What is a main drain test?

A main drain test is a quick water-supply verification performed at the sprinkler system riser. You open the main drain valve fully (typically a 2-inch valve near the base of the riser), read the static pressure before the valve opens and the residual pressure while flowing, then compare those numbers to the original acceptance test and the previous test. Per NFPA 25 §13.2.3.3, a pressure drop greater than 10% from previous results triggers an investigation. The test takes 2–5 minutes once the water is flowing and is the single most efficient check that the water supply hasn't degraded since the last inspection.

How often does a main drain test need to be performed?

Per NFPA 25 §13.2.3: (1) ANNUAL is the default for most sprinkler systems. (2) QUARTERLY is required if the sole water supply passes through a backflow preventer or pressure-reducing valve — per §13.2.3.2. The rationale for quarterly isn't really water-supply verification; it's to exercise the backflow or PRV internals so they don't seize up. If multiple risers are fed by the same backflow preventer, only ONE riser needs to be tested quarterly.

What is the difference between a main drain connection and a main drain TEST connection?

Per NFSA citing NFPA 13 §16.10.4.6: the main drain connection is used to drain the fire protection system at the system riser. The main drain TEST connection is the same physical connection, but installed so that the valve can be opened wide for enough time to actually conduct the test. Many field installations fail this distinction — a main drain piped to a floor sink may work for slow system drainage but cannot handle the full flow needed for a main drain test. The test connection must discharge to a location capable of handling full flow (typically the building exterior).

What triggers a main drain test failure?

NFPA 25 §13.2.3.3: a 10% or greater reduction in full flow pressure compared to the original acceptance test OR previous tests triggers an investigation. Common causes: (1) partially closed control valve upstream of the test point (the classic finding), (2) fouled or failing backflow preventer reducing supply pressure, (3) scale buildup in underground or riser piping, (4) city water main work reducing available supply, (5) a neighboring property added demand that reduced residual pressure at your service. The failed test doesn't diagnose the cause — investigation does.

What do I compare the test results against?

Per NFPA 25 §13.2.3.3 and §4.6.4, compare current main drain test results to BOTH the original acceptance test AND previous test results. The original acceptance documents the system's day-one condition; the previous test documents the immediate baseline. A drop from last year indicates recent degradation; a drop from the original shows long-term trend. NFPA 25 §4.3 requires the building owner to keep the original acceptance test results for the life of the system — missing records are a citable deficiency.

Do I need to test every riser in the building?

Not always. Per NFPA 25 §13.2.3.1: if multiple risers are on a SINGLE manifold, test only one riser. If there are several INDIVIDUAL risers (separately fed from the service), each riser requires its own test. For multi-riser buildings with a single backflow preventer feeding them all, the quarterly backflow-exercise requirement is satisfied by testing ONE downstream riser. The annual test still applies per riser where the supply configuration calls for it.

What if the main drain piping cannot handle the full flow?

This is a surprisingly common installation problem — main drain piped to an undersized floor sink or interior drain that overflows during a full-flow test. Per NFPA 13 §16.10.4.6, the main drain TEST connection must be installable so the valve can be opened wide for enough time to conduct the test. If your drain can't handle full flow, three options: (1) retrofit the drain to discharge outside, (2) petition the AHJ for an alternative test method, (3) accept that the test cannot be properly conducted and document the limitation each inspection cycle. The NFSA cites this as a common installation issue that persists because it was originally "approved or accepted by an AHJ."

What does a main drain test actually prove?

It proves the available water supply at the system riser today matches what was available at acceptance (or at the previous test). It is NOT a fire pump test, it is NOT a sprinkler performance test, and it does NOT measure sprinkler discharge density. It IS a fast, high-value check that nothing between the city main and your riser has quietly changed — a closed valve, a fouled backflow, a scaled underground, or a municipal supply reduction. Think of it as taking your pulse: the number doesn't diagnose anything specific, but a significant change from yesterday tells you something needs investigation.

