I might get my builders (who are doing the loft) to run the pipework and electrics now and then get a certified engineer in at the end to commission and connect it all up.
I'd be very wary of that unless you can be sure that they understand what they are dealing with. Builders come in all varieties, but most I would not feel confident entrusting this to.
Whatever you do, you need to keep the pipes capped to keep both moisture and dirt out. Moisture can be vacuumed out within reason, dirt risks blocking fine capillaries and/or valves.
The pipework also needs to be properly lagged with closed cell insulation, and with all joints sealed. Open cell insulation or open joints allows moist air in and then condensation forms on the cold pipes - eventually leading to waterlogged and ineffectual insulation and potentially decor damaging drips.
May I ask is it ok to have a run of the copper ac piping outside the building exposed to the atmosphere? Or does the piping have to enter the external unit almost as soon as it leaves the building?
You can run the pipes pretty much where you like - as long as you stay within the limits imposed by the installation instructions. Typically there will be a maximum pipe length AND a maximum height difference between indoor and outdoor units. Running up through the ceiling, across the loft, and then down to the outdoor unit should be no problem - subject to height and length limits.
And of course, the electrical work will be notifiable.
As to condensate drains. There are a variety of pump types, I've used several - all have their pro's and cons.
Some are a shallow tray that sits within the indoor unit, and when the liquid builds up a small pump fires up - the ones I've seen then make a distinctive (and probably annoying in a bedroom) "durrrr" noise while they pump for a few seconds.
Another type that may be worth considering is a peristaltic pump. These are like the ones you see in medical dramas on TV - where a rotor with rollers one runs round and round trapping a piece of rubber tube between the rollers and the casing. These will handle a negative inlet head (ie they will suck uphill) which means you could position the pump in the loft (if there's a convenient and accessible place) so it sucks the condensate out. However, I have heard that these can make a slurping noise while they are running. You also need to find a way of turning them on only while cooling, which may not be easy if the AC unit doesn't have a convenient connection.
I'd agree with the others though - if you can arrange a gravity drain, it will be the simplest and most reliable by far.
Disclaimer - I don't work with, and I've no qualifications in, A/C. I've just picked up a fair bit while it's been my responsibility at work.
) Obviously it will be easy to run the pipes up through the loft floor whilst it is all exposed up there.
