Pipes round outside of house ?

[Corgigrouch]

Generally speaking, a combi requires a lot more gas to heat the domestic hot water than it does to heat your house.. an average small to medium sized house can be heated with a 12Kw boiler.. to get a decent flow of water from a combi you would need upward of 24Kw, though even 24Kw is regarded as a bit limp wristed these days and a 30 Kw is usually the preferred minimum. All this output needs more energy input and for this you need a larger flow of gas and hence a larger gas pipe

 

[ChrisR]

I've just put 20m of 28mm round a house, following a decorative protruding brick line. Copper goes dull brown so doesn't look too bad, unless you use white plastic clips.
These

are made of copper so don't show up much. They can be hard to find in p's merchants though.

I've also just channeled 8m of concrete floors for 28mm pipe. Believe me, outside is better!

 

[plummerman]

if only your last post was true, our live would be so much easier. unfortunatly a combi require a greater amount of gas to function properly compared with an older boiler. we have all these wonderous formulas and pipe sizing to accomodate set guidelines on pressure drops etc. and this is why it needs to be increased. depending on which way your joist run internally you could have a lot of fittings which cause restriction so possiblity of greater pipe size and more expense for you not to mention the distruption it will cause to you through ripping floor boards up. Granted externally doesn't always look asthetically pleasing but it can be painted etc to disguise it somewhat