Anti-Siphon Valves: A Quick Guide

If a chemical tank keeps draining overnight after the metering pump shuts off, or your discharge line runs up and over before dropping to the injection point, the fix usually isn't a bigger check valve — it's a different kind of valve entirely. Here's the short version of what anti-siphon valves do, where they're expected, and the hose-barb sizes we stock.

What They Do

An anti-siphon valve is spring-loaded and requires positive pressure to open — not just forward flow, but pressure above what gravity alone can generate. When the pump strokes and builds discharge pressure, the valve opens. The instant pressure drops back to zero, the spring re-seats the valve immediately, breaking the liquid column before gravity can pull it into a siphon. That's a meaningfully different job than a standard check valve, which is built to stop reverse flow but generally opens too easily to stop a forward gravity siphon on its own. For the full physics, the standards involved (ASSE 1001 and related), and detailed sizing guidance, see our complete Anti-Siphon Valves Explained page — this guide is the short, product-focused version.

Where They're Expected

The most common case in our world is a chemical metering pump discharge line where the tubing rises and then drops to the injection point — without an anti-siphon valve, that elevation change can drain the chemical tank overnight and cost you pump prime on the next cycle. Anti-siphon and vacuum-breaker devices are also standard practice on hose bibb and irrigation connections under most U.S. plumbing codes, to keep a hose left in a bucket of chemical from siphoning contamination back into a potable line.

Hose-Barb Configurations in Stock

Both are built for direct connection to chemical tubing on one end and a threaded pipe or fitting on the other — no additional barb-to-NPT adapter needed. Both install in any orientation, vertical or horizontal.

Installation Note

Install on the discharge side, following the flow arrow on the body, and position the valve upstream of the highest point in the line so that when it closes, it isolates the descending column of liquid between that high point and the injection point. Installing it below the high point defeats the purpose — it won't be able to break the siphon path.

When to Ask Us

Both stocked sizes ship with a Buna-N (NBR) seal, which is the durable, cost-effective default for plain water and general oil or fuel service — the same seal-material logic that applies to any check valve. If you're dosing anything beyond plain water, tell us the chemical and concentration before you order; NBR is not built to resist oxidizers like sodium hypochlorite, and we'd rather confirm the right seal material than have a valve fail on a line that can't afford it. See Buna vs. Viton Check Valve Seats for the full elastomer breakdown.

Frequently Asked Questions

What's the difference between an anti-siphon valve and a check valve?

A check valve stops reverse flow but typically opens too easily to stop a forward gravity siphon. An anti-siphon valve requires positive pressure to open and closes immediately when pressure drops, which also stops siphoning. See our full anti-siphon valve guide for the complete explanation.

Do I need one on my metering pump?

If your discharge line rises and then drops to the injection point, yes — that elevation change is exactly the condition that causes a chemical tank to drain overnight without an anti-siphon valve in place.

What sizes are available?

We stock 3/8 in. (46531A) and 1/2 in. (46533A), both hose barb x male NPT, aluminum body with a Buna-N seal, rated to 150 psi.

Can I install it in any orientation?

Yes — both stocked sizes install in any orientation, vertical or horizontal, as long as the flow arrow points from the pump toward the injection point.

Related Guides

• Swing Check Valve Guide
• Check Valve Types Compared
• Check Valve Orientation & Installation
• Y Strainer vs. Basket Strainer
• Buna vs. Viton Check Valve Seats
• Strainer Mesh & Sizing Reference

Published performance data is general engineering reference, not a certification for your application. Confirm specifics against the manufacturer datasheet or ask us.

Written by the Apex Flow Solutions technical desk — twenty years of industrial distribution experience behind every guide.