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We have a 30 gal tanked heater and dual knob faucet (no anti scald). We use it once a week. It is on a hand crank 12 hour timer, set it, get 12hrs of hot water.

The thermostat is set low enough to not scald (110-120F) but a good temperature to grow legionella. We do not want legionella.

Noting that tankless heaters can't grow legionella because their water is room tempersture most of the time... does the same apply to us?

Can legionella develop if the tank is room temperature 6 days out of 7?

  • Is this an electric or gas fired tank? – Jim Stewart Oct 24 '17 at 1:29
  • When you leave for the week do you replace the hot water in the tank with cold so that the tank is at RT all week? I thought Legionella wouldn't grow at normal RT. – Jim Stewart Oct 24 '17 at 1:44
  • Is this dual knob valve in the shower? – Jim Stewart Oct 24 '17 at 2:01
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    Electric, it is not purged, just the normal cooldown, it cools in about 24 hours, and there is no shower. @JimStewart – Harper - Reinstate Monica Oct 24 '17 at 5:47
  • What is the temperature of the inlet water? Is the water purified, chlorinated, otherwise treated? Is there a tub for bathing? What is the hot water used for ? – Jim Stewart Oct 24 '17 at 15:45
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In Australia tanks must be set to bring the water to at least 60 C (140 F) and so it seems that thermostatic mixing valves at the output of the heater are promoted there to lower the temperature in the distribution lines to 50 C (122 F) or a little less. You might consider such a mixing valve on the output of the tank.

Legionella bacteria can be present in water and become a health concern when present in high concentrations. Water should be heated and stored above 60 C for at least 35 minutes to ensure Legionella bacteria are killed. http://www.yourhome.gov.au/energy/hot-water-service

I wonder if Australia goes overboard on regulations? For 20 years in Dallas before the Legionella regulations were out we kept our 40 gal NG fired tanks at ~120 F, and nobody got Legionella (that I know of).

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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.
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