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← Fire Protection Systems

PROCEDURENFPA 25 §13.2.3ANNUAL / QUARTERLY

Main Drain Test
The Simplest Test in Fire Protection

Open a 2-inch valve. Read two gauges. Compare to last year. If the pressure dropped more than 10% — investigate. It's the single fastest check that your water supply hasn't quietly degraded since the last inspection. But the details matter: main drain vs main drain test connection, annual vs quarterly frequency, and the baseline-documents problem that sinks half the inspections in the field.

By Samektra · 11 min read · Last updated April 2026

The Simplest Test with the Highest Signal

Of all the tests required by NFPA 25 — pump flow test, standpipe flow test, alarm test, 5-year internal — the main drain test is the simplest. Open one 2-inch valve at the base of the riser. Read two pressure gauges. Compare to last year. That's it.

It's also the test with the highest signal-to-effort ratio. A significant pressure drop from the previous test is one of the earliest warning signs that something between the municipal water main and your sprinkler system has changed — a closed valve, a fouled backflow preventer, scale buildup, or a neighboring property that added demand. The test doesn't tell you what is wrong; it tells you to investigate. And it's the first test a fire marshal asks to see results for.

Main Drain vs. Main Drain Test Connection — The Distinction That Traps Installers

Per NFSA Source 3, citing NFPA 13 §16.10.4.6: there is a subtle but important distinction between a main drain connection and a main drain TEST connection.

Main Drain Connection

Used to drain the fire protection system (controlled drain-down for maintenance, freeze protection, etc.). Located at the system riser. Can be piped to any drain capable of handling slow-drain flow.

Main Drain TEST Connection

The same physical connection, but installed so the valve can be opened wide for enough time to actually conduct the test. Must discharge to a drain capable of handling FULL flow. A floor sink that works for slow drainage may not handle the test.

This distinction is a common installation failure. NFSA flagged it in their 2021 post on evaluating installation issues: a main drain that can't handle full flow is “commonly not cited because it was approved or accepted by an AHJ” Source 4 — but the practical result is that the test cannot be properly performed, every year, forever, until the installation is corrected.

Anatomy of a Main Drain Assembly

What each labeled component does

Main Drain Assembly — purple-highlighted: the valve, elbow, and discharge pipe you operate during the test

Red Cast Iron Handwheel (Main Drain) — the handwheel you turn to open the main drain valve

Hose Thread Discharge Coupling (NHT) — threaded outlet; a 2½" NH fitting in most installs

Main Drain Discharge Pipe — routes discharge to a drain location, typically exterior

DN150 Flanged Fitting (riser) — ~6" flanged fitting joining the riser sections

Top Pressure Gauge (Air / Water) — reads system-side pressure (above the valve); used for dry-system supervisory and wet-system residual checks

Bottom Pressure Gauge (Water Only) — reads supply-side (below the valve); this is typically the gauge used for static & residual during the main drain test

Brass Isolation Valves (on each gauge) — let you replace a gauge without draining the riser

Coupling Bolts (×4 typical) — Victaulic-style mechanical grooved coupling fasteners

Main Coupling Nut & Sealing Gasket — retain the coupling and seal it against pressure

System Identification Mark — stamped or painted ID on the riser (zone, owner, system type)

Signage Chain (Stainless Steel) + MAIN DRAIN Sign — NFPA 13 §16.9.11.1 required identification tag secured with non-corrosive chain

How to Perform the Test — Step by Step

Main Drain Test Procedure

1Notify the FACP and monitoring center before starting. The pressure drop may trip the waterflow switch — coordinate so nobody calls the fire department.

2Verify the test discharge location can handle full flow. If the drain backs up during the test, the test is invalid and you're flooding the pump room.

3Record STATIC pressure from the system gauge before opening the main drain valve. This is your "before" baseline.

4Open the main drain valve fully — 2-inch valve, typically a 90° ball valve or gate valve at the base of the riser. "Fully" means all the way open, not cracked.

