3 bathrooms for flow 9L/m, 3bar pressure - possible solution

7 Mar 2014
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United Kingdom
Hi there.
We moved in this house about a year back. Currently it has vented system with old Ideal gas boiler, hot water cylinder in the airing cupboard and cold water tank in the loft. 3 beds, 1 bathroom. hold and cold water is pumped to the bathroom by a motorised pump.

I tested water pressure downstairs - 3 bar, flow 9 litres/minute.

I am building extension and will have 3 bathrooms and downstairs WC, potentially another en-suite in the loft at later stage if I ever do loft conversion, so plan to replace central heating system completely and install new boiler.

Pipes in the property are lead so plan to replace them as well.

I initially wanted to install combi boiler vaillant 937 and replace pipe from the boundary with a new blue pipe but checking with neighbours I am not sure this would give me enough flow to run say 2 showers at the same time and say washing machine or dishwasher...

The other solution i came across was to install system boiler and unvented hot water tank (megaflo was suggested) but then again it would give out same flow as it receives from the mains

I also read about so called accumulator - so maybe accumulator and combi boiler ?

I hope specialists here can suggest something

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Conventional cylinder and storage tank are the best choice for you. Best with system boiler.

Combi for 3 bathroom isn't really a good choice.

Btw why 3 bathroom? Seem excess.

yeah in fact it will be one en-suite with a shower, family bathroom with bath and shower and downstairs WC.
A 937 is a typical Google warrior /looks good on paper choice.

With your figures there is no reason not to have an unvented cylinder once the lead pipes are replaced for decent mdpe.

You can't possibly get an accurate plan until you know what the performance is like then.

Stand by for Tony to talk about dynamic flow :LOL:.

Open vented is fine, but space consuming, with the added issue of several hundred litres of cold water in the possibly non existent loft.
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Its generally expected that to take advantage of an unvented cylinder you need a minimum dynamic flow rate of 22 li/min @ 1.0 bar.

a little bump here...
I am getting my supply pipe changed this week to 25mm mdpe and expecting flow rate improvement.
Assuming I get decent flow rate with the new pipe I need to decide on a new boiler asap... Seems missus likes the idea of having hot water constantly so that kids could take shower/bath one after another rather than using two showers at the same time - so I am again considering combi after almost decided to go for a system with unvented storage.

I was looking at Vaillant 937 with 15l storage but I feel thsi won't be enough .... there is also a Worcester Greenstar Highflow 550CDi which provides 25l/min on paper.

There is another option I am considering now - Viessmann Vitodens 111-W 35kW Storage Combi Boiler, again good performer on paper - 20l/m flow rate and 46L storage cylinder with performance similar to 150l unvented cylinder system...it says

Have you got any experience with any of those a smaller one Worceste 440CDi - would this be a good solution for me? Also taking in to account difference in installation costs of a system with unvented tank or a a/m combi?

At last I heard some people add unvented cylinder to a combi - what's the benefit of this and is it safe / covered by warranty etc?
Lord knows. Storage combis can be great.... But you need at least 50 litres of store, and no more than 28kW input otherwise you are into mental sizing or crap performance for the cost.
Thanks Dan. I am looking towards Viessmann now but could not find any reviews - seems not very popular in UK.
Viessmann has 46L storage - almost 50L :D

they have two models with 46L cylinder - 26kW and 35kW
I figured that for my house I would need 30-35kW
Veissmann rated heating output range for DHW 8.0-35,
rated rated heat input 8.2-36.5

is it any good?
one shower, one bath+shower and downstairs WC
house is 3 bed semi extended to 5 half of the ground floor will have underfloor heating. 2 adults + 2 kids

there is a potential for loft conversion in future which would add one more shower room but this is not happening in nearest future if happening at all
Acid, stop dreaming combi boiler be it storage or über size output.

Keep it simple

Why not fit a 24 or ~30kw combi and let hot water from it supply kitchen, utility and couple it to an unvented cylinder, solar cylinder even. By doing this you are going for belt and braces job

A storage combi no doubt is a single box solution to your dreams, but nightmare is when it fails and then three bathrooms have no water

Suggestion- work out what the heat load is, fit combi that size. Pick an unvented cylinder with a solar coil if you have good sunny aspect on roof and fit cylinder to add solar panels later when funds allow.
Seems eveyone here hates combis especially storage ones lol...

yes this was another option I was looking at but have not found much about combining combi with unvented cylinder - what are the benefits vs unvented system - never runs out of hot water?
Will this be more complicated and expensive to install?

Veissmann has another storage combi with 100L tank - https://www.plumbnation.co.uk/site/viessmann-vitodens-222-f-35kw-storage-combi-boiler--200-ho1b-/

and also a 242F model where storage tank is already prepared for solar heating....

One more question for knowledgeable people out there - is it ok to ihave cylinder and boiler on the ground floor and all bathrooms upstairs?

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