Designing cold water to stay below the 20°C to 45°C Legionella growth band.
Keeping cold water cold
Legionella can multiply in water roughly between 20°C and 45°C when nutrients and other suitable conditions are present. This range does not mean growth is equally fast throughout it. The control aim for cold water is to keep it below 20°C wherever practical, so it stays out of that band. Below about 20°C the bacteria are generally dormant. Thermal disinfection needs sufficient temperature and exposure time; neither a cold reading nor a brief hot reading proves sterilisation.
Where cold water warms up
Cold water often fails this aim not at the cistern but on the way to the tap. Common causes:
- Cold pipes routed close to or alongside hot pipes, or through warm voids and risers.
- Storage cisterns sited in warm roof spaces.
- Long dead legs and infrequently used branches where water stands and warms.
Design to avoid these: separate and insulate cold pipes from heat sources, keep storage cool and turning over, and remove unnecessary pipe lengths.
Materials and temperature
Materials selection links to temperature too. Choose materials rated for the pressures and any temperatures they may meet, suitable for contact with wholesome water, and unlikely to taint it. This matters where a cold pipe could be warmed in service.
This is safety-critical: confirm temperatures and control measures against HSE guidance, the L8 Approved Code of Practice and HSG274, and BS 8558.
Legal limits and cistern protection
- For cold tap supplies, route and protect the pipe so the water does not warm above 25°C wherever reasonably practicable under the Water Fittings Regulations. HSE's legionella control aim is lower: below 20°C.
- Every storage cistern needs an overflow with a warning that excludes insects, a cover that excludes light and insects, and insulation against freezing and undue warming. In practice that means a rigid, close-fitting lid that is not airtight, with suitable screened openings so the cistern can breathe. Overflow and effective warning are required functions; accepted combined pipes or alternative warning arrangements depend on the cistern and installation.
- Pipe connections are placed so water circulates freely through the cistern rather than leaving stagnant areas, and the cistern stays accessible for inspection and cleaning.
- Pipes and cisterns in unheated spaces such as lofts are insulated and kept away from cold external walls and eaves, to reduce freezing risk. Insulation delays heat loss but does not guarantee protection through prolonged exposure to freezing conditions.
- Short, direct runs kept away from heat leave less water standing and less time for it to warm.