Free. At the start of letting; whenever there are changes to your water installation.
What, Legionella? Yup, you’ve got to do a risk assessment on that one, too, even if you have no clue what Legionella is. It’s actually a bacterium found in water that, if inhaled in a fine spray, can cause Legionnaires’ Disease – a potentially fatal form of pneumonia.
It breeds in standing water at 20–45C, so if you have water tanks, the cold one should be kept at under 20C. The hot tank should distribute at 50C and heat to 60C for one hour a day – killing the critters. The tanks’ lids should also close tightly so no adventurous animals can get in. A combi-boiler means you’re probably safe.
Also ensure there are no old pipes to disused appliances where water can stagnate, or attached hosepipes that can backwash into the house’s pipework. Let all the taps run a bit before letting out the property after it’s been empty for a while. Tell tenants not to adjust the water temperatures. You can get your clipboard out and write down that you’ve checked all this – there’s no need to pay an expert for tests (hse.gov.uk/legionnaires).