C Nash is correct.If your unit has a bypass for the water heater,just bypassing and draing is all that is necessary.
If it doesn't have a bypass then the safest method is using anti-freeze......Be aware that it will take around 6-10 gal.to fill water heater.........
I don't want to beat a dead horse here but....... If you don't have a by pass then take the cold in line and connect it to the hot out line. This in effect by passes the hot water heater. Now the trick here is this. You have to be able to drain the hot water heater!! I have a hot water heater on a boat that has the safety valve low on the tank. I stick a small plastic tube in it to open the valve and drain the tank. Remember if you by pass the tank you have to have a way to drain it or it will freeze. If you can't drain the tank you will have to winterize it.
Don't forget the battery!! A discharged battery can freeze and
burst :angry: In the 40s tonight and these old bones have already started aching Guess you can tell your getting old when your deordant smell like BEN GAY or ICY HOT :blush: