Hi,

We're looking after a house which has been vacant for about 6 months, and will be vacant for at least a few more. We check it every 2nd or 3rd day.

Nova Scotia, Canada, climate in winter is cold, often well below freezing, sometimes down to -20C or lower (about -6F).
5 year old hot air oil furnace set at about 68F, serviced this summer.

Two months ago, we turned off the main water supply valve, left all taps open and water drained, including outside taps, toilet tank empty. I forgot, so power flowed to the electric hot water tank until last week, and the tank was full.

Question: about draining the hot water tank, which now has the power supply breaker off:

Drain water, or leave full?

If drained, will the tank be more likely to rust from inside?
If full, and the furnace fails, I suppose the tank may freeze and crack.
If partly filled, what happens?
Currently it's almost completely drained.

Tank is in heated basement, inlet & outlet pipes go upward. The basement laundry tub valves are open.

Thanks!
Dugie