---
title: "Future-proof design: plumbing designs that minimise Legionella"
source_url: https://legionella.io/articles/future-proof-design-plumbing-designs-that-minimise-legionella/
canonical_url: https://legionella.io/articles/future-proof-design-plumbing-designs-that-minimise-legionella/
pillar: "Best Practice & Future of Legionella Control"
summary: "Cut Legionella risk at the drawing board: design out dead legs, right-size storage and run the design checks that matter before the drawings are signed off."
primary_keyword: "Legionella design"
date_published: 2025-11-09
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."
---

# Future-proof design: plumbing designs that minimise Legionella

The cheapest Legionella control measure is the one you never have to carry out, because the system was drawn so it never needed it. A dead leg you don't build costs nothing. The same dead leg, found three years after handover, costs a contractor's day, a section of stripped-out ceiling, and a flushing task that then recurs forever.

That is the argument for treating design as the first line of control rather than an afterthought. Get the pipework, storage and temperatures right on the drawing and you remove the conditions Legionella needs — warm, still water and a way to be breathed in — before the system ever holds a drop. Everything downstream, the monitoring and flushing and sampling, then has less to fight against.

"Future-proof" is the harder half. A building's water use changes: a wing is mothballed, a floor is sublet, a back office becomes a gym with showers. Good design assumes the system will be used differently from the way it was specified, and makes those changes safe by default instead of quietly turning a redundant branch into a reservoir.

## What good design actually buys you

UK guidance frames Legionella control around the same chain whatever the building: assess the risk, control it, monitor, keep records, review [1][2]. Design is where you make that chain easy or hard to hold for the next thirty years. There are really only three levers, and every item below pulls one of them — keep the standing volume of water small, hold temperature through the whole system, and design for flow and access so the water keeps moving and someone can actually check it.

HSE's own list of higher-risk features reads almost like a list of design mistakes: stored water, dead legs, long runs that lose temperature, fittings that throw an aerosol [4]. Designing against that list is the job.

## The checks to make before the drawings are signed off

Run this at concept design and again at detailed design, marking each line against the actual layout. It is grouped the way a system fails, not the way a merchant's catalogue is organised.

**Storage and hot water generation**
- Size cold water storage for genuine turnover, not for a worst case that leaves tanks part-full and warming; avoid duplicate or oversized tanks that sit stagnant [4].
- Site cold tanks somewhere genuinely cool, away from plant heat, and specify lids, insulation and screened vents.
- Size the calorifier or cylinder to real demand, so stored hot water is used and replaced rather than held warm and idle.

**Pipework layout and dead legs**
- Eliminate blind ends and redundant branches; where a spur must exist, take it off as close to the live main as practicable.
- Keep runs short and direct to far outlets, and design a balanced secondary return so the ends of the hot system stay hot, not just the riser.
- Separate hot and cold routes — never run a cold main through a warm duct alongside heating or hot pipework where it will pick up heat.

**Temperature and insulation**
- Insulate hot, cold and recirculating pipework so the system holds hot water hot and cold water cold along the whole route, not only at the plant [3].
- Design out the warm no-man's-land between blending and outlet by siting thermostatic mixing valves as close to the tap or shower as practicable.

**Outlets, materials and access**
- Only install outlets that will actually be used; speculative taps and showers become tomorrow's low-use risk.
- Specify wholesome-water-approved materials and fittings that don't feed biofilm, and minimise flexible hoses, which harbour it.
- Build in the things that let someone prove control later: drain points, isolation valves, and accessible sampling and temperature points at the sentinel outlets.

**Handover and the as-built record**
- Hand the duty holder accurate as-built drawings and an asset register, not the design intent — the two diverge on every site.
- Zone the system so a future empty wing can be isolated and drained rather than left to stagnate.

## Putting the checklist to work

A checklist only counts if it lands somewhere a decision actually gets made. Bring it to the design review, not the snagging list. For each line, mark whether the drawing meets it, and where it doesn't, write down the decision and who owns it. "Blended supply runs four metres to the changing-room showers; accepted on cost grounds; flagged as a low-use flushing point at handover" is worth far more than a tick in a box.

Those notes are the seed of the commissioning risk assessment and the water safety plan the duty holder will inherit. A design that knowingly accepts a long blended run is defensible, as long as the decision is visible and the resulting flushing task is recorded rather than discovered the hard way later.

## The decisions that quietly get value-engineered out

In my experience the same handful of items are the first to go when a budget tightens, and they are exactly the ones that bite.

Secondary return balancing is the classic. Skip it and the riser stays hot while the far bathrooms run tepid for a minute — a warm dead leg in all but name. Mixing-valve placement is the next casualty: one central blending valve is cheaper than many local ones, but it turns the whole downstream network into warm, blended water sitting in the growth range. Removing genuinely redundant pipework during a refurbishment gets dropped because capping is quicker than ripping out, and a capped, disconnected spur is a textbook blind end. Sampling and drain points get cut because nobody in the room is picturing the person who, years later, has to prove the system is safe.

If you protect only one thing from the red pen, protect the removal of dead legs. Stagnation is the root condition behind most of what goes wrong, and it is far cheaper not to build it than to manage it forever — the same lesson that runs through [neglected, stagnant systems](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/).

## FAQ

### Can a well-designed system remove the need for flushing and monitoring?
No — it lowers the burden, it doesn't lift the duty. Good design means fewer low-use outlets, shorter runs and steadier temperatures, so there is less to flush and fewer surprises. The requirement to assess, control, monitor and review still applies to every system, however cleverly it was drawn [1][2].

### We're refurbishing, not building from scratch — does any of this apply?
Especially then. Refurbishments create dead legs faster than new builds do, because outlets get stripped out and the supplying pipework gets capped and forgotten. Treat every removed basin or shower as a branch to trace back and disconnect at the main, not simply cap. Design-stage thinking applies to alterations as much as to new build [3].

### Where should thermostatic mixing valves go — centrally or at each outlet?
As close to the outlet as practicable. Blending centrally is cheaper but fills the distribution pipework with warm water in the Legionella growth range, which defeats the point of keeping hot water hot all the way to the tap. Local blending keeps the warm, vulnerable section down to the short tail between valve and outlet.

## Before you sign off

Design choices are not a substitute for a competent designer working to the relevant water fittings rules, or for the site-specific risk assessment that should confirm the system at commissioning and feed a water safety plan [5][6]. The temperatures, tank turnover and pipe sizing that make a design safe are set by that assessment and the building's real demand, not by a rule of thumb in an article. And no drawing, however clean, is a reason to ease off monitoring once the building is in use.

The practical next step is small: get this checklist in front of whoever is reviewing your next set of plumbing drawings — a new build, a wing refurbishment, even a single change of use — and make someone sign against each line before the design is frozen. That one habit prevents more risk than any amount of remedial flushing afterwards.

## Related reading

- [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/)
- [Design-stage Legionella risk assessment for new buildings](https://legionella.io/articles/design-stage-legionella-risk-assessment-for-new-buildings/)
- [Legionella and building regulations: design-stage considerations](https://legionella.io/articles/legionella-and-building-regulations-design-stage-considerations/)
- [Design flaws: how poor system design can cause Legionella problems](https://legionella.io/articles/design-flaws-how-poor-system-design-can-cause-legionella-problems/)

## 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] HSE, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[4] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[5] 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
[6] BSI, "BS 8680:2020 — Water quality. Water safety plans. Code of practice". https://knowledge.bsigroup.com/products/water-quality-water-safety-plans-code-of-practice