---
title: "Using water system schematics and asset registers in risk assessments"
source_url: https://legionella.io/articles/using-water-system-schematics-and-asset-registers-in-risk-assessments/
canonical_url: https://legionella.io/articles/using-water-system-schematics-and-asset-registers-in-risk-assessments/
pillar: "Legionella Risk Assessment"
summary: "How UK responsible persons use a water system schematic and asset register to surface hidden dead legs, unowned assets and the gaps that hide Legionella risk."
primary_keyword: "water system schematic"
date_published: 2025-10-03
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Using water system schematics and asset registers in risk assessments

A water system schematic and an asset register are the two documents that turn a Legionella risk assessment from a memory of a walk-round into something you can actually interrogate. The schematic maps how water moves. The register lists every thing that needs a control. Used properly, they don't just describe your system — they expose the parts of it nobody can account for, and that gap is usually where the risk sits.

Most sites treat both as compliance furniture: a drawing filed at the back of the report, a spreadsheet updated once and forgotten. The more useful approach is to read them as a fault-finding kit. Every place where the paper and the building disagree is a lead worth chasing.

## What each document is really for

A schematic shows flow and storage: where water enters, where it is heated and held, the route it takes to each outlet, and where it returns or dead-ends. It is the picture that lets you reason about stagnation, temperature loss and circulation before you ever pick up a thermometer.

An asset register is the inventory: tanks, calorifiers, TMVs, showers, spray taps, hoses, expansion vessels, strainers — every component that can grow or spread Legionella, with enough detail to know who maintains it and when. The schematic tells you how the system behaves; the register tells you what is in it and whether each item has an owner.

A competent assessment leans on both. L8 and HSG274 are the core UK references for the management chain around them [1][2], and BS 8580-1 sets out what a good Legionella risk assessment should contain, including an accurate description of the system [3]. The point of holding the schematic and register is not to satisfy an auditor. It is to answer, for any outlet: how does water reach it, how long has it been sitting there, and who is responsible for keeping it safe.

## When the drawing and the building disagree

Walk the system with the schematic in one hand and the register in the other, and discrepancies surface fast. Treat each one as a symptom, not an annoyance. Here is how the common ones tend to resolve.

| What you find on site | What it usually means | The check that confirms it | The action |
| --- | --- | --- | --- |
| An outlet that isn't on the schematic | An undocumented branch — often a forgotten spur or dead leg | Trace the pipe back to its tee; feel for temperature and run length | Add it as-built; assess for stagnation and remove if redundant |
| An asset on the register you can't physically find | A component decommissioned but never isolated, or buried behind cladding | Cross-check service history; follow the pipe; lift access panels | Locate and isolate properly, or remove from the register with a dated note |
| A loop on the drawing that is a dead-end spur on site | The schematic is design-stage, not as-built | Walk the pipe run end to end | Record the as-built reality and reassess circulation and temperature |
| A TMV or filter not listed anywhere | Fitted during a refit and never logged | Inspect for make, model and last service | Add to the register with a maintenance schedule and an owner |
| A far outlet failing temperature on a long run shown on the drawing | Poor circulation, undersized return, or a hidden dead leg | Measure flow and return temperatures along the run | Re-balance, shorten the run, or bring the outlet under specific control |

The pattern matters more than any single defect. If the same kind of discrepancy keeps appearing — outlets missing from the drawing in three different wings — you are not finding one-off errors. You are finding evidence that the documents have drifted away from the building, which is itself a control failure worth recording.

## Reading risk off the page

Once the schematic and register match reality, they stop being records and start being a risk-ranking tool. Work the most likely problems first.

Start with stagnation. Trace every run that ends without regular flow: capped spurs, outlets in voided rooms, an oversized cold tank feeding a half-empty building. Stagnant, tepid water is the classic condition for growth, and the schematic shows you where it is far faster than a cold walk-round can [4].

Then check temperature geography. Hot water that leaves the calorifier hot can still arrive tepid at the end of a long, [poorly insulated run](https://legionella.io/articles/pipe-insulation-and-its-role-in-legionella-control/) — the drawing tells you which outlets are most exposed before you measure a single one.

Finally, weigh exposure. Cross the aerosol-producing assets on the register — showers, spray taps, anything that throws a mist — against who actually uses them. A rarely-used shower serving vulnerable occupants outranks a busy tap that flushes itself through constant use. That ranking is what turns a flat list of findings into a defensible order of work.

## Keeping both documents alive

A schematic dated five years ago, drawn before two refurbishments, is worse than no schematic: it gives false confidence. The register has the same failure mode the moment an outlet is added, removed or re-piped and nobody updates the row.

Tie updates to the events that actually change the system — refurbishments, plant replacement, a change of use, new tenants, or any remedial work that touches pipework. The risk assessment review is the natural checkpoint to confirm the drawing and the register still describe the building in front of you. HSE guidance is clear that monitoring and testing follow the system and its risk assessment rather than a fixed calendar [5], so a current schematic is what keeps that judgement honest.

## A note on what a good drawing can't do

A schematic and a register are decision tools, not proof of safety. An immaculate drawing of a system you no longer control protects nobody. These documents support a competent, site-specific assessment carried out by someone who understands the building — they do not replace one, and they are not legal or engineering sign-off. Where a discrepancy reveals a real defect, the fix belongs in your written scheme and to your competent person, not in a tidied-up spreadsheet.

## Do this on your next site visit

Print the current schematic and asset register and carry them round the plant, not the report. Mark every disagreement between the page and the building directly onto the drawing as you go. Before you leave, count the discrepancies: if there are more than a handful, the documents — not just the outlets — are your first remedial action. From there, feed the corrected drawing back into the assessment; [BS 8580-1: what a good Legionella risk assessment should include](https://legionella.io/articles/bs-8580-1-what-a-good-legionella-risk-assessment-should-include/) covers what a complete Legionella risk assessment should contain.

## FAQ

### Does a Legionella risk assessment have to include a schematic?
A competent assessment has to describe the system accurately enough to identify where Legionella could grow and spread, and a schematic is the normal way to do that — BS 8580-1 treats an accurate system description as part of a sound assessment [3]. Whether a formal drawing already exists or you build one, the assessor needs to be able to show the route water takes and where it stagnates. Confirm the specific expectation against the current standard for your site.

### What's the difference between a schematic and an asset register?
The schematic is the map: it shows how water flows, where it is stored and heated, and where it dead-ends. The register is the inventory: every component that needs a control, with its location, owner and maintenance detail. You need both — the drawing to reason about behaviour, the register to make sure nothing is left without a responsible person.

### How detailed does the asset register need to be?
Detailed enough that someone who has never seen the building could find each asset, know which control applies to it, and know who maintains it. For Legionella that usually means location, asset type, the control measure that applies and the responsible person — not exhaustive engineering data. The honest test is whether it would let a new responsible person take over without a site tour.

## Related reading

- [BS 8580-1: what a good Legionella risk assessment should include](https://legionella.io/articles/bs-8580-1-what-a-good-legionella-risk-assessment-should-include/)
- [Training staff for Legionella compliance: what is expected?](https://legionella.io/articles/training-staff-for-legionella-compliance-what-is-expected/)
- [Pipe insulation and its role in Legionella control](https://legionella.io/articles/pipe-insulation-and-its-role-in-legionella-control/)
- [Design-stage Legionella risk assessment for new buildings](https://legionella.io/articles/design-stage-legionella-risk-assessment-for-new-buildings/)

## Sources

[1] HSE, "Legionnaires' disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)". https://www.hse.gov.uk/pubns/books/l8.htm
[2] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] BSI, "BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice". https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1
[4] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[5] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm