Legionella in water tanks

Question

A house has been empty for several months and the taps not run. What is the risk of Legionella bacteria growing in the hot and cold water tanks. How/who would test it and if found how is it treated to make it safe

Josh · Accepted Answer

For a house that has just been empty, testing usually isn’t required.

Standard recommissioning procedure:

1. Flush the system

Run every outlet:
	•	Hot taps
	•	Cold taps
	•	Showers
	•	Outside taps

For at least 5 minutes each.

⸻

2. Clean shower heads

Remove and descale.

Soak in disinfectant.

⸻

3. Heat disinfect the cylinder

Raise hot water temperature:
	•	60–65°C minimum
	•	Run hot taps after heating.

Legionella dies rapidly above 60°C.

⸻

4. Inspect the cold water tank

Check for:
	•	Dead animals
	•	Sludge
	•	Broken lid
	•	Poor insulation

If dirty:
	•	Drain
	•	Clean
	•	Disinfect

John · Answer

All taps and showers should be run for 15-20 mins. First at a very low flow before turning it up full. Opening the windows will also help disperse any aerosols.

Owen · Answer

The chances are high. What to do?  is drain the cold system out. Turn the hot water on to sixty five degrees let it get to temp and then flush out the hot water system

Dan · Answer

You can get tester kits to check if your water has bacteria, if the hot water reset is 65° then it'll immediately kill the bacteria but we would always advise to drain the tank of any buildup
Also something to consider is your cold water tank should have a lid on top of it. It stops any dust and Debrie getting inside the tank.

You can clean these out by taking off the water emptying the tank and then just using disinfectant to clean any Build up of scale

Jordan · Answer

The chances are extremely high. Turn the hot water on to sixty five degrees let it get to temp and then flush out the hot water system, you could also have a professional treat the cold water tank with chlorine tablets then neutralise it then flush the system

Mikey · Answer

I wouldn’t worry too much but I would check the water tank for any clear problem signs then I would make sure the hot water is on and at 65’ then would run all outlets for around 20mins to ensure majority of water has been replaced by new treated mains water

Legionella in water tanks

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