Hot water goes cold

[Gijs]

I'm hoping somebody here could point me in the right direction. I've got a Vaillant Ecotec Plus vhr 25-32cs 1-5. Everything in this system is new since August this year.

Since a few weeks the hot water goes cold during showering, stays cold for a bit and then comes back again. Sometimes it's once, sometimes multiple times. I get the following s codes:
S92 Water circulation volume self-test
S326 Hydraulic and actuator test.

While trying to find the problem I heard air going true the pump. I could bleed the pump via the pressure relief so we could shower. Figured it would be the automatic air valve but after replacing this the problem stays. Still a lot of air in the system.

I hope someone can help me out finding the problem.

 

[terryplumb]

Do any hot taps suffer from the same problem ?

 

[Gijs]

I'm not sure, the problem is intermittent and we never use the hot water tap long enough to become apparent. Flow rate is more than double of the minimum required.

 

[Gijs]

I adjusted the bypass pressure a bit higher, I figured it might bleed better with more pressure. So far so good.

 

[DP]

I take it you have a system boiler that heats the water in the cylinder.
Is the system sealed or open vented.

If open vented, I would suspect air drawn from the F&E cistern
If sealed system, then pressure could be low.

The bypass valve should only open or pass when the motorised valves are closed and boiler pump has no circulation path except the bypass valve.

Not been able to look at the boiler literature