• 16 parts
  • Checked September 2026

Parts of a Backflow Preventer

A reduced pressure zone backflow preventer in the water line, then cut open to show its two check valves.

Parts of a Backflow Preventer

Name Date

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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Write the name of each numbered part.Answer key

  1. Body
  2. No. 1 shutoff valve
  3. No. 2 shutoff valve
  4. Relief valve
  5. Air gap
  6. Strainer
  7. No. 1 test cock
  8. No. 2 test cock
  9. No. 3 test cock
  10. No. 4 test cock
  11. First check valve
  12. Second check valve
  13. Reduced pressure zone
  14. Check valve spring
  15. Check valve seat
  16. Check valve cover

Drawn for this site on 2026-09-30 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: the parts of a gate valve and a ball valve

All 16 parts on the diagram

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In the water line 10

  1. Body Both check valves and the relief valve are built into it.
  2. No. 1 shutoff valve Closed to test or open up the assembly. Its twin at the outlet closes too.
  3. No. 2 shutoff valve It shuts the assembly off from the pipes it feeds.
  4. Relief valve Opens to keep the zone at a lower pressure than the supply.
  5. Air gap Water let out above falls through open air into the drain.
  6. Strainer It keeps debris in the water from fouling the check valves.
  7. No. 1 test cock A tester connects gauge hoses to the four of them.
  8. No. 2 test cock The high side hose goes here to test the first check valve.
  9. No. 3 test cock It opens into the zone. A gauge here reads the zone pressure.
  10. No. 4 test cock It is on the outlet side of the second check valve.

Cut open 6

  1. First check valve Its spring holds it shut until the supply pressure pushes it open.
  2. Second check valve It closes if water tries to flow back from the outlet side.
  3. Reduced pressure zone Kept at least 2 psi below the supply pressure.
  4. Check valve spring The first check valve has the heavier spring of the two.
  5. Check valve seat Debris caught here keeps the check valve from closing tight.
  6. Check valve cover Taking it off gives access to the spring and the check valve.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it: the US EPA Cross-Connection Control Manual (https://www.epa.gov/sites/default/files/2015-09/documents/epa816r03002_0.pdf); a maker's installation, testing and maintenance instructions for a reduced pressure principle assembly, read in the Internet Archive copy (https://web.archive.org/web/20240627154415/https://www.zurn.com/media-library/web_documents/pdfs/installation/issm975xl2-pdf.pdf); a second maker's technical manual (https://archive.org/details/manualzilla-id-6133616); and Wikipedia, text only: Reduced pressure zone device, Double check valve and Backflow prevention device. No maker is named on the page. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; a schematic of no maker's assembly, nothing traced. Licence: own work

How it works

The parts of a backflow preventer, shown on a reduced pressure zone assembly: two check valves in a row with a relief valve between them. The first drawing is the assembly in a water line; the second is its body cut open.

Read the full guide

Outside: follow the water

Water comes in from the left. It passes the strainer and the No. 1 shutoff valve, runs through the body and leaves by the No. 2 shutoff valve. The four test cocks are numbered the same way, 1 to 4. The relief valve hangs under the body, over the air gap and the drain.

Inside: two checks and a zone

Each check valve is pressed onto its seat by a spring under a cover. Water has to push the first one open, then the second. The space between them is the reduced pressure zone. If the pressure there climbs too close to the supply pressure, the relief valve opens and lets water out.

  • A double check valve assembly has the two check valves, the test cocks and the shutoff valves, but no relief valve.
  • The numbers run with the flow: No. 1 is always at the inlet end.

Questions and answers

What are the parts of a backflow preventer?

A reduced pressure zone assembly has two shutoff valves, four test cocks, two spring-loaded check valves, the zone between them and a relief valve that opens to the air. A strainer ahead of it and an air gap under it complete the installation.

What does the relief valve on a backflow preventer do?

It keeps the zone between the two check valves at a lower pressure than the supply. If that pressure rises, because a check valve leaks or water pushes back, the relief valve opens and the water falls out through the air gap.

What are the test cocks on a backflow preventer for?

They are small valves where a certified tester connects the hoses of a differential pressure gauge. The readings show whether each check valve holds and whether the relief valve opens when it should, without taking the assembly apart.

What is the difference between an RPZ and a double check valve?

Both have two check valves in a row. A reduced pressure zone assembly adds a relief valve between them, which lets water out if a check valve leaks. A double check valve assembly has none and is not used where the hazard is high.

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