Installing an unvented boiler.

Hey! I am going to install a direct unvented boiler in my loft for my upstairs bathroom. 

the current boiler is currently downstairs at the back and the bathroom is upstairs at the front so the hot water takes a while, which is why i'm doing this. 

I just wanted to know what size cable I would need for this? Also what type of conduit as it will go outside to reach the boiler. 

Sorry if this is vague and I can update if anyone has any questions? 

What type of boiler is this?

If it's, gas you will need a boarded loft, with a permenant light and a proper loft ladder ....

Is this gas or electric boiler? 

Sorry this is an electric cylinder not a boiler. 

This is the one i'm looking at:

https://www.screwfix.com/p/rm-cylinders-prostel-direct-unvented-cylinder-180ltr/66534?_requestid=194103 

Sorry for any confusion. 

Not sure this should be fitted by a diyer....

I believe these things have an expansion vessel  either built in or separate ...not sure ...  Personally I wouldn't tackle fitting it   without  sound  professional advice .

Looking at your Screwfix link  I see two immersion heaters ,  I'd guess the bottom one is intended for Off Peak  (E7)  supply & the top one for 24hr supply .  

I'd advise  25mm conduit  (black)  outside  with 2 X  2.5 mm  6242Y  cables  from your meter / consumer unit position .  

Are you planning on heating this purely via the immersion heaters or are you linking this to your gas boiler? 

This is a direct, electric only cylinder. Each immersion heater takes 3KW or about 13 Amps.  You would normally only use one at a time.

The electrical side is relatively trivial.

HOWEVER an unvented cylinder comes with a LOT of stringent plumbing requirements. Expansion vessel, over pressure and over temperature blow off valves, correctly piped discharge piping so if the vents do blow off they do so correctly and safely.  They are only supposed to be installed by someone competent to do so.  Do you meet that criteria? 

You might also want to check with building control, you might just need building control approval to fit one on it's own.

Get the electrical side of it wrong, you might get a shock or start a fire.  Get the plumbing side of it wrong and it becomes a bomb that could blow the end of yout house out.

Sorry to throw in yet another doubt, but have you considered the structural requirements to put that in your loft?

I have one of those for my solar hot water, it has the immersion heaters in it for when there's no sun, you need a separate expansion vessel, and pressure relief valves and there's all sorts of things to work out to do with the expansion of hot water, such as length and diameter of pipe from tank to outlet,as this governs the type of pressure relief devices needed. I've done a lot of plumbing over the years including central heating systems, but I wouldn't install one of these, get it wrong and basically you have a bomb! Ours was done by the guys who did the solar and we have all the certificates, these can make a vast difference if it all goes wrong, not least as to whether your insurance pays out or not.