What backflow is
Water normally flows one way, from the street into the building. If pressure in the supply drops, as in a main break or heavy fire hydrant use, water can reverse. Anything connected to the pipe, such as a hose in a bucket or a sprinkler system, can then be drawn backward into the drinking supply.
A backflow preventer is a valve assembly that blocks that reversal.
Where the problem shows up
| Setup | Why a preventer is used |
|---|---|
| Lawn irrigation | Sprinkler lines can hold fertilizer, soil and standing water |
| Boiler or hydronic heating | Boiler water can contain treatment chemicals |
| Commercial kitchens and car washes | Equipment that mixes chemicals or contaminants with supply water |
| Fire sprinkler systems | Standing water in the system can be stale or treated |
| Outdoor hose connections | A hose left in a pool or bucket can siphon back |
Which devices are required, and how often they must be tested, is set by the local water supplier and the plumbing code. Ask the supplier what applies to your property.
When the symptom is the device
A preventer that drips or discharges from its relief port is doing something. Often that means a fouled seat, a failing check or a pressure swing. The fix is cleaning or replacing parts, and it should not be ignored.
A device that has never been tested is a different problem. If you do not know when it was last checked, assume it is due.
- Water discharging from the relief valve.
- A device that is frozen or cracked after winter. See frozen pipe repair.
- A test report that was never filed with the water supplier.
- A new irrigation system installed without a preventer.
Testing and installation
Suppliers usually require the test to be done by a tester certified for backflow assemblies, so ask on the call whether one is available. The test checks that each part of the assembly holds. If it fails, the plumber repairs or replaces the faulty parts and retests. The results are filed with the water supplier where required.
A new device is installed on the supply line with correct clearances above the ground and drainage for any discharge. See commercial plumbing for building-wide requirements.
What to have ready for the call
- The water account number, if the supplier sent a notice.
- The make, model and size on the device tag.
- The date of the last test, if you have a report.
- Photos of the assembly and the pipe around it.
A plumber who has these can usually tell you what is needed without a site visit. A test notice from the supplier often lists a deadline, so check it before calling.