UFH pipe diameter 12mm or 15mm?

[Adsibob]

Apologies if this has been discussed on the forum before, but reading through the various results for "pipe diameter" didn't answer my question.

 

I'm laying wet UFH throughout the house. On the ground floor it will be embedded in 55mm of screed, whereas on the first and second floors we are laying Cellecta XFLO Micro Foil Faced boards in the bedrooms and XFLO Micro TB in the bathrooms. These boards are made from very dense extruded polysterene and have the advantage of allowing the finished floor to be laid/tiled directly on top, with no spreader plates or gypsum boards required. I attached the spec for both if anyone is interested. You will see they come in different thickness, with the pipe channels already drilled out. Cellecta advise using the 25mm board for 15mm pipes and the 20mm board for 12mm pipes. In either case, Pipe centres are 150mm apart.

 

My question is what is the difference between running 12mm and 15mm pipes? Presumably the larger the pipe, the more water and therefore the higher output, so am I right in thinking that a thicker pipe gives me the option of running:

1. a less responsive system at a lower temperature; or
2. a more responsive system at a higher temperature?

 

In terms of floor finishes, on the first floor we are having 9mm pergo laminate in the bedrooms and tiles in the bathrooms. The tiles vary between 1cm thick in our bathroom and 2cm thick in the family bathroom. We were planning on having a separate thermostat for each zone, but only using one manifold for each floor, which i understand will mean i will have to use the same temperature water across the floor.

 

On the ground floor, we will have:

1. engineered wooden board 15mm thick glued directly onto the screed in one room; and
2. everywhere else, Topcret Baxab (which is a type of extra strong microcement that is only 3mm thick) laid directly on to the screed. 

Because of the dramatic differences in conductivity of these two floor finishes, we plan on having a separate manifold for each, as well as them being on separate zones. I've only got room for 100mm of PIR under the screed.

 

The house is an upgrade 1930s semi, so good insulation and windows throughout, but nothing close to passive standard.

XFLO Micro FF & XFLO Micro TB.pdf