Backflow Preventer Guide for Idaho

What Is a Backflow Preventer?

A backflow preventer is a mechanical device installed on your irrigation system's connection to the potable water supply. Its purpose is simple but critical: prevent irrigation water — which may contain fertilizers, pesticides, animal waste, or soil bacteria — from flowing backward into your home's drinking water if water pressure drops.

Backflow events happen more often than you'd think. A water main break, fire hydrant use, or even multiple faucets running simultaneously can create a pressure drop that sucks irrigation water back into the supply. Without a backflow preventer, that contaminated water could reach your kitchen tap.

Idaho follows the Idaho Rules for Public Drinking Water Systems (IDAPA 58.01.08) which require backflow prevention on all cross-connections, including irrigation systems. Individual cities may have additional requirements — we cover the major ones below.

Three Types of Backflow Preventers

There are three backflow preventer types commonly used for residential irrigation in Idaho. Each offers a different level of protection, cost, and installation complexity.

Idaho City-Specific Requirements

Annual Testing Requirements

Most Idaho cities require annual backflow preventer testing by a certified tester. This is separate from your regular sprinkler maintenance and is a legal requirement, not just a recommendation. Here is what the process looks like:

• When: Typically due each spring, often by June 1. Many homeowners schedule testing as part of their spring startup.
• Who: Must be performed by a certified backflow assembly tester. Your irrigation contractor or a plumber with backflow certification can do this.
• Cost: $50-$100 per test. Some contractors include it in their spring startup service.
• What happens: The tester uses a differential pressure gauge to verify each check valve is sealing properly and the relief valve (RPZ) opens at the correct differential.
• Results: The tester files a report with your city water department. If the device fails, it must be repaired or replaced and retested.
• Penalty for non-compliance: Cities can shut off your water service until the backflow device is tested and certified. Fines vary by municipality.

Winterization and Your Backflow Preventer

In Idaho's climate, your backflow preventer is one of the most freeze-vulnerable components. PVBs mounted above ground are especially at risk. During winterization, the backflow preventer must be properly drained or blown out. Failure to winterize can crack the body, destroying the device — a $200-$800 replacement.

Steps for winterizing your backflow preventer:

• Shut off the water supply to the irrigation system
• Open both test cocks on the backflow preventer (small brass valves)
• Open the ball valves to a 45-degree (half-open) position
• Blow compressed air through the system as part of the full system blowout
• Leave test cocks open and valves at 45 degrees all winter to allow drainage
• Some contractors wrap exposed PVBs with insulation — this provides extra protection during shoulder-season freezes

Installation Considerations for New Systems

If you are installing a new sprinkler system — whether on a new construction home or retrofitting an existing property — backflow preventer selection should happen during the design phase, not as an afterthought. Consider these factors:

• Lot grade: If any sprinkler heads sit higher than where the backflow device will be mounted, you cannot use a PVB. You need an RPZ or DCVA.
• Aesthetics: PVBs are visible above ground. If that matters, a DCVA in a below-ground box is less visible (where code allows).
• Water pressure: RPZ devices reduce pressure by 10-15 PSI. If your supply pressure is marginal, this matters for system design. Check your pressure with our water meter guide.
• Future plans: If you might add a fertilizer injector later, install an RPZ now — it will be required.

For detailed cost breakdowns of all system components including backflow preventers, see our cost guide. For pipe material selection, check our pipe types guide.

Building a new home? Coordinate your sprinkler system with your builder early — see our new construction guide. For other home finishing needs, check Basement Finishing Idaho and Idaho Yard Pros.