Installing a Vacuum Breaker for a Sprinkler Line

[crokett™]

I want to install a proper line for a future sprinkler system.  My current system is run off a hose spigot that comes off the line after the water filter.   My choices are

1.  cut into the main water line at the well.  Advantage is the splice can be done at the well head (no digging).  However, I'd have to trench across my driveway to where I want the water line to go.  
2.  cut into the line under the house.  no digging for the splice but I'd be working under the house and I'd have to go through the foundation with the water line, plus that puts the shut off valve under the house also.
3.  cut into the line just before it enters the house.  Digging, but no cutting the foundation and I can easily trench by hand to where I need to go.    My plan is to cut into the line and cut out about a foot or so.  I will come off the line 90 degrees, then another 90, then a tee for the sprinkler, then 90s to return to the main line.  

Option 3 is my current plan.  

What is the best way of preventing the  backflow valve from freezing in the winter, if it is installed above ground?  I am thinking a box with a valve to turn off below ground, and some kind of fitting to allow the line to be blown out.  The vacuum breaker could then be installed above ground.  Officially the frost line here is 12".  In 30 years I am pretty sure the ground has never frozen below an inch or so.

[goaliedad]

Are you planning on blowing out the system? If so the vacuum breaker will be blown out if you attach the air prior to it.
If not blowing it out, but a shut off valve with a drain after the vacuum breaker??

[blackhat]

I’m going to make a couple assumptions. You are using a brass bodied, thread connection atmospheric vac breaker. Your existing line is PVC

For illustration purposes I will say your line runs north into your house from the well.
Cut the line as proposed in option 3. Install a tee with the branch on your existing supply. The run of the tee will face east and west. Choose one end of the run to create a swing as you proposed and reconnect the house. On the other end install a valve, a tee with the branch facing up and a second valve in the open end of the tee. This second valve will be a drain for the vertical pipe that comes up from the branch of the tee. This vertical pipe will have a union and a threaded adapter that screws into the vac breaker. The union will have the nut on the top portion. Out of the vac breaker, a thread adapter, a 90 pointing back into the ground and a union. This union will also have the nut on the top section. 
To remove the vac breaker for winter storage or repair/ replacement, simply undo the 2 unions and take it away. 
To winterize, close the first valve, remove the vac breaker and open the second valve. A third union can be used to make a blow out adapter to connect to the stub going into the ground to your sprinkler piping. A glue by thread bushing into the nut end of the third union and then whatever you need for an airline connection screwed into that. 
The vac breaker documentation should list how high the valve needs to be above the sprinkler heads. Mind that height for the valve to work properly.

[crokett™]

I think I understand what you are saying. The water line actually runs east-west with water flow to the west. It's about 9" from the foundation on the south side as measured in the crawlspace so I will have to add anything to the north. I assume any valves I install will have to be in a box buried in the ground. What I am thinking is come off the main water line to a box with a valve and drain downstream of the valve, then come up and out to the vacuum breaker. make the breaker removable. Take it off in the winter and blow out the sprinkler lines, then open the drain and drain the 24" or so of vertical pipe. the line that feeds the valve shouldn't freeze since it is below ground.
That said on the north side there's the electric line to my shop and the main service to the house plus the feed to the cable modem. The main service should be deep enough to not be an issue. I think what I will do is find the water line then trench under the electric to my shop to where there's room for a buried box then tee in like I planned but run the line to the box with a valve.

[blackhat]

Sounds like you have it figured.