Although some orchids aren't supposed to like temperatures below about 15ºC (~60ºF), mine all happily overwinter at a minimum of 10ºC (~50ºF), including various phals and other vandaceous things that are supposed to sulk when cold; that said, the sun generally then raises the temperature by some 5 or so degrees C or more during the day. One thing you may find is that cold spell prompts them all to spike a few weeks after you get back - a late wedding present.
Houses in South Africa are generally not centrally heated (nor insulated), so it's pretty common for them to sit at 50-60ºF in winter {or less!}; plenty of my LOS people keep all their plants outdoors (mostly in shadehouses rather than greenhouses/tunnels) and it regularly gets to 10ºC (or lower) at night - although they all sit on the coast and are more buffered from extremes than we are ~60km inland and a few hundred meters higher up.
Point-heating (i.e. seedling mats) is often more energy efficient than space heating [you can work this out by comparing wattages] - akin to that campfire tale of native americans making a small fire and getting close to it, while paleface make
big fire to warm whole world!
Given that's only about a week, most phals will be happy with a good watering before you leave - but if you have mounts, you may need someone to give them the odd sprinkling (this makes our collection need a caretaker when we go away).
---------- Post added at 11:57 AM ---------- Previous post was at 11:55 AM ----------
Oh, one thing I meant to say but forgot - what orchids *won't* like is a sudden change from a house kept at 80 or whatever and then suddenly dropped to 50 overnight. If you can bear it, try dropping the thermostat by about 5 degrees a week until you get to whatever you're planning to set it to while you're away.
Keep your fans on to stop cold spots by the windows where plants generally sit. My wife thinks it's madness to have fans on in the winter, but she forgets she's warm blooded (despite her reptilian tendencies to bask near sources of heat), and that reducing the boundary layer when you're already at the ambient temperature won't make you "colder" (unless of course you're wet, in which case, watch out for evaporative chilling, which *can* be a problem in winter).