backflow preventer leaking with house water shut off

I've got a confusing situation. We had the sprinkler system winterized and the backflow preventer is still leaking a little even though the house water line to the sprinklers is off.

I thought maybe the valve inside the house was broken and allowing some water through, but when I open the spigot inside a little air is released, but no water. When I open the outside spigot an ounce or two drains out. the inside shutoff and spigot are below ground level.

This sounds weird, but could there be something that is causing pressure to build up in the sprinklers and push water from the yard back towards the house?




Thanks

 

It is extremely difficult to get all the water out of the lines so some water is not that unusual. As for the pressure how much is there? Changes in temperature and atmospheric pressure could force water back to the house but not with a lot of pressure. It's possible that there is pressure in the lines if they used compressed air to blow them out for winterization.