recommendation And advice are welcome.

[Old Account]

No but once the 20 litres of stored hot water has gone it’s just a Vaillant 838 Combi so about 15 lpm of hot water available

The cylinder won’t run empty. As explained earlier it’s capable of making as much as a conventional cylinder.

 

[Old Account]

When I am there it all looks like it is doing what it is supposed to mate. Only happens about once every 2 weeks

Just have a look next time harvest. It’s a common fault that the shump pump has an intermittent fault.

 

[Last Plumber]

Don’t fit the 938 mate. I’m having a mare with one and the hot water at the minute. Boiler and unvented for me

What's the issue? (curiosity).
EDIT:
Sorry just read the thread!

 

[Last Plumber]

Does it cut out or do they just struggle to get DHW?

 

[Harvest Fields]

The cylinder won’t run empty. As explained earlier it’s capable of making as much as a conventional cylinder.

No it isn’t mate. Once the 15ltr store is empty it shifts into combi mode

 

[Harvest Fields]

Does it cut out or do they just struggle to get DHW?

Runs cold then they have to turn the tap off and back on again. The only thing I can think of is the flow switch isn’t recognising the demand from store to combi

 

[Last Plumber]

Runs cold then they have to turn the tap off and back on again. The only thing I can think of is the flow switch isn’t recognising the demand from store to combi

Do they have any mixer valves?
Showers, TMV's etc?
Could it be a passing valve?
You might have ruled that out already but I thought I'd mention it as I came across a similar issue the other week.

 

[Harvest Fields]

Do they have any mixer valves?
Showers, TMV's etc?
Could it be a passing valve?
You might have ruled that out already but I thought I'd mention it as I came across a similar issue the other week.

All of them. And isolated each 1 by one. All brand new as I put them in. What was your issue.

 

[Last Plumber]

All of them. And isolated each 1 by one. All brand new as I put them in. What was your issue.

Did you try turning the cold to the Boiler off underneath and running a hot tap? there should be nothing flowing as you will undoubtedly know.

My issue was that the Hot water was running but more often than not tepid. Occasionally it was hot and OK.

The Boiler appeared to be running OK and doing what it should do and at first I suspected other things. Turned out to be a passing valve. Not always the same though due to fluctuations in pressure.

 

[Harvest Fields]

It is a 938 mate so with no taps on the stores still empty.