Vaillant Combi boiler VUW 282 no hot water (upstairs only)

[Jimbojunk]

Scratching my head here.

Boiler is fitted down stair , heating is fine.
Down stairs Kitchen DHW works fine.
Upstairs sinks and shower are cold
Upstairs bath is hot..

When upstairs sink tap or shower is turn to hot the white lever on the diverter valve moves slightly but not enough to depress the micro switch. Note: This operates correctly for the kitchen.

The diverter valve is fairly new and has been replaced twice during the life of the boiler.
The heat exchanger has never been replaced, and I am feeling this may be the cause of the problem restricting flow such that the upstairs is the first to see the failure. Prior to this failure I have experienced showers which fluctuate hot warm which I understand can also point to heat exchangers.

I wanted to run my logic past some experts before I spend £140+.
I have measured flows rates at various points:

Downstairs
Cold tap kitchen: 10.5 l/min
Hot tap kitchen: 8.4 l/min

Upstairs
Cold tap bath: 10.4 l/min
Hot tap bath: 7.2 l/min

Cold sink: 7.2 l/min
Hot sink: 4.8 l/ min

Shower flows seems to be closer to bath but still not sufficient to demand DHW.

The flow reduction upstairs (between bath & sink) seems to be consistent and at ~ 30% - good or bad I'm not sure. Note: Plastic pipe is used for all house plumbing.

Interestingly, if sink and shower turned to hot at the same time the boiler will fire up.

Any help gratefully received,
Jim

 

[Bunnyman]

You have a problem with your shower mixer or a non return valve passing, definitely not a boiler problem

 

[Jimbojunk]

You have a problem with your shower mixer or a non return valve passing, definitely not a boiler problem

Thank you for this steer, this is something I had not considered but makes a great deal of sense.
I don't believe I have any non-return valves in the system but two upstairs sinks and the shower have mixers, with the shower being non-thermostatic type. I can easily isolate the main sink hot and cold which should be a start in trying to isolate the problem.

Any tried and tested methods you have for tracking down the mixer at fault will be much appreciated!

Thanks again

 

[Bunnyman]

Try turning off the boiler cold isolation valve, then open a hot tap , or a shower on hot only an see if you get water flowing .( u shouldn't get any)

 

[Agile]

I am still suspicious!

Turn hot tap on in kitchen and SLOWLY increase the flow until the boiler JUST fires up.

Then measure the actual flow rate! It should be just about 2.5 litres per minute.

I suspect it may be far more!

Tony

 

[Bunnyman]

Tony this indicates different..When upstairs sink tap or shower is turn to hot the white lever on the diverter valve moves slightly but not enough to depress the micro switch. Note: This operates correctly for the kitchen.

 

[gazdaz36]

How fairly new is the diverter? I've had a few problems of diverter valves going wrong after 18 months. If your getting hot water it isn't going to be the plate heat exchanger.

 

[Jimbojunk]

Try turning off the boiler cold isolation valve, then open a hot tap , or a shower on hot only an see if you get water flowing .( u shouldn't get any)

Ok done this and no flow on hot from any taps or shower..

 

[Jimbojunk]

I am still suspicious!

Turn hot tap on in kitchen and SLOWLY increase the flow until the boiler JUST fires up.

Then measure the actual flow rate! It should be just about 2.5 litres per minute.

I suspect it may be far more!

Tony

Ok tried the flow demand seems to be around 6 l/m however did manage to ease the tap gradually to full without the boiler even coming on! So it looks more likely to be a sticky diverter valve, or perhaps diaphragm issue.

I have seen it is possible to buy the diaphragm or the interns (moving parts) only of the diverter valve is this advisable or is it better to get an entire unit replaced.

Thanks again

 

[gazdaz36]

Full unit

 

[Jimbojunk]

Ok will check diaphragm first, wishful win. Followed by diverter replacement, again!
Thanks to all for this great forum and the expert help from those who know and take pride in what they do and give up their free time to support others, much appreciated.
Jim (wannabe plumber).

 

[Jimbojunk]

Fixed! The diaphragm had a tiny split this part was only two years old too.
So really happy as no need to drain down etc, and the best bit is I had a spare diaphragm so cost to me was zero! My 11 year old lad was keen to help and learn about boilers so managed to turn a chore into a pleasure and still have the afternoon to play.

Thanks again all, keep up the good work..

Jim

 

[Agile]

Yes, that is what I was suspecting!

Commercially I always replace the whole unit then I know that the boiler should work well for several years more.

But I have encountered them where they have failed after only a very short time.

I was hoping to do a post mortem on one recently but the customer wanted to keep it to show to his landlord or freeholder. He did say I could have it later but never contacted me again.

Tony