Vertical vs horizontal unvented hot water cylinder

chris1012

I have been looking into purchasing an unvented hot water cylinder system for when we have our loft conversion (we will have a 3rd bathroom), which is starting next week.

Due to storage constraints for the location of the cylinder, the plumbers that have viewed the property have advised either to put the cylinder (250l-300l) in the eves of the loft (which would cause a lot of disruption to the rest of the house) or in the cellar (far less disruption).
We also can put it in the hallway and box it in the future, although we were going to be using that space to build a coat/shoe cupboard.

While I have been looking online at various cylinders, I have come across some forums stating that Horizontal cylinder are not as good as vertical and should be avoided unless no other choice.
One thing I could not find in addition was the exact reasons for this?
We do have a choice of having a vertical option but it would mean having two 150l cylinders side by side in the cellar or a large one in the hall way.

We are likely to stick with out combi boiler (33kw) and have it modified so it runs as a heat only boiler - apparently we need to have one tap that is used regularly for this setup to work (something to do with the diverter valve working properly?).

The plumber we are thinking of going with has advised we would still need to run a 22ml pipe from the cylinder to the new third bathroom, where as others have said they can feed off existing heating pipes from the first floor.
Which is correct and the best method?

We also have a twintec s3 water softener that will be feeding the boiler/cylinder.

Any one have any experience of setting up an horizontal unvented cylinder, especially in the cellar?
The head height we have in the cellar for the cylinder is 1.5m and width 2m

Our water supply to the property is via a 25ml blue pipe and the boiler would be located at leat 5m-10m from the cylinder if within the cellar.

Thank you