5Wait for pressure to stabilize — typically 10–30 seconds depending on system volume.

6Record RESIDUAL pressure while water is flowing. This is your "during" reading at full flow.

7Close the main drain valve slowly. Closing too fast can cause water hammer in the supply line.

8Verify static pressure returns to the pre-test reading once the valve is closed.

9Document: static, residual, ambient conditions, date, technician name, and license. Compare to original acceptance AND previous tests.

10Reset FACP and notify monitoring center that the test is complete.

Reading the Results — Static, Residual, and the 10% Rule

The test produces two numbers that matter, plus a third comparison:

Static pressure: system pressure before the main drain valve is opened. Reflects the resting water supply pressure at the riser.
Residual pressure: system pressure while the main drain is flowing at full open. Reflects the pressure the water supply can maintain under load.
Pressure drop (static − residual): the difference between the two. A bigger drop = weaker supply under load.

⚠️ The 10% rule (NFPA 25 §13.2.3.3)

“When there is a reduction of ten percent of full flow an investigation shall be conducted to determine the reason.” NFPA 25 §13.2.3.3 The 10% is measured against the original acceptance test OR previous test results — per §4.6.4, all flow test results should be compared to BOTH. A drop from last year signals recent change; a drop from the original shows long-term degradation.

Worked example

Original acceptance (2015): Static 85 psi → Residual 65 psi → Drop 20 psi
2024 test:                   Static 82 psi → Residual 48 psi → Drop 34 psi

Residual pressure change: 65 → 48 psi = 26% reduction — significantly over the 10% threshold.
Flow-equivalent reduction can be estimated from √(residual ratio) using hydraulic approximations, but the 26% residual drop alone clearly triggers §13.2.3.3 investigation.

Frequency — Annual vs. Quarterly

NFSA gives the clearest authoritative answer Source 3:

ANNUAL — the default

Standard requirement for most sprinkler systems. One main drain test per year per riser (or per manifold). Per NFPA 25 §13.2.3.

QUARTERLY — when backflow / PRV is present

If the sole water supply passes through a backflow preventer OR a pressure-reducing valve (PRV), the test is quarterly per §13.2.3.2. Reason: exercise the backflow / PRV internals, not just verify supply.

💡 The “which riser” rule (§13.2.3.1)

Multiple risers on a single manifold: test at only one riser. Multiple individual risers: test at each. Multiple risers fed by the same backflow preventer: quarterly test required on only ONE downstream system. The point of the quarterly test isn't redundant supply verification — it's to exercise the backflow components.

What a Failed Test Points To

A significant pressure drop (>10%) doesn't diagnose the cause — it just tells you to investigate. Common findings in that investigation:

Partially closed upstream valve

The classic finding. A control valve somewhere between the city main and the riser is not fully open. Often a PIV, OS&Y, or post-indicator that was closed during maintenance and never fully reopened. Check every valve upstream of the riser.

Fouled backflow preventer

RPZ or double-check device with failing internals restricting flow. Typically shows up as pressure drop across the backflow device itself. Requires §6.2.5 internal inspection to confirm.

Scale / debris in piping

Long-term buildup in underground or riser piping reduces effective cross-section. Exceeds the scope of a main drain test to confirm — may need an obstruction investigation under NFPA 25 Ch. 14.

Municipal supply reduction

Water utility work, new demand on the same main, or a water tower level change affecting the pressure zone. Call the utility — they'll confirm current system pressure at your service tap.

Neighboring property demand

Adjacent building installed a new sprinkler system, industrial process, or irrigation that shares your service main. Shows as persistent lower residual at your test.

Check valve partially stuck

Reverse-flow check valve in the line between supply and riser is not fully opening under flow. Confirmed at the 5-year internal inspection.

The Baseline Documents Problem

Per NFPA 25 §4.3, the building owner is legally responsible for keeping the original acceptance test results for the life of the system. Without those records, every main drain test is orphaned data — you can compare to the previous year, but you can't compare to day-one performance. Missing original records is a citable deficiency.

