recommendation And advice are welcome.

[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.

 

[Last Plumber]

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

What do you mean?
I meant that if you turned the water off to the Boiler and water still runs from a hot tap, it must be getting in elsewhere. If that is happening, when you run a hot tap with the Boiler working, it will mix and cool the DHW. It can also out balance the hot totally and allow only cold to run, depending on the system. There will always be slightly more resistance through the Boiler than direct.

If we are talking at cross purposes, let me know?

 

[Harvest Fields]

I miss understood. When the boiler cold is isolated the 2 x storage vessels empty then the water stops is what I meant. Sorry.

 

[Last Plumber]

I miss understood. When the boiler cold is isolated the 2 x storage vessels empty then the water stops is what I meant. Sorry.

Oh I see. If the flow stops then no you're right, it won't be a passing valve lol.

 

[Scott_D]

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

Do you mean with a blending valve it’s the equivalent of a conventional tank?
It won’t run empty but once the 20 litres is used it just works as an 838 then reheats the cylinder when it gets chance

 

[sammathias]

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

Don't talk dirty about our beloved vaillants.

What's wrong with it?

 

[Harvest Fields]

Don't talk dirty about our beloved vaillants.

What's wrong with it?

It’s all detailed in the previous posts mate. I would never bad mouth Vaillant’s. They are all I fit normally.

 

[Old Account]

Do you mean with a blending valve it’s the equivalent of a conventional tank?
It won’t run empty but once the 20 litres is used it just works as an 838 then reheats the cylinder when it gets chance

That’s correct I may misunderstood you scott. Yes it would be equivalent to a conventional tank of 70l

 

[Harvest Fields]

That’s correct I may misunderstood you scott. Yes it would be equivalent to a conventional tank of 70l

Am I missing something here?

 

[Last Plumber]

Am I missing something here?

Lol.

 

[Harvest Fields]

The store on a 938 is 15ltrs. How will that be like a 70ltr tank?

 

[Scott_D]

The store on a 938 is 15ltrs. How will that be like a 70ltr tank?

You just add 55 litres of cold

 

[Harvest Fields]

I am just putting this out there before I completely get confused.

The storage on a 938 is 7.5 ltrs a vessel, so 15ltrs total, producing upto 20ltrs/m. Once the vessel is cold it switches to a combi @ approx 16ltrs/m.

 

[Last Plumber]

I am just putting this out there before I completely get confused.

The storage on a 938 is 7.5 ltrs a vessel, so 15ltrs total, producing upto 20ltrs/m. Once the vessel is cold it switches to a combi @ approx 16ltrs/m.

Yes.

The combi is an 838. It will only perform with the same ability as the 838 (because that's what it is), but with the additional pre heated (secondary water) store.

In theory as you draw off water the store is being replenished and therefore there is a delay in time before you get to be using purely the combi's performance.

There are figures about that state the performance being the same as an unvented cylinder but I personally tend to ignore the magic figures and see it for what it is..... A combi with a bonus!

My opinion.

 

[Harvest Fields]

Cheers @Last Plumber I thought that I was going mental

 

[Last Plumber]

The next thing is the central heating pipes from the boiler are starting in 22mm but after 2 m it reduces to 15mm

Those 15mm pipes will only carry 22000-25000 Btu's comfortably in my experience. At a push you may get 30000 but you will be lucky. It will present problems and noise is one of them.

 

[Last Plumber]

15mm.at the moment they only have downstairs rads and one added upstairs bathroom

What outputs?
You need to add them up and work it out.

 

[Last Plumber]

( most of them are large 1400x600 )

What kind? P1, P+, K1 or K2?
It all makes a difference especially if it's off 15mm

 

[Last Plumber]

Combi's are fine and even large ones like the 938 and they do perform well whilst the store is there but they are still only combi's once it has gone.
In houses with more than one bath / shower room and families, I would always advise having a modern unvented cylinder. The reheat times are very good as you all know and the back up of an immersion heater is there too. If sized, fed and piped correctly, you can run what you like without issue.

 

[Old Account]

Am I missing something here?

Yeah I did a mistake in terms of saying it would not run empty obviously it does that’s the writing mistake I made.

 

[Old Account]

The store on a 938 is 15ltrs. How will that be like a 70ltr tank?

Mate, just have read online... it’s german I know but I don’t know how else I can explain that. It’s clearly written that’s equivalent to a 70liters cylinder