System Drains
The Unsung Heroes
Main drains, auxiliary drains, and drum drips — the fittings that make the difference between a healthy dry system and a frozen, burst riser.
Three Kinds of Drain
The Main Drain Test
NFPA 25 §13.2.5 requires an annual main drain test at each system riser. The procedure is simple: record the static pressure from the supply-side gauge, fully open the main drain valve, wait for a steady residual pressure reading, then close the drain valve slowly.
The test exposes two failure modes that nothing else catches. First, a partially closed control valve upstream will cause a dramatic drop in residual pressure compared to previous tests — often the only warning that someone left a valve not-quite-open after maintenance. Second, a deteriorating underground main or closed curb cock at the city tap will trend the static pressure downward over years of tests. Compare to the baseline from acceptance, not just to last year.
Drum Drips in Cold Weather
Winter procedure: Drain auxiliary drum drips before the onset of freezing weather and again after each trip test. On an older system without low-point drains at every dip in the pipe, an undocumented sag can accumulate half a gallon of condensation over a year — enough to freeze and split the pipe. If an auxiliary drain is producing noticeably more water than expected, the pipe has a sag or there's a slow air leak somewhere allowing humid air in.
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Watch on YouTube →References
1. NFPA 13 (2022), §16.7 — Drain installation requirements.
2. NFPA 25 (2023), §13.2.5 — Annual main drain test.
3. NFPA 25 (2023), §13.4.4.3 — Auxiliary drain maintenance on dry systems.
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Discussion (2)
Great breakdown of the technical details. The NFPA 25 maintenance table is exactly what I needed for my ITM schedule.
Really clear explanation. Would love to see a companion video walkthrough of the inspection process.