What an automatic purge cycle is

An automatic purge cycle is a programmed event where the faucet runs water for a defined time or volume without a user present. The intent is usually to reduce stagnant water in small branches, help maintain disinfectant residual and water quality at points of use, exercise valves after idle periods, and in some strategies rinse drains (site dependent).

Why stagnation matters in real buildings

Stagnation can show up in new construction when occupancy is phased, wings are rarely used, tenant fit-outs stall, or low-flow designs reduce turnover in long branches. Stagnant water can contribute to disinfectant residual decay and odor complaints, scale settling that later clogs strainers and outlets, temperature drift in branches and mixing valves, and higher risk of pathogen growth under the right conditions.

Public health guidance generally treats stagnation control as part of a broader water management program, not a single product feature. When risk profiles call for it, align purge cycles with a documented WMP.

Purge cycles are a control measure, not the whole plan

A practical way to treat purge cycles is as a point-of-use control measure that supports a WMP. They tend to make sense in schools with breaks, airports with uneven concourse usage, stadiums between events, offices with vacant floors or hybrid schedules, and healthcare only with infection-control review and validation.

Match purge volume to pipe volume

The most common purge mistake is guessing a duration that “sounds reasonable.” If the branch volume is larger than the purge volume, you may not exchange the stagnant water you are trying to address. Tie purge duration (or target volume) to the volume of water in the branch serving the faucet.

For defensible specifications, calculate purge duration from verified branch volume and measured outlet flow rather than relying on arbitrary timer settings.

Table 1: Purge sizing inputs to document

Purge parameters to specify

A purge cycle has three levers: frequency, duration or target volume, and timing window. Coordinate across floors and restroom banks so you do not purge everything at once.

Table 2: Practical purge targets by building condition

Commissioning checklist for purge cycles

A. Pre-commissioning checks

• Confirm drains are clear and traps are functional
• Confirm stop valves and filters are accessible
• Verify outlet flow rate on site
• Confirm mixing strategy, especially with thermostatic mixing valves
• Confirm a WMP exists (or is being developed) if purge is justified for hygiene risk control

B. Program settings and document them

• Purge frequency
• Purge duration or target volume
• Time window (after-hours vs occupied hours)
• Per-restroom-bank coordination

C. Validate flushing actually exchanges water

• Measure outlet flow (gpm)
• Confirm purge time produces enough volume to clear the local branch
• Follow the WMP monitoring approach if water-quality metrics are required

D. Confirm the purge does not create new problems

• No overflow, no persistent oversplash outside the basin
• No nuisance activations that confuse users
• No unacceptable after-hours noise or drain gurgling
• No repeated short cycling that wears valves

Manual flushing vs automatic purge

Table 3: Manual fixture flushing vs automatic purge cycles

Do and do not list for AEC teams

Do

• Tie purge settings to a clear purpose: turnover, compliance, post-idle recovery
• Size purge duration to branch volume, not guesswork
• Commission drains and overflow risk
• Document settings in closeout with a simple “what was set and why” record
• Review purge schedules after occupancy changes

Do not

• Run purge cycles without confirming drain function
• Set all faucets in a bank to purge at the exact same minute
• Treat purge cycles as a replacement for a WMP when risk profile calls for one
• Assume more flushing always equals better hygiene

Category pages and key references

Category pages requested

Water hygiene and water management references

Connected platform examples referenced