Vaillant Turbomax 837 Boiler - Help!

[Mulberry View]

We've got a 15 year old Vaillant Turbomax 837 boiler, it's a hefty beast and has been pretty reliable. It's recently developed an intermittent fault.

 

I'd say 1 in 10 times, possibly less, it doesn't fire on demand for hot water. There are no obvious errors and often shutting the tap off, waiting a couple of minutes and then going again will get it going.

 

I've not noticed an issue on the heating side, although I realise that'd be less obvious.

 

Any ideas where I could start looking?

[ProDave]

Flow switch would be my first check.

[Jeremy Harris]

AS @ProDave flow switch is one cause, as can be a diverter valve fault.  We had a diverter valve fail on our old Vaillant, when it was around 7 or 8 years old.

[Nickfromwales]

 

 

It’s item 19. 
 

[Nickfromwales]

It’s a small propeller type spindle that you can buy as a spare for a few £10’s. Easy enough to change too. 
 

 

https://rover.ebay.co.uk/rover/0/0/99?loc=https%3A%2F%2Fwww.ebay.co.uk%2Fp%2F2256002575%3Fiid%3D173512321204

 

Try that first and go from there ??

[Nickfromwales]

Impeller even ?

[Mulberry View]

11 minutes ago, Nickfromwales said:

It’s a small propeller type spindle that you can buy as a spare for a few £10’s. Easy enough to change too. 
 

 

https://rover.ebay.co.uk/rover/0/0/99?loc=https%3A%2F%2Fwww.ebay.co.uk%2Fp%2F2256002575%3Fiid%3D173512321204

 

Try that first and go from there ??

 

Nick,

 

Thanks for this, all very helpful. I've done a bit of work on the boiler anyway, so am *fairly* confident. I've had the DHW Heat Exchanger out a couple of times (once to clean it and once to replace it), I'll take a look at this over the weekend.

 

?

[Mulberry View]

Further to this, I've noticed that some refer to the flow sensor as the small 'impellor' shown above, some as the whole brass affair. Is the impellor part of the larger brass assembly?

 

I'm guessing the impellor itself has no electrics in it, logic would tell me that it either works or it doesn't?