Why Sizing Matters

An incorrectly sized backflow enclosure is worse than no enclosure at all. Too small and it cannot be installed — or if forced into place, it prevents technician access for annual testing. Too large and you pay for capacity you do not need, and may run into installation clearance limits around the assembly.

Sizing an enclosure requires measuring the entire installed assembly — not just the preventer body — and then adding clearance for testing access. This guide walks through every step.

Step 1: Identify Your Assembly Type

• RPZ — Reduced Pressure Zone Assembly: Has a distinctive relief valve body (dome or rectangular) on top of the main body. Used on high-hazard cross-connections including most commercial and irrigation systems. The relief valve body adds meaningfully to the height measurement.
• DC or DCVA — Double Check Valve Assembly: More compact than an RPZ, with no relief valve body on top. Used on low-hazard cross-connections. You will see it written both ways — DC and DCVA refer to the same device, so a residential "DC" and a commercial "DCVA" are the same assembly type at different sizes, not different products.
• PVB — Pressure Vacuum Breaker: Identified by a tall bonnet with an air inlet valve on top. Codes typically require installation a set height above the highest downstream outlet — commonly 12 inches — so verify the requirement in your local code. Relatively tall assembly.

Look for the manufacturer label or body stamping on your assembly. If the assembly has already been inspected by your water authority, your permit or inspection record will identify the type.

Step 2: Measure the Full Installed Assembly

Here is what to include in each measurement:

Length (Along the Pipe Run)

Measure from the outer face of the inlet shutoff valve (the valve on the upstream, supply side) to the outer face of the outlet shutoff valve (the valve on the downstream side). This full dimension includes:

• Inlet shutoff valve
• Any union or nipple between the valve and the preventer body
• The preventer body itself
• Any strainer (on some assemblies)
• Any union or nipple on the outlet side
• Outlet shutoff valve

Width (Widest Point)

Measure the widest dimension of the assembly perpendicular to the pipe run. On most assemblies the widest point is at the test cock handles, which may extend laterally beyond the preventer body. Also account for any pressure gauges that project outward.

Height (Lowest to Highest Point)

Measure from the bottom of the lowest pipe centreline (or the base of the lowest fitting if the assembly sits close to the ground) to the top of the highest component. On RPZ assemblies that is the top of the relief valve body, not the top of the preventer body. On PVB assemblies, measure to the top of the bonnet.

Step 3: Add Clearance on Every Dimension

Example calculation:

• Assembly length 24 inches, so required interior length 30 inches minimum
• Assembly width 8 inches, so required interior width 14 inches minimum
• Assembly height 14 inches, so required interior height 20 inches minimum

When in doubt, round up. A slightly oversized enclosure is far better than one that is even slightly undersized, and the extra interior air volume also contributes to thermal buffering when the enclosure is heated for freeze protection.

Step 4: Note Your Pipe Size

Enclosures are built around specific pipe size ranges, and product listings state the pipe size the model is designed for. Confirming compatibility avoids oversized cutouts or field modifications that compromise the enclosure seal. For larger assemblies, enclosures are available with multiple pipe openings, and written quotes and volume pricing are available for large or custom units.

Step 5: Use the Backflow Enclosure Size Finder

Enter your required inside length, width, and height in the Backflow Enclosure Size Finder and it returns every enclosure whose interior dimensions meet your requirements. That removes the need to compare spec sheets by hand and ensures you only see options that will actually fit your assembly.

Eastgate Supply stocks hundreds of sizes across the Safe-T-Cover aluminium line (100 through 1000 Series) and the AquaShield marine-grade aluminium line, in heated and non-heated configurations, so the constraint is usually finding the right size rather than whether one exists.

Common Sizing Mistakes

Why Annual Testing Requires Proper Clearance

Backflow preventer assemblies must be tested annually by a certified backflow prevention assembly tester in most jurisdictions. The tester connects a differential pressure test kit to the test cocks on the assembly and measures the pressure differential across each check valve.

This process requires:

• Unrestricted access to each test cock — the tester must be able to turn the test cock handles to the partially open position and attach test kit hoses
• Room to work — the tester must be able to reach inside the enclosure with both hands at once
• Visibility — the tester must read the gauge faces and observe component positions during testing

An enclosure with inadequate clearance means the tester has to remove it entirely before testing, which adds time and creates a situation where the enclosure may not be properly reinstalled afterwards. The goal is an enclosure that allows testing to be completed in place with the access door open — which is why enclosure lines are designed around easy access, removable panels, and lift-off or multi-door configurations.