Vaillant 937 Hot water problems?

[eveares]

Hi all; my 1 year old Vaillant Eco tech 937 boiler has started playing up today where when I use hot water, the water only comes out lukewarm and not hot.

If relevant, my system is part of an extension and is a mix of new pipe and rads and couple of old pipes and rads.

More specifically; the hot water comes out at it's normal temperature for about 15-30 seconds then goes cool/lukewarm-ish, probably form stored heat in the storage tanks.

I have also gone into the installers menu and looked at the "DHW Flow rate" under the diagnostic menu; not sure how accurate this it, but it was showing 6.2L/min when i measured 11.9L/min coming out of the hot tap.
Faulty impeller sensor seems like the most likely cause to me, but could it be a dodgy PCB (not reading impeller sensor properly) or some other fault? No fault codes show up.

Also my grandparents have had a new vaillant ecotech pro 24 installed recently as part of a new build and their boiler has magnetic filter/inhibitor on the hot water return (may be flow) and also on the cold mains in which mine dose not have, are these gadgets or are the a necessary piece of equipment?

So in essence, should i get Vaillant out and what is the most likely cause and why has it happened? Also, I have no magnetic filters as said, are they worth it?

 

[ScottishGasMan]

whats most likley happening is that you have a mixer shower/tap in the house that is faulty and passing cold water into the hot. Thus why you have 6l/min of hot out the boiler mixing with 6l/min of cold. Does the 937 you have have a mixing valve fitted to mix the hot/cold?

 

[eveares]

No, it has no mixing valve as our plumber did not fit one,

The problem occurs with all hot water outlets, including single hot taps.

 

[Bunnyman]

As Scottish said prob mixer shower passing. It does not matter what tap you open, if the shower is passing it will pull cold water along the back of it causing it to mix at whatever outlet you open hence 11 litres at the tap. The d36 code on the boiler is very accurate if it says 6.8 that's what it is.

 

[eveares]

Have I understood right?

So cold water is passing from the cold side to the hot side on the shower mixer, allowing cold water into the hot water pipes. The hot water pipe from the boiler feels hot as normal so i guess it is this.

I have more than 1 shower mixer, how do i find out which one it is without isolating their cold/hot supply?

Also the problem of the boiler firing up when a cold tap is turned off as mentioned in this post: //www.diynot.com/forums/plumbing/vaillant-eco-tech-plus-937-issues-problems.369352/#2811198; is that related or is that problem still to do with trapped air?

 

[Tipper]

Could it not be something to do with the storage cylinders ('back pack') recharging system/pump/pcb?

It's still under guarantee so why not get Vaillant in?

 

[eveares]

I'm just going to get vaillant in as something is not right; boiler was firing up in the night every 30 mins when heating was off and no hot water was being drawn off. Surely tanks should be able to hold the heat for longer than 30 mins!

Shower mixers or boiler problem, it wont hurt to have one check over the boiler.

 

[eveares]

Fixed problem, was the shower mixer - turned the show mixer temperature control a bit on my mums shower and the problem went away.

I can't recreate the problem, so was it a fluke or does the mixer need replacing?