Vailant VCW sine 18 T3 WF No hot water.

13 Nov 2016
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United Kingdom
Hi, first post on here.
I have an issue of no hot water when a tap is opened on the valiant VCW sine 18 t3 WF, no sounds or response whatsoever but, the central heating works perfectly. I've tried to pick the bones out of several threads on here as to what the problem is but don't seem to have had much luck.
Following one line of investigation recommended, I took the aluminium cover off what I think is the diverter valve (lower left hand side, with two microswitches) On turning on the hot tap there is no sign of any movement of the shaft which operates the microswitches as suggested in one post. If I open the hot tap and press the plate down manually, this presses the right hand switch which fires the boiler and the tap water gets hot. If I trip the left hand switch manually (as the plate on the shaft would if it moved up) nothing happens. Tried a drop of WD40 on the shaft in the hope it may free things up.
Just wondered if anyone could point me in the right direction please?
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At the base of the diverter valve there is a wax capsule which expands when hot water is required, this expansion pushes the rod up to operate the micro switches. I think you need a new wax capsule but unfortunately I think they are now obsolete.
Is this one?

No, that is a differential pressure valve. The wax capsule is called a thermostatic element.
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Sorry to seem a little dim, but what is the sequence of events from turning on the hot tap and thus, what, and at what point in the sequence does it tell the burner to fire? Is the flow valve the part that senses a tap has been opened or is that the differential pressure valve? (though I assumed that was a gas component)

I seem to be getting confusing component names too like, is the diverter valve also called the flow switch or is that something different? I thought the diverter valve was the water valve? yet the parts catalogue shows the "Thermostatic element (wax capsule?) located in the flow switch.

I have no intention of touching anything gas related but feel sure this is a mechanical issue.I feel sure if I know the sequence in which things should happen, through a process of elimination i should be able to at least work out if it is repairable or 2 to 3K of a job.

Thanks for such quick responses guys, I really appreciate it.
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At the base of the diverter valve there is a wax capsule which expands when hot water is required, this expansion pushes the rod up to operate the micro switches. I think you need a new wax capsule but unfortunately I think they are now obsolete.

Actually, the wax contracts when it cools!

It is cooled by a flow of DHW and that allows the upper part ( if it is free ) to move down and divert the boiler water into the DHW heat exchanger.

But this operation is only to follow either:-

A demand for DHW

A DHW preheat request.

While the wax stat is officially obsolete, I think the whole assembly is still available,

I still have a few left for my own customers!

Hi Tony,
Based on my description of the problem, would I be taking a big risk just buying and fitting a replacement in the hope this would solve the issue of no hot water, or would you suggest any solid tests or alternatives I could do first?

I don't want to throw a bucket load of money at it unless I'm fairly sure as It's not actually my boiler, it's an elderly close friend who I'm just trying to help out.

I would suggest elderly close friend seeks a qualified engineer who is comfortable repairing this excellent boiler.

Deciding factor is going to be what state other components are, which only someone who is familiar with this boiler would be able to check.

If this were my boiler, deciding factor if an engineer should proceed with repair, would be how he removed the cover after the control knob is removed:whistle:
The way this boiler works is rather unique.

Few engineers are very familiar with it.

They are now over 20 years old and although a very solidly built boiler, they are very over engineered and there are few good reasons for not replacing them.

I do offer customers to repair them but only after asking them if they really do want such an old boiler repaired.

To me its more of a fun thing like restoring old cars!

I would never recommend that model for DIY repairs! Particularly for anyone who is not the owner!

What bothers me is, the same old story that crops up on here time and time again, three people have looked at this boiler and said, "Ouch, this looks like a new boiler or, you can't get parts for these anymore" etc. and I appreciate that they can't spend loads of time systematically working through an obsolete piece of kit but, I am a retired engineer / toolmaker an have all the time in the world to tootle on and find out what's making this little old girl poorly.
Besides, reading between the lines on here, this is the best boiler ever built, which speaks for itself. I have a 1931 vintage vehicle that constantly needs cuddling and even parts making but we still manage to keep it running.

Can anyone suggest a good tech in the Yorkshire dales area maybe?
It is a very complicated boiler using a wax element in the diverter valve. Only one other boiler did that and it was not really very successful there.

One byproduct of it's complicated design is that it is very fault tolerant and will continue working when several parts have failed. It is rare that I ever go to one which does not have at least two faults.

Hi Tony,
I have spent all evening trying to find a wax capsule or complete flow valve (which I assume is sometimes called the diverter?) but with no luck.

is it likely this is the problem or is there a possibility it could be the Diaphragm and stuffing box on the water valve and I am barking up the wrong tree?

Regards, Paul.
PUl, as said earlier, engage services of an engineer who knows this boiler. You may we'll be able to get the wax capsule, I will even sell you one, but replacing it is going to be challenging as you need other seals which may of may not be available

If you were to but a full diverter, you would need to replace some seals, again problematic if you are doing it for the first time

Let us say you have manages all that, boiler will need primed or it will not work
It is difficult for me to guess what is wrong with your friend's boiler.

To diagnose they need an engineer who understands them at the boiler.

There are all sorts of causes. Some I have been to have had a working wax element but a jammed shaft for example.

What does it do when you select hot water preheat?

The boiler does not seem to respond in any way from opening a tap (when CH is off) but when the central heating has been on, there seems to be warm water for about 5 to 10 seconds which I assumed was coming from the small tank on the bottom left, in front of the diverter.

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