Can legionella develop if the tank is room temperature 6 days out of 7?
In brief, yes.
- If "room temperature" is >= 68F (20C)
- If the heater is set to less than 140F (60C).
In the UK, the Health and Safety Executive (HSE) issue the following guidance:
The bacteria multiply where temperatures are between 20-45°C and nutrients are available
The bacteria are dormant below 20°C
The primary method used to control the risk from Legionella is water temperature control.
Water services should be operated at temperatures that prevent Legionella growth:
- Hot water storage cylinders (calorifiers) should store water at 60°C or higher
- Hot water should be distributed at 50°C or higher (thermostatic mixer valves need to be fitted as close as possible to outlets, where a scald risk is identified).
- Cold water should be stored and distributed below 20°C.
Stagnant water favours Legionella growth. To reduce the risk you should remove dead legs/dead ends in pipe-work, flush out infrequently used outlets (including showerheads and taps) at least weekly and clean and de-scale shower heads and hoses at least quarterly. Cold-water storage tanks should be cleaned periodically and water should be drained from hot water cylinders to check for debris or signs of corrosion.
So, according to this guidance
- If Legionella are present, at less than 68F (20C) they are just dormant, not killed. They can multiply when the water is warmed up (anything above 68F). So if the temperature is never high enough to kill them, they might multiply in weekly stages over a period of months?
- Your tank temperature is too low, it needs to be at least 140F (60C)
- After a week of no use, you should probably flush water away at the faucets until the temperature at the faucet (or mixer inlet) is at least 120F (50C)
- To avoid scalding fit a thermostatic mixer valve before the faucet.