Vaillant Ecotec Plus - Luke warm water

[Johntheo5]

This is odd. The DHW at boiler is hot to touch, i.e. the 43 degrees D.04 reading being given is correct. If I grip the pipe it becomes uncomfortable after 5-7 secs.

However, 2.5m away, at the kitchen HW tap, the water’s lukewarm!

There’s a tee beneath the boiler leading to a WC, and closer to the sink, another tee to the bathroom. I’ve closed the WC basin’s HW isolator (it’s a mixer tap). This doesn’t make any difference to the temp of water from kitchen tap.

I’d isolate the mains CW at the kitchen tap but unfortunately there’s no isolator fitted. I’ll fit one later today - however, given above strongly suggests there is a fault with the kitchen mixer tap?

Looks that way, installing a isolating valve should narrow down (if) the problem.

Also, faulty mixer tap in kitchen would explain why the D.36 readings don’t reflect the actual flow rates I’m measuring at the kitchen tap (8.5 LPM, D.04 is 4) or bath tap (16 LPM, D.0.04 is 9)?

9LPM @ 42C + 7LPM @ 19C = 16LPM @ 31.9C.

Lastly, probably a separate issue. I notice if I isolate mains CW immediately beneath the combi, with a quarter turn using an Allen key, it doesn’t work. DHW at kitchen sink continues to flow.

The boiler shouldn't even fire up in DHW mode if the mains into it is actually isolated. If it doesn't fire up and there is still "HW" flowing from any tap then definitely a mixer problem.

 

[Daljit]

FFS! It’s the thermostatic shower! That’s why when I’ve been closing the CW supply at the boiler, all the hot taps in the house still run at the same flow rate (with CW, of course)!

Thanks @Johntheo5 for all your persistence and advice.

 

[Hairymelon]

Edit. Too late

 

[Johntheo5]

FFS! It’s the thermostatic shower! That’s why when I’ve been closing the CW supply at the boiler, all the hot taps in the house still run at the same flow rate (with CW, of course)!

Thanks @Johntheo5 for all your persistence and advice.

Glad to hear you found the culprit.

What type of thermostatic shower?, is it a single lever or two lever?, thermostatic showers are supposed to have NR valves to prevent cross over in the event of thermostat failure.
When your shower was working normally, if you turn(ed) it to fully cold did/do you get just fully cold water without the boiler firing up? which I assume should happen as the thermostat HW port should be fully shut off by spring action?. I might start a new thread on thermostatic shower failure later.