When we went for a couple weeks a few years back I set it to 52 degrees, It was our first time going away in the winter and I was worried there could be a problem.
I also made an Arduino temp check thing that would record the temp every 5 min and send an alert if the temperature fell when below 45. The house stayed perfect at 52, and we had a real bad cold snap that year while we were gone -18 and -22
If I were to do it again I think would set the temp to 45 and Arduino warning at 40
One other thing, We have a detached garage that we heat (oil forced hot air) I normally keep that at 42 all winter. When I use to keep it at 40 on real cold snaps the far corner of the garage could hit 32 I saw ice in buckets. Raising it 2 degrees keeps the far corner above freezing.
Leave all of the basement vents open. You have to remember, with the lower thermostat setting and no doors being opened, your furnace won't be running near as often as when you're living there. Heat rises, some of the heat from your basement will make its way upstairs; therefore, you're not adding much additional cost to your gas bill.
Also, I assume if you live in a cold climate, you already turn off and drain all of your outside water bibs for the winter anyway.
I left my thermostat setting at 55°; this was in northern Wisconsin, where we averaged 28 below zero days (low temp.), with "occasional" days not reaching 0° (high temp.) during cold snaps; average high temp in the low-mid 20s in Dec-Jan-Feb.
If you have block basement and clay soil you also want to keep enough heat in the basement to prevent the ground in contact with the walls from freezing. I have seen several unheated basements buckle from the clay soil freezing and expanding. A poured basement or sand soil don't have the same problem.
When I had a home sitting unheated and electric disconnected for a winter I drained the plumbing. Put antifreeze in all the drains and toilets, and where the water pipe came in through the basement floor I built an insulated box open to the floor around the pipe up to above the shut off and drain for the rest of the house. The idea for the box was the ground would never freeze that deep and box would trap the 55 degree heat from the floor and keep it around the pipe. Shutting off the water or well would prevent flooding if a pipe were to break.
A few years back a local family headed south for the winter. While they were gone a pipe broke and kept flowing. It filled up the entire basement and broke out the windows. Then the top froze. The plumber had to drill a hole through the ice in the stairwell to get a pump in to pump out the water. It destroyed everything in the basement.
.Thanks to all who replied . Gonna try it set at 40 . #2 son lives in Apt over detached garage and will check house before and after work and make any adjustments that might be needed .It will only be for 3 weeks so what could go wrong HA HA HA