What hot water/CH system for big showers and 10 rads please?

[youngwill100]

I really need some help as I'm a novice at this...
I need to install a new hot water and CH system for my 4-bed house, which is currently gutted (i.e. easy to install pipes in floors etc).
As well as central heating with 10 rads, I want the capacity to run two big showers at once for 15 mins at least (for the early morning rush!) without running out of hot water. One shower is 12 litres/minute and the other 10 litres/minute. The showers need at least 3 bar to get a decent flow, according to their spec sheets.
My mains (standing?) pressure is 4 bar. I assumed I could get a big combi boiler (perhaps the Vaillant Ecotec Plus 937 with 15 litre store)
BUT... I had a guy from British Gas around to do a quote, and he said that a combi wouldn't give me enough flow for what I wanted - he said that while my standing pressure was 4 bar, the combi would only output 0.5 bar and about 7 litres a minute largely because I have an old lead mains pipe rather than a modern (22mm?) plastic one. Is this right?
He recommended a conventional boiler with a mains vented (or should this be unvented??) pressurised tank. He said that there are twin coil ones which would give me good flow and good recovery times.
Can anyone comment on his comments and conclusions?
As I mentioned earlier, my main priority is being able to run two big showers together in the mornings at a really decent pressure.
Many thanks
William

 

[Dan Robinson]

British gas guy is correct that a Combi will not do what you want - but is otherwise talking out of his chocolate starfish...


A very high powered combi might offe the flow rate you need, but idealley you should have a good system boiler and Unvented cylinder - or what I have which is the Multi 24/80+.

A twin coil one is for use with a solar panel. If the house is already igtted, then you should really change the lead pipe to MDPE.

You need to understand the difference between pressure and flow to a lot of what will follow to make sense.

Now stand back for plenty of different boilers/cylinder combinations - perhapos even an Thermal Store or two. I think the general concensus will be to avoid a Combi.

 

[ChrisR]

10 an 12 l/min aren't high flows. Sure they aren't more?
A 270 or 300l cylinder would be fine.
Forget combis.
Or one Rinnai 50 would do them both, for as long as you like. External mount if short on space.

 

[Dan Robinson]

With a system boiler for heating...?

 

[youngwill100]

Thanks for all the replies so far!
Yes, physics/flow/pressure was never my thing at school!
The shower flow rates I have estimated from doing a crude trial - I ran the shower at a good rate for 10 seconds into a measuring jug - it was 2 litres, so that's how I got 12 litres/minute. If I could have a higher flow that would be even better!
I can't change the lead pipe mains as it runs through a big section of concrete floor and then under a garden and pavement to the connection in the street. The BG guy said that as long as I had the flow to run 3 taps at 25 litres a minute simultaneously, I had enough for the unvented cylinder system. Seems a bit bizarre to me!
To be fair to him, he said that the twin coil cylinders were for a solar heat conection, but that I could actually use the second coil for the boiler as well (??)
Looking forward to more suggestions so I can get a consensus. The other limiting factor is that the only place I can put the HWC is in an old coal store under the stairs, which is 1.4 meters high, so I need a short wide cylinder!
William

 

[youngwill100]

The Rinnai seems an interesting solution. I have a Worcester 924 boiler that was removed during the renovation and is in OK condition.
1) Could I use this as the central heating boiler with the Rinnai for hot water?
2) Is this a good model of boiler, and will it run 10 rads?
3) Is it legal to re-fit a boiler like this? I'm not sure if this model is a condensing boiler.
Sorry for all the questions !
William

 

[gas4you]

BG dont fit Vaillants that why he put you off this as well.

You need definite proof of your water flow rate.

If you've got enough to supply your needs from an unvented then the 937 will give you 20 litres/min for 10 mins.

It does sound like an unvented is your best choice though. Most are approx 550mm diameter and at 1.4M max height I feel this is may be restrictive for your needs.

However you could at a push get a unistor 180 in as this is 1339mm high. Couple this with an ecotec plus system boiler and a VR65 contrller and you will have what is probably the most intelligent and up to date efficient system on the market at this moment.

The next one down is the 155. Both have a external pressure vessel so will need space for this.

With 4 bar pressure and 20 litrs/min incoming mains you should get around 17 - 18 litres/min out of the hot taps.