Winterizing a Trailer Procedure

mike-s wrote:
Not sure where you get that 1/4" as a requirement, or even typical. If you look at made-for-purpose RV blow-out plugs, the brass ones have an orifice much smaller than 1/4", and others have Schrader valves which also very much restrict flow.

It would be a very large RV to have more than a couple of gallons of capacity in the water lines. I know a single gallon will fill the lines on our (relatively modest) one with more than enough left to fill the traps. It's more volume than pressure that's needed to blow out lines - you really only need a few psi to push the water through. A 2 gallon air tank at 40 psi will deliver over 3 gallons of 10 psi air.

And really, you don't even need that - just constant flow. Open the valves one at a time starting closest to the source. After you get rid of all the constant water flow (so they just sputter), let the system pressurize, then go back and open one at a time, the air pressure in the lines will blast out most of what remains.

A 1/4" is the standard ID of an air fitting like a Milton quick connect. The effective ID of a Schrader valve is 0.21" so the difference really isn't worth talking about.

Your point about volume was exactly what I was getting at, let's says a typical RV has 20' of 1/2" pipe in each of hot and cold circuits. A 1/2" pipe has a volume of 0.0102 gallons per foot, so 20' of pipe would hold 0.204 gallons. Now hook up a tankless (battery powered tire filler?) to it and charge it to 40 psi.

When you open the tap you have 2/10ths of a gallon of air trying to displace the water before the pressure drops back to zero.

That won't do very much if anything.

Pressure ONLY counts when there's a solid plug of water ahead of the air, once there's even a tiny way past that water air takes the path of least resistance.

It is only continuous air flow at that 40 psi that drags all those remaining drops along and out the tap.