---
title: "Heat pumps and Legionella: keeping low-carbon hot water hot enough"
source_url: https://legionella.io/articles/heat-pump-hot-water-legionella/
canonical_url: https://legionella.io/articles/heat-pump-hot-water-legionella/
pillar: "Water Temperature Control"
summary: "Heat-pump cylinders store hot water cooler to stay efficient, so a scheduled high-temperature cycle handles Legionella control. Here's what to check."
primary_keyword: "heat pump Legionella"
date_published: 2026-04-28
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."
---

# Heat pumps and Legionella: keeping low-carbon hot water hot enough

A heat pump warms your hot water cylinder more gently than a gas boiler, and storing that water cooler is part of what makes the system efficient. The catch is that cooler storage sits closer to the temperature band Legionella grows in. The control that keeps it safe is a scheduled high-temperature cycle — and it only works if it is set, running and reaching the whole cylinder.

So, do heat pumps increase Legionella risk? Not inherently. This is a control-strategy point, not a verdict on the technology. A heat pump hot water cylinder is designed around a periodic disinfection cycle doing the job that a boiler's hotter everyday storage used to do quietly in the background. Get that cycle right and the system is as controllable as any other. Leave it switched off and you have warm water stored in the worst possible band.

## Why a heat-pump cylinder stores cooler

A boiler burns fuel and can hold a cylinder hot all day at little penalty. A heat pump moves heat rather than making it, and it works hardest — least efficiently — the higher the temperature it has to reach. Every extra degree of storage spends more of the efficiency that was the reason to fit it. So designers commonly specify a heat-pump cylinder to store at a lower everyday temperature than a boiler-fed one, and to top the whole tank up to a disinfection temperature only on a schedule.

That trade is deliberate and sensible. It simply changes where your Legionella control comes from.

## The problem cooler storage creates

Legionella multiplies in standing water in roughly the 20°C to 45°C band, and the core of control is keeping stored hot water hot enough that the bacteria cannot establish, while keeping cold water cold [1][2]. A continuously hot boiler cylinder does the hot half passively — most of the store sits above the band most of the time. Stored water that drifts into the band can seed the outlets, and Legionella reaches the lungs as a fine aerosol from a shower or spray tap [3].

Lower everyday storage moves a heat-pump cylinder closer to, or partly into, that band. The coolest part of any tank is the bottom, where incoming mains water enters and the heat lags behind the top — [Temperature stratification in hot water cylinders](https://legionella.io/articles/temperature-stratification-in-hot-water-cylinders/) sets out why the base runs cooler than the dial at the top suggests. That is the whole worry behind "do heat pumps increase Legionella risk" in one line: not the heat pump, but the cooler stored water it favours and the cool layer at the foot of the tank.

## The high-temperature cycle is your actual control

The design answer is a periodic high-temperature run — labelled on controllers as the anti-Legionella cycle, the pasteurisation cycle, or the ASHP disinfection cycle. Its job is to bring the whole stored volume, not just the top of the tank, up to the disinfection temperature your risk assessment specifies, at the frequency it sets [2]. Because the heat pump on its own may struggle to reach that figure efficiently, the cycle usually calls on a backup electric immersion to finish the job.

Two things make or break it. First, it has to actually run — enabled on the controller, with a working immersion, not switched off by an installer or owner chasing a lower bill. Second, it has to reach the bottom of the cylinder, where stratification leaves the coolest water. An immersion or sensor set too high in the tank can report success while a cool layer sits underneath.

## Boiler-fed vs heat-pump cylinder: what changes for control

The fundamentals of stored hot water do not change when you swap the heat source. What changes is the temperature strategy, and therefore what your control regime has to watch.

| What to check | Boiler-fed cylinder | Heat-pump cylinder |
|---|---|---|
| Everyday storage temperature | Usually held continuously hot | Often kept lower to protect efficiency |
| Where disinfection comes from | Passive — the store stays hot | Active — a scheduled high-temperature cycle |
| Coolest zone between heat-ups | Small; most of the store stays hot | The cylinder base can sit cool until the next cycle |
| How it fails | Loudly — a fault usually means no hot water | Quietly — a disabled cycle still gives water hot enough to bathe in |
| What your regime must verify | Periodic outlet and storage temperatures | That the cycle runs and the whole volume reaches temperature |

The row that deserves a second look is "how it fails". A boiler problem announces itself: cold showers, complaints, a callout. A disabled or mis-set disinfection cycle leaves the water feeling perfectly warm at the tap, so nothing prompts anyone to investigate. The failure is invisible until someone checks the controller — which is exactly why it has to be checked.

## Three situations, three things to check

**Homeowner on a retrofit.** The most common failure is also the simplest: the anti-Legionella cycle left disabled, or set so rarely it does little, usually to keep running costs down. Find the setting on the controller, confirm it is on, and confirm the backup immersion works.