For facilities with lost original documentation (common on any building over 20 years old), the pragmatic fix is a retro-baseline: document the current static/residual/flow pressures as the new baseline, with a letter to the owner acknowledging the original is lost and this is the new reference. Not perfect, but defensible and better than nothing. Fresh installs should have the acceptance packet scanned and archived in three places before the contractor leaves the site.

Documentation Requirements

Per NFPA 25 §4.6.1 and §4.6.4, every main drain test must document:

Static pressure (before opening the valve)
Residual pressure (during full flow)
Date and time of test
Technician name and license number (in Georgia, GAC 120-3-05 licensed)
Comparison to previous test and to original acceptance — percent change on both
Pass/fail determination based on the 10% threshold
Notes: any abnormal conditions, alarm trips during test, water supply work in progress, etc.
Corrective action if investigation was triggered and not yet resolved

Keep the test report filed with the building's fire protection ITM records. Per §4.3, these are the owner's responsibility and must be available to the AHJ on request.

▶ Watch: Main drain test — field walkthrough

Source: Field technique · Open on YouTube ↗

References

1. NFPA 25 (2023): Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, §13.2.3 (main drain test), §13.2.3.1–.3, §4.3 (owner records), §4.6.1 (components test), §4.6.4 (comparison requirement).

2. NFPA 13 (2019): Standard for the Installation of Sprinkler Systems, §16.10.4.6 — Main drain test connection installation.

3. NFSA: Main Drain Tests for Fire Sprinkler Systems — authoritative explanation of test connection rules, annual vs quarterly frequency, per-riser requirements, and the 10% threshold.

4. NFSA: To Cite or Not to Cite — Evaluating Installation in NFPA 25 — on citing test connections that cannot handle full flow.

5. IFC 2024: International Fire Code, §901.6 — Inspection, testing, and maintenance of fire protection systems.

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Discussion (3)

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Julio M.Fire Sprinkler Inspector, NICET III· 4 days ago

The #1 main drain test finding I write isn't a pressure drop — it's that the owner can't produce the original acceptance test results. Without those baseline numbers, there's nothing to compare current results against. NFPA 25 §4.3 makes the owner legally responsible for keeping those records for the life of the system. I've walked into 30-year-old buildings where the original paperwork is long gone, and now every main drain test is effectively orphaned data.

▲ 21Reply

SamektraSafety Management & Training· 3 days ago

Exactly right — and it's a $5,000+ problem when the insurance carrier finds out post-incident. For facilities we service, we retro-baseline the system at our first visit if the original is lost: document the current static/residual/flow pressures as the 'new baseline' with a letter to the owner explaining why. Not perfect, but defensible. Fresh installs should get the acceptance packet scanned and archived in three places before the contractor leaves the site.

▲ 10

Ginny S.Property Manager· 1 week ago

Our main drain was piped to a floor sink inside the pump room. When we opened it fully for the 2024 test, the sink couldn't handle the flow and we flooded about 200 sq ft of the room before getting it shut. NFSA's Aug 2022 blog post explicitly warned about this: the main drain TEST connection has to be able to handle full flow, not just slow drainage. Rebuilt ours to discharge outside — problem solved, and now the test actually runs per spec.

▲ 17Reply

BEA

Building Engineer, AtlantaMid-rise office· 2 weeks ago

Quarterly vs annual tripped us up last year. We have an RPZ on the riser — didn't realize that bumped us from annual to quarterly per NFPA 25 §13.2.3.2. Fire marshal wrote us up for three missed quarterlies. The AHJ reasoning wasn't about water-supply verification — it was about keeping the RPZ internals exercised. Worth checking your installation: if there's a backflow preventer or PRV in the line, you're quarterly whether you like it or not.

▲ 14Reply

Main Drain TestThe Simplest Test in Fire Protection