recommendation And advice are welcome.

[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