This is a photo of a damaged back flow preventer taken during a home inspection in Aurora Colorado by my company, Magpie Property Inspections. It appears it is worn. likely has has frozen and become damaged due to a lack of proper maintenance or winterization. It needs repair or replacement by a qualified technician.
The backflow preventer is a device that prevents your water system from becoming contaminated from water flowing backwards into your supply lines. If you have a sprinkler system which is fed by fresh water lines from your home, one should be installed where the sprinkler line feeds into the sprinkler valves located in the valve box underground.
The backflow preventer, works by having the water siphoned through a branch line in the unit. Water flows in one direction freely through the line but is not allowed to move in the opposite direction. If water is allowed to back up in the lines after routing to the sprinkler system the water brings with it whatever is in the line which might include weed killer, fertilizer, and other contaminants in the grass and soil. Without a properly functioning backflow preventer on the sprinkler system cut-in, your water system can inadvertently draw those toxins into your water supply and present a major health hazard.
It is important to check your system and back flow preventer regularly and to have the system checked each fall during winterization and in the spring when it is activated to make sure it is in good working order.