What backflow actually is
Water in a supply system is meant to travel one direction. Backflow is when it reverses — and the concern is what it carries back with it.
Two things cause it. Back-siphonage happens when supply pressure drops suddenly — a main break, a fire hydrant opened, heavy demand nearby — creating suction that pulls water backwards. Back-pressure happens when pressure on your side exceeds the supply side and pushes water into the main.
On its own that would be a curiosity. It matters because of what the water has been sitting in. An irrigation system's pipes lie in soil that may have fertiliser, herbicide or animal waste on it, and sprinkler heads sit in puddles. If that water gets drawn back into the main, it does not just affect your house — it enters the supply your neighbours drink from.
That is why this is regulated rather than recommended, and why the test is annual rather than occasional.
Who needs testing
The trigger is a cross connection — any point where the potable supply could meet something non-potable.
- Irrigation and sprinkler systems. By far the most common, and the reason most Boerne homeowners encounter this at all.
- Fire suppression systems.
- Pools and spas with a direct fill line.
- Wells connected to a house that also has city water — a serious cross connection.
- Commercial equipment including boilers, chemical injection and commercial kitchen equipment.
- Rainwater collection tied into household plumbing, which is increasingly common in the Hill Country.
Worth knowing
If your irrigation was installed by a licensed contractor, a backflow assembly was almost certainly fitted — it is required. The usual surprise is not that it exists but that nobody told the new owner it has to be tested every year.
What the test involves
Short, non-destructive, and it either passes or it does not.
The tester attaches a calibrated differential pressure gauge to the assembly's test cocks and isolates it from the supply. The assembly's internal check valves and relief valve are then verified to open and close at the correct pressure differentials — confirming they will actually stop reverse flow rather than merely being present.
It takes 15 to 30 minutes. Water to the irrigation system is off during it; household water is unaffected.
The results go on a state-mandated form filed with your water purveyor. Keep your copy — it is what proves compliance if anyone asks, and it saves an argument when the property changes hands.
In Texas this must be done by a licensed tester holding a current backflow prevention assembly tester certification. It is not general plumbing work and not something to have done by whoever mows the lawn.
Why they fail
The assembly sits outside, above ground, full of water, doing nothing for most of the year. That is a hard life.
Freeze damage is the leading cause here. The assembly is entirely exposed, and a hard freeze cracks the body or damages the internals — which is why insulating it before winter is worth the few minutes it takes.
Debris on the check valve seats. Grit, sand or scale prevents a valve seating fully, so it fails the pressure differential test even though it looks fine.
Worn rubber. Seats and discs harden and deform over years. This is normal wear and usually a repair kit rather than a replacement.
Hard water scale on the internal working surfaces, for the same reason it affects every other valve in the area.
A failed test is generally repairable. The assembly gets rebuilt with a kit and retested, and only rarely needs replacing outright.
The practical bits
Test in spring, before the irrigation season. That way any freeze damage from winter is found before you need the system, rather than during the first hot week when everyone is booking.
Insulate the assembly before winter. An insulated cover costs very little and prevents the most common failure.
Keep the paperwork. New owners routinely have no idea when the last test was, and the water purveyor's records are the fallback rather than the first resort.
And if you are buying a property with irrigation, ask for the most recent test report along with everything else. It is a small thing that occasionally reveals a system nobody has maintained in years.