recommendation And advice are welcome.

[Old Account]

hi lads,

I just went today to a customer which wants me to convert his back boiler and hot water cylinder to a sealed system. He wants me to install a Vaillant 938 with 11 rads ( most of them are large 1400x600 ) and future 2 bathrooms.
He is worried that the pressure will drop significantly but I assured him due to my experience and based on Vaillant that it won’t happen and that the 938 will be fine. He is upgrading the mains water supply to 32 mdpe.
Will this type of boiler enough for future zoning? He is planning on splitting the zones in to 3 zones which are ufh downstairs, upstairs rads, downstairs rads.

However, at the moment his last radiator in the bathroom upstairs is only working half way ( vertical radiator). The difference between the F&E tank and the vertical Rad is about 10-20cm at max. Is that the problem why it doesn’t work in full?

Thank you for your help

 

[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.