What is an air bleeder valve, and how does it winterize pipes?

Question

I found this message attached to a pipe in the first house I bought

Caution

Winter Drain Instructions:

1. Close valve in October before winter freeze
2. Open air bleeder valve on side of main valve
3. Open exterior hose bib
4. Pull down pin in centre of back flow preventer until all the water has drained

I'm not sure what the winter drain is.

What will happen if the valve is closed?

What is the air bleeder?

What are the rest points?

What do these points mean and what for?

Answer 1

This is for an outside valve that can freeze. Frozen water can crack pipes and valves. Destroying them.

An air bleeder will look like a small round knob sticking out of the side of the valve, under the handle.

These instructions are to make sure most of the water in the outside pipes/valve/hose bib is drained so it does not crack the valve or pipe.

Answer 2

An air bleed valve looks like this

You'll note

1. The directional arrow
2. The cutoff valve (red handle)
3. The air bleed valve (the knob past the cutoff)

The reason for this process is that once you shut off the water, there's nothing that will relieve the pressure. You can open the other end and nothing will come out because there's nothing to replace the water with. By turning that valve, you allow air to enter the line. You need to then open the other end of the setup (a hose bib in this case).

The other step is you have a backflow preventer (which is a good thing!). A pin is an odd thing, so I assume it is probably an external unit. A backflow preventer prevents any sort of pipe burst from sucking potentially dirty water back into your potable water pipes. The pin they mention is probably to release the water from the mechanism. Normally this is a device on the hose bib itself (which would have no pin) so I suspect your setup is more involved than a simple hose bib.

As crip659 pointed out, this is for winterizing your pipes. Given the oddity of the backflow preventer step, my bet is this is a larger water line that goes to an out-building of some sort (like maybe a garden shed). It's also likely you're somewhere that gets truly hard freezes (northern US or Canada), where it's impractical to merely protect the pipes with insulation. Draining the pipes ensures there's no water sitting above ground, where it can burst your pipes.