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Most facilities that have a Legionella problem already checked the cooling tower. The tower wasn’t the source.
Legionella risk in building water systems extends well beyond cooling towers — and most facility managers are running a program that only covers part of the exposure ASHRAE 188 requires them to manage. The gap between what the standard covers and what’s actually being monitored is where incidents happen.
This guide breaks down which water systems are at risk, what ASHRAE 188 actually requires you to document, and what facility managers most frequently get wrong when auditing their programs.
ASHRAE 188 is the industry standard governing Legionella risk management for building water systems. It requires facility managers to inventory every water system in a building that could amplify or disperse Legionella pneumophila — and to include each one in a written Water Management Plan (WMP) with documented control measures, monitoring protocols, and corrective action procedures.
The standard covers any system where water can stagnate, warm to temperatures between 77°F and 113°F (the Legionella growth range), and create aerosols that could be inhaled. That definition applies to far more than just cooling towers.
The following systems are commonly identified as high-risk under ASHRAE 188 and should be included in every facility’s WMP:
| System | Risk Level | Why It Matters |
|---|---|---|
| Cooling towers | High | High volume, aerosol-generating, warm water — primary ASHRAE 188 target |
| Hot water storage tanks | High | Water at 110–120°F is in the Legionella amplification range; low flow worsens risk |
| Hot water distribution piping | High | Dead-end branches and low-use fixtures allow stagnation and temperature drop |
| Decorative water features | Medium-High | Aerosol-generating, recirculating, often minimal disinfection |
| Eyewash stations | Medium | Low use, stagnant water, no thermal disinfection — frequently missing from WMPs |
| Ice machines | Medium | Internal reservoirs can harbor Legionella if water sits and cleaning lags |
| Emergency safety showers | Medium | Infrequent use means water sits for weeks; temperature varies widely |
| Low-use restroom/break room fixtures | Low-Medium | Especially a risk in partial-occupancy or seasonal-use buildings |
Most facility water management plans are well-developed for cooling towers and deficient everywhere else. When we walk a facility’s water systems, the pattern is consistent: the tower program has documented control limits, testing records, and chemistry reports. The hot water distribution system has a maintenance schedule that hasn’t been updated in two years. The lobby fountain hasn’t been tested since installation.
If your hot water distribution system or non-tower fixtures aren't in your current water management plan, a free on-site system walk is the fastest way to see what's missing.
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Legionella pneumophila multiplies most rapidly between 77°F and 113°F. Hot water storage tanks operating at 110–120°F sit at the edge of that range — warm enough for amplification when temperatures fluctuate, cool enough that bacteria survive in protected biofilm sites even when bulk water temperatures are slightly higher.
Distribution piping compounds the risk. Long horizontal runs, dead-end branches, and low-flow fixtures allow water to cool as it moves away from the heat source. A fixture at the end of a 200-foot run in a wing that sees minimal daily use may see water temperatures in the 90–100°F range — squarely in the growth zone — without anyone monitoring it.
Per the CDC, approximately 70% of Legionnaires’ disease cases are associated with building water systems other than cooling towers. Healthcare facilities, hotels, office buildings, and manufacturing plants with complex plumbing all carry this exposure if non-tower systems aren’t included in the water management plan.
ASHRAE 188 requires a WMP to include: a written description of the water system (with flow diagrams), identification of all water use points and hazardous conditions, specific control measures for each hazardous condition, monitoring parameters and control limits, corrective action procedures when control limits are exceeded, verification protocols, and a documentation and recordkeeping system.
The most common WMP failures we see in the field aren’t missing sections — they’re sections that exist on paper but aren’t being actively maintained. Control limits that haven’t been revisited since the plan was first written. Sampling records that show the tower was tested but nothing else. Plans that were last updated before a building renovation that changed the water system.
A plan audited by a regulator, an insurer, or a Joint Commission surveyor needs to demonstrate active compliance — not just existence. That means current test results for all covered systems, documented corrective actions when readings were out of range, and evidence the plan was updated when system conditions changed.
If your documentation, sampling records, and corrective action logs wouldn't hold up to a regulator today, this is the moment to address that gap — not after an investigation forces the conversation.
Schedule Your Free Water Analysis →June through August represents peak Legionella risk for building water systems. Warmer ambient temperatures push hot water distribution temperatures into the amplification range more easily and make it harder to maintain cold-side temperatures below 68°F. Low-flow and low-use fixtures that weren’t a concern in winter become significantly higher-risk as baseline water temperatures climb throughout the building.
Cooling towers are also carrying their highest biological load during this period — higher evaporation rates concentrate both dissolved and suspended material faster, and biocide demand increases with temperature and system load. A biocide program adequate in April may not provide sufficient residual in July.
Legionella treatment services aren’t one-size-fits-all. The right control approach for a hot water distribution system is different from what applies to a cooling tower or a decorative feature. Water management plans developed without accounting for the full system inventory can’t be adjusted correctly for seasonal conditions — because the baseline was wrong to begin with.
ChemREADY offers a free on-site water analysis — we'll walk your system, test your water, and give you a plain-English report on what we find. No obligation.
Schedule Your Free Water Analysis →800-229-6801
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