Backflow Testing in Los Altos: What Cal Water Requires From Homeowners
If you own a home in Los Altos, your water almost certainly comes from California Water Service, better known as Cal Water. Its Los Altos Suburban District also covers parts of Cupertino, Los Altos Hills, Mountain View and Sunnyvale. And if your property has a backflow preventer, Cal Water expects it to be tested every year.
Plenty of homeowners don’t realize they have one until a letter or a closing document mentions it. So here’s what the device is, what Cal Water actually asks of you, and why the test comes around every single year.
First, what is that thing by the meter?
A backflow preventer is a set of valves, usually brass and usually above ground near the water meter or the front of the house. On residential properties it’s most often protecting an irrigation system or a fire sprinkler line.
Its job is simple. Water is supposed to flow one way: from Cal Water’s main, through the meter, into your house. But pressure in the main can drop suddenly, for example during a main break or when firefighters open a hydrant. When that happens, water in your pipes can be pulled backward toward the street. If the irrigation line runs through soil treated with fertilizer, or a hose is sitting in a pool or a bucket, that backward flow can carry whatever it touches into the public water supply.
The backflow preventer stops that. Inside are spring-loaded check valves that close when water tries to reverse. The more protective type, a reduced pressure assembly (often called an RP or RPZ), also has a relief valve between the checks that opens and dumps water if both checks start to leak.
What Cal Water requires
Cal Water’s rules are short, and they’re spelled out on its cross-connection control page.
The device is your responsibility. Cal Water puts it plainly: property owners are responsible for making sure backflow assemblies and air gaps on their side of the meter are properly installed and maintained. It has to be tested every year by a certified backflow prevention assembly tester, and air gaps have to be inspected every year as well.
Only approved, testable assemblies are allowed at the meter. The little vacuum breakers that screw onto a hose bib are fine for protecting your own plumbing, but they don’t stand in for the assembly at the meter. And if your device ever needs replacing, the new one has to be the type Cal Water’s local cross-connection control specialist calls for. Swapping an RP for something cheaper isn’t an option.
Cal Water also says what happens if you ignore all this: water service can be discontinued until proper protection is in place.
None of this is unique to Cal Water. California’s State Water Resources Control Board has a statewide Cross-Connection Control Policy Handbook, in effect since July 1, 2024, and every water supplier in the state has to follow it. Cal Water’s program is how that plays out in Los Altos.
Why every year? The device seems fine.
It’s a fair question. The assembly sits there for months doing nothing you can see. But the parts that matter are inside, and they wear out quietly.
Each check valve seals with rubber discs and seats held closed by springs, all working against constant line pressure. Over time the rubber hardens and takes a set, and springs lose some of their tension. None of that changes how the valve looks from the outside.
Debris is the other usual suspect. Whenever there’s work on a water main, sand and flakes of pipe scale can travel down the line, and a single bit of grit caught on a valve seat is enough to stop it sealing.
Irrigation makes it worse. Many residential backflow preventers serve sprinkler systems that barely run in winter, and valves that sit still for months can stick. Then spring comes, the system starts drawing hard, and that’s exactly when you’d want the valves working.
The real problem is that a failed device usually looks perfectly normal. A check valve leaking internally gives no outward sign at all. The one exception is an RP whose relief valve is dripping or dumping water, and by then something inside has already gone wrong. Putting a test gauge on the valves once a year is the only way to know they’ll close when it counts.
And it isn’t only your water at stake. The device protects the shared supply your neighbors drink from too, which is why the state and Cal Water treat the yearly test as non-negotiable.
How the test works
For most homes this is quick and doesn’t need you to be there.
- You hire a certified tester. Cal Water only accepts tests from certified testers with calibrated equipment and a signed tester agreement on file with Cal Water.
- The tester needs access to the device. If it’s in a locked cage or behind a gate, leave a key or a code. Most residential devices are outside, so you don’t need to be home.
- Water is shut off at the assembly for a short time. The tester connects a calibrated differential pressure gauge to the test ports and checks that each valve holds pressure and, on an RP, that the relief valve opens at the right point. On one easy-to-reach device, the routine test is short.
- The results go to Cal Water. Testers working in Cal Water’s service areas record results in Cal Water’s own mobile testing application, so the report reaches Cal Water directly. You should also get a copy for your records.
- If it fails, it gets repaired and retested. Most failures come down to worn rubber parts, which are replaced with the manufacturer’s repair kit for that model before the device is tested again.
If the body of the device is cracked or frozen, or parts aren’t available anymore, it has to be replaced. Replacement is plumbing work that usually needs a permit and a licensed plumbing contractor, and the new device has to be the type Cal Water specifies.
A few practical tips
Don’t plug a dripping relief valve or cap it to stop the mess. That drip is the device telling you it needs attention, and blocking it defeats the whole point.
Keep the area around the device clear. Shrubs grow, and a tester who can’t reach the test ports can’t do the test.
Keep your test reports. A simple record of past tests is handy if Cal Water ever has a question, and it’s one less thing to dig up if you sell.
If you just bought a home in Los Altos and aren’t sure whether you have a device, look near the water meter and along the front of the property for a brass assembly with two shutoff valves and small test ports on the side. If you find one, it’s on Cal Water’s list.
What it costs
At BackflowCare a standard backflow test is $199 per device, and new customers get $25 off the first test. We also offer an annual service plan if you’d rather not think about it again. Any fees a water supplier charges on its own are separate.
If your Los Altos backflow test is coming up, you can book online or see the Los Altos service page for the details of Cal Water’s requirements.
Common questions
Does Cal Water require backflow testing in Los Altos?
Yes. Cal Water requires backflow assemblies on the customer's side of the meter to be tested every year by a certified backflow prevention assembly tester, and air gaps to be inspected every year.
Who is responsible for the backflow preventer at a Los Altos home?
The property owner. Cal Water says owners are responsible for making sure backflow assemblies and air gaps on their side of the meter are properly installed and maintained.
How are Cal Water backflow test results submitted?
Testers working in Cal Water's service areas record results in Cal Water's own mobile testing application, so the report goes to Cal Water directly.
What happens if I don't test my backflow device with Cal Water?
Cal Water says water service can be discontinued until proper backflow protection is in place.
Can I replace my backflow preventer with a different type?
Not on your own say-so. Cal Water requires a replacement to be the type its local cross-connection control specialist specifies.