panel off there are 2 lights underneath. An orange one that appears to denote the burners lit and a green one that illuminates as soon as hot water is called but then fades slowly out about 5 seconds before the boiler extinguishes. The green light then lights again about 10 seconds later and the boiler imediatly fires again.

Repeat ad nauseum

 

[45yearsagasman]

My crystal ball seems not to be working.so I can't tell what model you have but it could be something as simple as a a restricted plate heat exchanger.
After reading your additional comments it may not be that.More info required please.

 

[vaillantmark]

I'm with the 45yearsman, sounds like the plate is severely blocked restricted. However, we'll all be in the dark until we find out what model. Sounds like a VCW 242/282E?
Have a look here and see if you can see it?
http://www.vaillantservice.co.uk/VAILLANT/Identify_Your_Vaillant_Boiler.html

 

[speshman]

I'm with the 45yearsman, sounds like the plate is severely blocked restricted. However, we'll all be in the dark until we find out what model. Sounds like a VCW 242/282E?
Have a look here and see if you can see it?
http://www.vaillantservice.co.uk/VAILLANT/Identify_Your_Vaillant_Boiler.html[/QUOTE]

According to that it is a VC/VCW but I'll be buggered if I can find any identifying anythings on the boiler itself

 

[vaillantmark]

The fact that the green neon drops in and out may indicate the water section is shafted... Check the hydraulic piston rises and remains steady when the hot water tap runs..
You've got one of the most hydraulically complex domestic boilers ever built.. Good luck

 

[speshman]

When you say hydralic piston do you mean the part central in the boiler that has a lever arm and a microswitch?

If so it appears to be working correctly

 

[speshman]

one thing that springs to mind is the part on the back left of the boiler (water heat exchanger?) was changed a good while back as it was originally plastic I think and was totally knackered, its now a stainless item.

I also live in a very very soft water area and have a 14year old kettle with zero scale so I'm pretty confident its not a scale issue.


Also the expansion vessel in the back of the boiler is long dead and it has now been fitted with an external item that seems to work as well, if not better

 

[vaillantmark]

Well, the green neon does extinguish with a decrease of voltage to the diaphragm pump, then the modulation is controlled by the Flow ntc, which varies the voltage to the diaphragm pump via the PCB, which has the DHW overheat stat wired in series..
Which points to a rapid rise in temp on the plate!
We'll be here all night, and I'm afraid food and a nap beckons.. I'll be checking up on your progress later

 

[Andygasman2010]

As already said... you have a blocked plate heat exchanger. The green neon dimming then extinguishing makes this pretty much a cert!

 

[speshman]

Is the offending heat exchanger the small one at the bottom left of the boiler at the back?

Thanks for the continuing help, most appreciated

 

[Andygasman2010]

Yea that's the one. Take it out and soak each side in something like kettle descaler.

 

[speshman]

Looks like a job for next week to me, fiddly stuff doesn't happen on the weekends

 

[speshman]

Holy thread resurrection batman

Well since I first posted this I replaced the dhw heat exchanger and it made bugger all difference, had 2 other plumbers come and suck air through their teeth so I'm going to have anothe crack at it.

I've re read a few things posted above so now have a few more questions.

Where is the overheat stat and is it possible to test it? I'm assuming it's one of the thermistors I see about the boiler and I suspect it's the one Isobel by the ontrol panel to the left? Looks to be in series with something up in the burner compartment too (which, being a spark and not a plumber I'm rather reluctant to touch).

Also as an update on my aging boiler in general it's now also being rather reluctant to run the ch on anything less that 8 on the small stat as it just fails to do anything aside from sitting there with the pump running.

 

[speshman]

Oh also I found the model number as vcw282gb