Holding time index as a risk screen

Holding time — and related residence-time or stagnation screens — appear in Legionella and building-water risk discussions for a reason: Legionella prefers places where water sits, cools into a growth range, and loses disinfectant residual. A holding-time index is a way to flag those conditions. It is not a diagnosis, and it is not a water management plan.

What the screen is trying to capture

In distribution and premise plumbing, risk clusters around:

• long residence times in tanks, dead legs, and low-use branches,
• temperatures that enter the organism's growth window,
• loss of oxidant residual,
• nutrient and biofilm conditions that support amplification.

A holding-time or similar index compresses flow, volume, and usage patterns into a comparative score: which nodes look "stale" relative to the rest of the system? That prioritization is valuable when an account has hundreds of assets and a limited sampling budget.

What it cannot replace

ASHRAE 188 / water management program structure. A screen does not invent a team, a schematic, a control limit table, or an append-only corrective-action trail. Program software exists so those artifacts are generated, owned, and inspectable.

Temperature and disinfectant control. Holding time without temperature and residual data is half a story. A short residence time at a bad temperature can still amplify; a long residence time with tight temperature control and residual may be lower priority than the index alone suggests.

Culture / PCR results. Laboratory evidence remains laboratory evidence. Indices route attention; they do not substitute for confirmed detection and response.

Jurisdictional rules. Monitoring frequency, notification, and documentation requirements vary. A generic holding-time flag does not encode every pack — and should not pretend to.

Using the screen honestly

1. Treat holding time as a risk screen in a broader model: temperature bands, residual, asset type, and usage.
2. Drive sampling and flushing priorities from the screen, then close the loop with lab results and corrective actions.
3. Keep an audit trail — who changed a control limit, when a finding closed, which version of the plan applied — because inspectors ask for provenance as much as engineers do.
4. Do not publish a single site-wide "safe / unsafe" color from holding time alone.

AquaAdvisor's Legionella program packs are built around jurisdiction-aware rules, generated monitoring plans, lab results, corrective actions, and an append-only trail. Holding-time style screens belong inside that program as prioritization tools — never as the program itself.