As may be obvious, water is the primary issue when winterizing a property that will not be heated.
1. Turn off the water to the entire unit.
2. Shut-off valves may be found under a sink or in a basement that will shut off water to an upstairs unit. In a single family home, turn it off in the basement at the water meter where it comes from the street.
3. Drain the water from the hot water heater, and the boiler if there is hot water or steam heat. Leave the valves open.
4. Open all the faucets and valves on sinks and bathtubs, both to drain water from the valves and to allow air to enter the lines.
5. Flush all toilets to drain the water from the toilet tank.
6. Water remains in the U-shaped traps under the sinks, bathtub shower and any floor drains. Pour enough nontoxic antifreeze, often called RV antifreeze (not the type used in your car radiator) down the drain to displace the water from the trap. The RV antifreeze will not freeze and expand causing the trap to burst. Two cups should be enough in each drain.
7. Pour nontoxic antifreeze in the toilet bowl to displace any water left in the bowl or trap. Pour a little RV antifreeze into the toilet tank too. It will prevent any residual water there from freezing and cracking the toilet tank.
8. Many shut-off valves have little round freeze plugs on the bottom that can be loosened with pliers by unscrewing. That will allow any last remaining water to drip out the small hole.
9. On reoccupying the rental, close the water heater and boiler valves. Tighten the freeze plugs on any valves that have them. Then, just turn on the water. The toilet tank, the furnace boiler, and the hot water heater will refill themselves automatically. Air and water will spurt from the sinks and the bathtub. When the water is flowing smoothly, turn off the water faucets on the sinks and tub.
10. The rental is ready to be occupied.