**Landlord with a heat-pump let.** The duty to assess and control Legionella applies to a rented home whatever heats the water [4]. A net-zero retrofit changes the basis of that control — from passive hot storage to a scheduled cycle — so your risk assessment and records should say so, and should note that the cycle is enabled and verified. The boiler-to-heat-pump swap is itself a reason to revisit the assessment rather than assume the old one still describes the system.

**Housing or estates M&E lead running a programme.** At portfolio scale the silent-failure mode is the one that bites: hundreds of cylinders, an unknown number with the cycle quietly off or mis-set, none of them obviously broken because the water still runs warm. Standardise the cycle settings across the stock, decide how you will confirm each one reaches temperature through the whole cylinder, and treat "cycle disabled" as a reportable fault rather than a tenant preference.

## The energy trade-off is real — and not the part to cut

The appeal of a net-zero retrofit is lower carbon and a lower running cost, and the disinfection cycle spends some of that saving by design — it runs the cylinder hot, immersion included. That tension is precisely why the cycle gets switched off. [Energy savings vs Legionella risk: finding the balance](https://legionella.io/articles/energy-savings-vs-legionella-risk-finding-the-balance/) makes the general case; the heat-pump version is blunt. Trim standby losses, insulation, timing and flow temperatures all you like. The disinfection cycle is the one control you do not quietly remove to save energy without a risk assessment that justifies an alternative.

## How this differs from systems you already know

A combi sidesteps the problem by storing nothing — it heats on demand, so there is no warm reservoir to manage at all, as [Combi boilers and instantaneous water heaters: do they remove Legionella risk?](https://legionella.io/articles/combi-boilers-and-instantaneous-water-heaters-do-they-remove-legionella-risk/) explains. A heat pump goes the other way: it keeps a stored cylinder, like the boiler systems in [Hot water storage: preventing Legionella in tanks and cylinders](https://legionella.io/articles/hot-water-storage-preventing-legionella-in-tanks-and-cylinders/), but runs it cooler day to day. Same storage fundamentals, different temperature strategy — which is why the periodic cycle carries more weight here than on a system kept hot around the clock.

This is general guidance, not a design or an assessment of your installation. The storage temperature, the disinfection-cycle setting and how often it should run that are adequate for a particular building are judgements for a competent person working from a current, site-specific risk assessment under ACoP L8 — not figures to copy from an article or change on a hunch. Do not disable or rewrite a manufacturer's anti-Legionella cycle without that assessment behind you. Treat this as general information, not legal, medical or design advice.

## What to do this week

Go to the heat-pump controller and find the anti-Legionella or pasteurisation setting. Confirm it is enabled and that the backup immersion works. Then, on the next scheduled run, check the temperature it actually reaches low down on the cylinder, not just at the top outlet — that is where a cool layer hides. If you cannot show the cycle runs and brings the whole stored volume up to the figure your risk assessment sets, that single gap is the one to put to your installer or assessor before anything else.

## FAQ

### Do heat pumps increase Legionella risk?

Not by themselves. A heat pump hot water cylinder stores water cooler to stay efficient, which shifts disinfection from passive hot storage to a scheduled high-temperature cycle. Risk only rises if that cycle is disabled, set too rarely, or fails to heat the whole cylinder. With the cycle working and verified, a heat-pump system is as controllable as a boiler-fed one [1][2].

### What is the ASHP disinfection or pasteurisation cycle, and do I need it?

It is a periodic run that lifts the whole stored cylinder up to a disinfection temperature, usually with help from a backup immersion because the heat pump alone may not reach it efficiently [2]. On a cooler-stored cylinder it is the main thing standing between you and water held in the growth band, so on a stored heat-pump system you generally do need it — set to the temperature and frequency your risk assessment specifies.

### Can I switch off the anti-Legionella cycle to save money?

Not without a risk assessment that justifies a different control. The cycle is the active replacement for the passive protection a continuously hot cylinder used to give, and turning it off leaves stored water sitting cooler for longer [2]. Save energy on insulation, timing and flow temperatures instead, and keep the disinfection cycle as the one fixed point.

## Related reading

- [Hot water storage: preventing Legionella in tanks and cylinders](https://legionella.io/articles/hot-water-storage-preventing-legionella-in-tanks-and-cylinders/)
- [Temperature stratification in hot water cylinders](https://legionella.io/articles/temperature-stratification-in-hot-water-cylinders/)
- [Combi boilers and instantaneous water heaters: do they remove Legionella risk?](https://legionella.io/articles/combi-boilers-and-instantaneous-water-heaters-do-they-remove-legionella-risk/)
- [Energy savings vs Legionella risk: finding the balance](https://legionella.io/articles/energy-savings-vs-legionella-risk-finding-the-balance/)

## 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, "Hot and cold water systems". https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
[3] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html
[4] HSE, "Legionella and landlords' responsibilities". https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm