In reference to your mention of using a blowout plug for winterizing, I used to use compressed air for winterizing as well, but was told that this method doesn’t get the water out of the check valves in the water pump and could subject the pump to rupture. I would be most interested in your comments on this. —Bob K.
Keep in mind, the entire fresh water piping system is factory-tested by the coach manufacturer and tested again by the dealer at pressures around 80 PSI. So anything less than that will not create a problem within the system. In fact, in previous years the test standard was 100 PSI. The backflow preventer located at or near the water pump outlet becomes nothing more than a closed valve when air pressure is introduced through the city water inlet, so there’s no danger to the pump from that side.
Now, it is important that the water pump indeed has a check valve located at or near the pump outlet. If a pump does not come equipped with an internal check valve, the manufacturer will install a separate check valve fitting at the pump outlet.
All said, I’ve never seen a water pump damaged by injecting air into the system through the city water entry. But you make a good point about the remaining water inside the pump on the other side of the check valve! One remedy, after emptying the fresh water tank, is to remove the outlet tubing on the pump and then briefly run the pump to eject as much moisture as possible. Running dry for those few seconds will not damage the modern RV water pump. What little moisture that may remain inside the pump head will not likely cause any damage should temps dip below freezing.