Legionella

Short definition

Legionella is an aquatic bacterium that proliferates in warm stagnant water (77–113°F) and is inhaled as aerosol from showers, humidifiers, and hot tubs. It causes Legionnaires’ disease — severe pneumonia. Hot water heaters held below 140°F are amplification sites, especially with sediment, dead legs, or recirculation loops set too low.

What it is

Legionella pneumophila is a bacterium found naturally in fresh water at low concentrations. It grows aggressively in a narrow temperature range — 77°F to 113°F (25–45°C) — and amplifies in any plumbing system that combines warm water, stagnation, and surfaces (sediment, scale, biofilm) where it can colonize.

People don’t get sick from drinking contaminated water — they get sick from inhaling aerosol containing the bacteria. Showers, decorative fountains, humidifiers, and cooling towers are the typical exposure routes. Older adults, smokers, and people with weakened immune systems are at higher risk.

Temperature is the main control:

• Below 77°F: Legionella is dormant.
• 77–113°F: Growth zone. The hotter side of this range is where it multiplies fastest.
• 113–122°F: Slow growth, slow die-off.
• 140°F: Lethal — kills 90% of bacteria in about 32 minutes.
• 151°F: Kills instantly.

Why it matters to a homeowner

Most US homeowners never think about Legionella because the typical setup — a 140°F tank with hot water cycling through use every day — keeps it under control. But there are scenarios where it matters:

• Vacation homes. A house unused for months sits in the bacterial growth zone. Pre-occupancy mitigation: heat the tank to 140°F+, then run every hot fixture for at least 5 minutes to flush the lines.
• Long-term rentals between tenants. Same risk as vacation homes.
• Recirculation loops set too low. A loop that returns at 110°F is a Legionella factory.
• Immune-compromised residents. The recommended setup is to store at 140°F+ and use a thermostatic mixing valve (TMV) at the heater to deliver 120°F at fixtures — combines bacterial control with scald safety.
• Dead legs and rarely-used branches. Stagnant warm water in a remote bathroom or capped stub is a textbook growth site.

When a plumber says “I’m going to install a TMV and bump the tank to 140°F,” that’s the modern standard for households with vulnerable residents.

When you’ll encounter this term

• A news outbreak references Legionella in cooling towers or building water systems.
• An immune-compromised family member’s doctor recommends raising heater temperature and adding a TMV.
• A long-vacant property pre-occupancy checklist includes “shock the hot water system.”
• An infection control nurse asks about your home’s water heater setup before a discharge from the hospital.

Common variants and what Legionella mitigation is not

• Legionella vs. other waterborne bacteria. Legionella is the only common waterborne bacterium that infects through aerosol. Pseudomonas and others matter at the point of use but follow different control strategies.
• 120°F tank with no TMV vs. 140°F tank with TMV. 120°F is scald-safe but Legionella-permissive. 140°F + TMV is safe both ways. The TMV mixes hot tank water with cold to deliver 120°F at the fixture.
• Chlorine shock vs. thermal shock. Both work; thermal is easier in residential settings. Public water systems use chlorine.

Washington note

WA RCW 19.27A.060 requires new water heaters to be preset at or below 120°F (or the lowest available setting if the heater can’t go lower). The intent is scald prevention, especially for children and elders.

That setpoint puts WA homeowners directly in tension with Legionella control: 120°F is below the lethal threshold. The standard mitigation is exactly what the RCW allows — store at 140°F+ and use a thermostatic mixing valve to deliver 120°F at the point of use. The TMV requires a one-time install ($150–$400 in WA labor) and balances both risks.

CDC guidance: store at 140°F or higher; recirculate at 120°F or higher. WA AHJs generally accept this configuration when a TMV is installed.

For WA vacation homes (Whidbey Island, Hood Canal, Methow Valley, Lake Chelan) that sit unused for months, the pre-occupancy procedure is:

1. Raise the tank to 140°F+ (most heaters allow 150°F max).
2. Run every hot fixture — including outdoor showers and unused guest baths — for at least 5 minutes each.
3. Optional: a chlorine shock through the cold supply if the home has been unused for a year or more.
4. Return setpoint to your normal operating temperature after exposure.

FAQ

Is 120°F safe for water heaters?

For scald risk, yes — 120°F is the WA-required preset and dramatically reduces burn injuries. For Legionella risk, 120°F is below the lethal threshold (140°F+ kills the bacteria within an hour). The best-of-both setup is a tank set to 140°F or higher with a thermostatic mixing valve at the heater that delivers 120°F at fixtures.

How do I disinfect a water heater for Legionella?

Raise the tank to 150°F or its maximum setpoint and let it stabilize for several hours. Then run every hot fixture in the house for at least 5 minutes — let scalding-hot water flow through the lines and out of every showerhead and aerator. Be careful: 150°F water causes severe burns in seconds. Use gloves and clear the area of children and pets.

Should I worry about Legionella in my home water heater?

For most healthy households with normal use patterns, no. Daily hot-water use keeps lines turning over and storage temperatures usually high enough to suppress growth. The risk goes up with vacation homes left unused for months, recirc loops returning below 120°F, dead legs, or immune-compromised residents — those are the cases where TMV-plus-140°F-tank becomes worthwhile.