What DHW flow rate do I need on new combi boiler please?

[JohnHuson]

Currently have Worcester 26 CDi managing around 9 litre a minute.
House with students has one bathroom with elec shower, and one shower room on different levels using main combi boiler. Out of the four basins only the kitchen one is used a lot. 12 rads.
I do not currently have a supply problem on CH the 26CDi copes fine with the current set up.
I would like to have the elctric shower taken out and supply both showers off the new combi.
Is this going to work and what Kw combi do I need please and flow rate?

 

[glasgowgas]

wont work, flow rate is too low (from this boiler)

probably better with an unvented cylinder

 

[kevplumb]

you need to check the incoming flowrate / pressure

before you do anything

if it aint coming in you aint gonna get it out fella

thanks for the e mail GG

 

[JohnHuson]

Thanks for that....I am looking to have installed a new combi so am asking what spec to go for please?
There is no issue with incoming supply, but I will check tomorrow.
If the supply is as expected what KW boiler do I need please?
I am currently looking at the following;
Vokera Unica 36 at 14.7 @35 degrees.
Ideal Isar HE35 .....14.4 " "
Ravenheat CSI 120...13.2 " "
Sabre (Vokera) 14.4 " "

I do not want a Worcester again have had six of them to deal with over the years and cost a fortune to keep going.
Thanks.

 

[glasgowgas]

as Kev has already stated you MUST check incoming flowrate and pressure

if you have 18 litres per min you can fit any of your mentioned combi`s


if you only have 10 litres per minute incoming water mains, there is no point fitting a 14 l/p/min combi

 

[glasgowgas]

do NOT fit and Ideal anything

 

[JohnHuson]

Thanks Glasgow gas for that comment........ what I am after please is whether the 35Kw will suit rather than going for a system boiler.
The plumber I use is happy to fit but prefers me to make the choice of boiler hence the request to know what capacity I should be looking at and prefer to stay with a combe system if possible.
Thanks for your help....and to others who reply.

 

[glasgowgas]

until you can post your incoming mains water pressure and flowrate,

we really cant advise wether a 35 kw is best for you

if you only have 10 litres per min in,you wont get 14.7 litres per minute out!!

if the hot water usage is quite high, you really cant beat an unvented cyl

plus with the combis you have listed, none of them will power 2 showers at the same time

an unvented cyl will

 

[dextrous]

Your plumber should really advise you what specifications you can work with - not only water as previously advised but also what gas pressure is currently available. There's no point installing a big boiler if there's not enough juice to run it.

 

[JohnHuson]

You guys were right to ask me to check incoming water supply. What appeared to be excellent is in fact only 12L a minute......can I get the Water supplier to change this? Daft question I guess!
The outside stop cock is connected to a lead supply for a few feet then married to plastic pipe to the house supply.

 

[Hugh Jaleak]

You have responsibility for the incoming supply pipe from your boundary to the property. If you relay this from the house up to the boundary, as long as it is to the required spec, then the water authority should connect it to their main.

Regarding Boiler, Vaillant, Viessmann, Atag seem some of the better choices, if you want something reliable then Ideal are far from it....

 

[MarkRainbow]

I have to agree about Ideal... I've had a lot of bad experiences with them. In my opinion, stick to something German. I'm surprised to hear you've had bad experiences with Worcester.

I would think that fitting an unvented cylinder wouldn't be the way to go if you have poor cold supply. As was pointed out earlier, you can't get more out than you put in, unless you store water.

Grundfos make a very neat flow/pressure booster that contains its own break tank. Pricey though at £1000 trade and you need space.