Try this simple experiment for yourself...fill a glass with water then put a soda straw in the glass. Now tightly cover the top of the soda straw with a finger and lift it straight up above the glass so the bottom of the straw is well above the water. You will note that even though the bottom of the straw is wide open to atmosphere, there is still a substantial amount of water in the straw.

The same thing can occur in your plumbing system. Simply turning the main water shutoff valve to off position and disconnecting a union at the main water valve or opening a valve at the bottom of the system does not necessarily insure that the system is adequately drained.

With the lines open at the lowest point you need to then go through the entire house and open all faucetts to allow air to vent into the lines, and even then it is very iffy whether the system is drained. You still have service lines to appliances such as a dishwasher, ice maker or humidifier and quite often those lines are made up with a coil of copper pipe behind the appliance to permit pulling the appliance away from the wall for service. You need to then flush all toilets and hope the fill valve will allow enough air into the system to allow the supply lines to drain, but some toilet fill valves have a backflow preventer that would prohibit the air from entering. The best thing is to literally disconnect the supply tube from the toilet tank, (But be sure to make a note of what you disconnect and attach a copy of that to the main water shutoff valve to remind you to close the valves or reconnect the lines when you are ready to turn the water on at the beginning of the next season.)

All the faucetts, outside hose bibb and the valve or connection at the lowest part of the system should remain open all winter so if any water happens to leak past the main water shutoff valve, or any residual water remaing in the system can continue to drain when the structure is unattended, and even after you have taken all of these precautions, there is still no guarantee that the system is completely drained.

When plumbing a seasonal home that will be unattended during the cold season great care must be taken to insure that all supply lines are installed with a downward pitch, the same as drain lines, and you must be very careful not to have a horizontal line with a vertical riser on each end that would hold water like a drain trap. You must be very careful installing pipe hangers to make sure your system will not develope sags on horizontal lines.

You could open the system at the low point, then go through the house with an air compressor and literally blow out each line, but even then it is doubtful if you would get all the water out of the system.

Now some will argue that PEX tubing will tolerate freezing, which is true, but in most instances a line freezes at a joint or fitting. If the freeze occurs in the PEX tubing the tubing will expand and later return to size when the ice melts, but if the freeze occurs at a fitting the expanding ice will enlarge the crimp ring on the PEX fitting and once the ice melts you are left with a loose fitting that may blow out under pressure. It should also be noted that the PEX fittings are made of copper or brass and they will freeze and crack the same as copper does.

Boats, RV's and campers have a holding tank to provide water when they are not connected to a hookup. Those holding tanks have an inlet system where you can attach a garden hose or you can manually pour water directly into the tanks from a bucket. To winterize those systems they pour "Food Grade Anti-freeze" into the holding tank, then open all the facetts to circulate the water-antifreeze mix throughout the system.

Note:food grade anti-freeze is available at all Rv & Camper or Marine supply stores.

You can make a cheap antifreeze injector that will inject the food grade antifreeze into your structure, then you could leave water in the system and just do a simple flush at season startup.

It should also be mentioned that when shutting down your structure at the end of the season you should put some food grade antifreeze in the toilet tanks, the bowls and in every drain to protect the traps from freezing.

After experiencing these problems many times in the past I have been through each and every one of these issues time and time again. I have had cracks appear in vertical pipes with drains underneath where it is 100% impossible for water to have pooled there. I am convinced that cpvc is not freeze proof with or without water in it.