A New Water Heater Was Linked to Legionnaires’ Disease. Here’s Why That Matters to Every Facility Manager.

When a facility replaces an aging water heater with brand-new equipment, the assumption is usually simple: the system should now be safer, cleaner, and more reliable.

A recently published Legionnaires’ disease investigation shows why that assumption can be dangerous.

In July 2026, the CDC's Emerging Infectious Diseases journal published an investigation into cases of Legionnaires’ disease in the Netherlands associated with newly installed residential water heaters.

The findings provide an important lesson for healthcare facilities:

New equipment does not automatically mean microbiologically safe water.

What Happened?

The investigation began after two clusters of Legionnaires’ disease occurred in separate apartment buildings in the Netherlands during 2022 and 2023.

Investigators eventually identified 23 Legionnaires’ disease cases.

Of those 23 patients, 21, or 91%, had the same brand of water heater installed less than six months before their illness began.

Environmental sampling was conducted at the homes of 21 patients. Legionella pneumophila sequence type 37 was detected in 20 of those 21 homes, or 95%.

Even more importantly, investigators were able to genetically match clinical and environmental isolates in four cases.

None of the 31 control patients interviewed had recently installed a water heater.

The statistical association between Legionnaires’ disease and the newly installed heaters was substantial. Investigators reported an odds ratio of approximately 542, although they also cautioned that the magnitude of that estimate could have been inflated because some controls could not be contacted.

The evidence was strong enough that the manufacturer implemented control measures.

How Can Brand-New Equipment Become a Problem?

This is where the story becomes particularly relevant to hospitals, skilled nursing facilities, assisted living facilities, and other healthcare environments.

Legionella does not care how old a piece of equipment is.

It responds to conditions.

CDC identifies four key factors affecting Legionella growth in potable water systems:

Sediment and biofilm

Temperature

Water age

Disinfectant residual

CDC refers to these factors collectively as STAR.

Replacing a water heater changes part of the mechanical system, but it does not necessarily correct the conditions throughout the entire water distribution system.

The piping is still there.

The recirculation system is still there.

Low-use fixtures may still be there.

Biofilm may still exist.

Dead legs may still exist.

Disinfectant can still decay as water travels through the building.

Water can still stagnate.

Temperatures can still enter ranges favorable for Legionella growth.

That is why equipment replacement should never be confused with verification of water quality.

The Water Heater Is Only One Part of the System

A building's potable water system should be viewed as an interconnected ecosystem.

CDC recommends evaluating the system from the point where water enters the facility all the way to where it exits through a fixture or device.

That includes water heaters, storage tanks, recirculation loops, mixing valves, piping, filters, softeners, showerheads, aerators, faucets and other components.

CDC specifically notes that heaters and other water system components can contribute to the decay of disinfectant residual. It also warns that storage tanks can create unfavorable temperature gradients if they are improperly sized or operated.

This is why simply checking the temperature at the water heater is not enough.

What matters is what is happening throughout the building.

Temperature Matters, But It Is Not the Whole Story

Legionella grows best between approximately 77°F and 113°F (25°C to 45°C).

CDC recommends storing hot water above 140°F (60°C) and maintaining circulating hot water above 120°F (49°C), while also following applicable state and local requirements for preventing scalding.

But temperature is only one control measure.

CDC explicitly states:

No single control measure ensures the control of Legionella in potable water systems.

Water age, disinfectant residual, sediment, biofilm, system design, flow and usage patterns all matter.

Renovations and Equipment Changes Should Trigger Verification

This is the larger lesson healthcare facilities should take from this investigation.

Whenever a building undergoes a significant change to its potable water system, the water management team should ask:

What changed?

Did the change alter water temperature?

Did it change flow or recirculation?

Did it increase water age somewhere else in the system?

Were new sections of piping installed?

Did construction create temporary stagnation?

Was the system properly flushed?

Is disinfectant residual reaching distal fixtures?

Are temperatures where we expect them to be?

And most importantly:

Did we verify what is actually happening after the work was completed?

A successful mechanical installation does not necessarily mean successful water-system commissioning.

This Is Where Water Management Programs Matter

CDC recommends comprehensive water management programs for healthcare facilities within the scope of ASHRAE Standard 188.

A water management program is not simply a document sitting in a binder.

It should identify where hazardous conditions can occur, establish control measures, monitor those controls, define corrective actions when limits are not met, and continuously evaluate whether the program is actually working.

Equipment replacement, construction, renovation, prolonged shutdowns and major plumbing changes should all cause the water management team to look closely at what those changes may have done to the system.

Don't Assume. Verify.

At Legionella Specialties, this is one of the reasons we focus heavily on understanding the overall health of the water system.

We look at conditions throughout the building, including temperature, disinfectant residual, pH, ORP, ATP and other indicators, to identify areas where water quality may be deteriorating.

When appropriate, Legionella testing provides another level of verification.

The goal is not simply to ask whether the equipment is working.

The better question is:

Is the water system working the way we think it is?

There is a major difference.

A new water heater can be installed correctly.

A renovation can pass inspection.

A plumbing project can be completed exactly according to plan.

And the resulting water conditions can still create an environment where Legionella can grow.

The 2026 investigation from the Netherlands is an important reminder that water safety cannot be assumed based on the age of the equipment.

New does not automatically mean safe.

Water safety comes from understanding the system, controlling the conditions that allow pathogens to grow, monitoring those conditions, and verifying that those controls are actually working.

That is what an effective water management program is supposed to do.