Hot water problem - old Vaillant boiler.

[Mike1501]

Hi,
I have an old Vaillant VCW-sine 18 T3 W. Yes I know it's ancient but I would like to squeeze a few more years out of it if possible.
The central heating works fine but it has developed a fault concerning the domestic hot water - when a tap is turned on, the tap water does not get as hot as usual. It is the same whether or not the central heating is on.
The water starts off warm, then goes stone cold for say 20 seconds then heats up again but never gets really hot as it used to. If I turn up the thermostat knob on the boiler, it makes no difference to the temperature of the hot water.
Anyone know what might be at fault?
By the way, the boiler had a new electronic control board installed two years ago so that, at least, should be in the clear.
Cheers
Mike

 

[AfterburN003]

Possible worn diaphragm in the divertor valve maybe. Diaphragm not tight enough to switch on microswitch. A divertor valve service kit is available i think.

 

[The831Bunny]

Hi

From a cold start run the hot tap, does the heating flow pipe (far left hand of the five) get hot after a few seconds? If so, the Flow Switch (Diverter Valve) needs to be replaced. You used to be able to replace the wax capsule, but thats now obsolete

Bunny

 

[Mike1501]

Hi Bunny,
Thanks for the advice.
Tried your suggestion but left hand large pipe remains cold whilst the domestic hot water pipe (second left, narrower) warms up but not very hot.
Mike

 

[Mike1501]

Hi AfterburN003,
Thanks for the reply. Excuse my ignorance but when you say a diaphragm in the diverter valve, is that the flow switch or the water section valve?
Cheers
Mike

 

[Agile]

I think there is a bit of mis information going on here!

The DHW flow is sensed by a diaphragm operated switch underneath.

The diversion is performed by a wax element in a similar way the Puma used later on.

Without seeing it I cannot come to any comclusion whats wrong.

Tony

 

[Mike1501]

Hi Tony,

Thanks for the reply.

The diaphragm-operated switch you mention – is that what Vaillant refer to as the water section valve? If it is then this appears to be working OK. When the hot tap is turned on the boiler fires up so I assume the diaphragm is operating the microswitch. Presumably if it was not the boiler would not fire at all?

Looking at the diagram in the little Vaillant leaflet I have, it looks like the wax element you mention is in the base of the flow switch (described by Vaillant as a thermostatic element). Am I looking at the right part? I assume this expands and contracts according to temperature and affects opening and closing of valves in the flow switch so it looks a likely suspect although, as I mentioned to Bunny, the heating flow pipe remains cold. In your experience, when these components start to go do they usually cause the DHW to underheat? My only experience is with car thermostats and they usually end up stuck wide open or fully closed.

My other concern is a possible fault on the (recently new) electronic control board. Apparently there is a DHW potentiometer on there that is pre-set for 82degreesC and my DHW is nowhere near that temperature. I have no idea how this actually controls the water temperature but I would assume it does by affecting the gas flow? Certainly when I run the hot water the gas roars away all the while but the water only gets up to about 38C.

Mike

 

[Agile]

You are looking at the right parts.

The diverter valve can stick in any position.

Until thats solved ( with difficulty ) its not easy to come to any conclusion about any other faults which might be present.

Tony

 

[howmuchlonger08]

The only thing that controls the water through the diverter valve is the wax capusle in the base of the diverter, you can see the valve spindle moving by removing the aluminium cover from the top of the diverter valve, it should pull down when you open a hot tap, and then slowly move back up when tap turned off due to heat in he base of the Div Valve.
Check that the micro switches are connecting when spindle moves.

 

[Mike1501]

Many thanks for the replies.
I shall do some more checking as suggested.
The thing that really puzzles me is that the gas is roaring away, the CH flow pipe is not heating up but the DHW is barely hot either so where is all the heat going other than out of the flue?
Cheers
Mike

 

[The831Bunny]

Hi again

Two things need to be checked. Hot water flow rate and gas rate. The gas rate can only be checked by a Gas Safe Registered engineer. Hot water flow rate is easy to do though.

With age the flow restrictor can let through too much water. As this boiler only gives out 19.5Kw the flow rate for hot water has to be around 6.5 litres per minute to achieve about 50C at the taps. Theres no great problem trimming the flow at the cold isolation knob till the hot water is hot enough. You will be doing well if the plastic knob is actually there, and you will be doing really well if you don't break it If so, just turn the spindle in with pliers till the right temperature is reached. You may be surprised to find it ends up very nearly turned all the way in to reach the right temperature. You can cross reference the flow using a 10 litre (2 gallon) bucket and timing for 1minute 33 secs. That should be the time for 10litres.

The fact that the flow pipe remains cold when DHW is drawn is a good sign! It shows the wax capsule (thermostatic element) is still OK.

Bunny

 

[Agile]

You can measure the water flow rate with buckets and a watch.

You can also measure the boiler input power at the gas meter. See the site FAQs under "gas rate".

The boiler also has a DHW preheat. If this is not on then the boiler will have to spend a bit of time heating up the boiler's own water content.

Tony

 

[Mike1501]

Many thanks Bunny and Tony for your advice.

I have checked the water flow rate and it is taking approx 1 minute to fill a 10 litre bucket so it looks as though it is flowing to fast. I will adjust it and see. By the way, the white knob is still intact! For the moment at least.

I looked up "gas rate" under the FAQs (for checking the boiler input power) but unfortunately I cannot do this, as I have no gas meter. I am on LPG and all I have is a contents gauge on the tank.

By the way, is it worth getting hold of a wax capsule replacement kit while they are still available? i.e. do they always pack-up eventually?

Cheers

Mike

 

[The831Bunny]

Ah LPG!

You may be also suffering from blocked injectors on the burner manifold. Again you need a Gas Safe Engineer to check this. Common problem on LPG injectors that reduces the already small orifice caused by deposits from the gas.

The wax capsule on its own is very hard to source now. Part No. is 140106 and yes they do eventually fail, although 15-20 years is a good life expectancy for one. Your boiler does qualify for the boiler scrappage scheme incidentally.

Bunny

 

[Mike1501]

Success! I have adjusted the flow at the isolation valve and this seems to have cured the problem. The DHW is now at a decent temperature and taking a shower has been transformed! My wife is delighted. Many thanks for the advice. You were right about having to turn it almost all the way in. I don't know what type of valve it is but it certainly isn't a very progressive one. I will get the boiler serviced and cleaned during the summer and will mention the possibility of blocked injectors.

I have found a couple of on-line suppliers who still have stock of the wax capsules (part No. 140106) available at around £30. I think I had better get one - I doubt if mine has ever been replaced and the boiler is now 27 years old.

I am not a great fan of LPG; my wife calls the tank a "huge potential bomb in the garden" and the price that Calor charge me has more than trebled in the last 9 years. If I updated the boiler is there any merit in considering a switch to oil? I believe that oil is more energy efficient than LPG and I understand that oil fired, through-the-wall combi boilers are now available (it would need to be a combi in my case) but are they any good or just asking for trouble?

Again, many thanks to Bunny and Tony. I owe you both a pint!

Mike