Lower boiler/water temp and legionella - how much of a concern?

Discussion in 'Getting Started FAQ' started by noidea678, Jan 29, 2023.

  1. noidea678

    noidea678 New Member

    Hi everyone,

    I have a standard condensing boiler which was installed maybe 6-7 years ago and heats a hot water tank.

    The boiler has always been set to the "max" temperature (it doesn't have a screen showing temperature so I don't know what that is but it's a Baxi Solo 15 HE and apparently the "flow temperature" which I think is what the dial on the boiler adjusts, is 55-78C

    However, A few years ago I set the hot water thermostat to be around 40-45C, high enough for a shower or doing washing up but not high enough that (I thought) it wastes unnecessary energy or I risk being scalded by it. I also have it set to come on for like 30 minutes in the morning, 30 minutes in the evening and 10 minutes at night before bed.

    Yesterday I was looking to see if I could turn the temperature down, since I thought it might save some costs (even by trying to be conservative with the heating, it's costing almost £10 per day (£2 from elec, £8 from gas). However when looking online to see if I should adjust it, I read about Legionella, and realised that this setup has probably been the perfect breeding ground for years.

    Apparently the water should be set at 65C, and if it's lower than that (say 50C) it takes several hours to kill Legionella, so having the boiler heat up the water tank a couple times a day for 30 minutes at a time max and being around 30-40C this is nowhere near what they recommend and seems like it's dangerous?

    I'm not saying I don't believe it, or that "I haven't noticed any issues in the last few years so nothing will ever happen" but is it possible that because the water is being used often and the tank gets emptied that explains why I've been fine?

    Have I been OK because the boiler temp was set at max? I did read an article ( here ) which did say that an electric heating setup should be set to 60C, but a gas heating set up can be set to 49C. Not sure why though, I assumed the heating would be the same regardless of type?

    Anyway, should I lower my boiler temp to say "medium" so it's more effective and then increase the water temp to 50C? Or higher? And should I have the hot water stay on much longer?

    Thanks in advance
  2. AnotherTopJob

    AnotherTopJob Screwfix Select

  3. The Happy Builder

    The Happy Builder Screwfix Select

    There’s a thermostat actually on the hot water cylinder to control the temperature of the hot water, if the boiler flow is set at a lower temperature than the cylinder temperature the boiler will just keep cycling on an off as the target temperature will never be reached.

    So bearing that in mind, what temperatures have you set on thermostats and where are they?
  4. noidea678

    noidea678 New Member

    Thanks, that was an interesting (and re-assuring read)

    The boiler was set to max. It doesn't display a temp.

    The hot water cylinder thermostat was set to like 40-45C. Just checked about 30 mins ago and it was ~37C (hot water has been off since this morning).

    I don't really want to leave the hot water on all day to keep topping it up as it will cost a fortune, so I was wondering if I actually need to do that. Seems like from the link above it's unlikely that I do though
  5. Alan22

    Alan22 Active Member

    In the summer I turn my temperature down to below the figures mentioned, showered every day of my life and still alive, is there not chlorine or something in the water to prevent legionella?
  6. The Happy Builder

    The Happy Builder Screwfix Select

    How many people live in the house,?
  7. The Happy Builder

    The Happy Builder Screwfix Select

    One possible option is to increase the temperature of the water in the cylinder to above 60 degrees and move cylinder stat up to heat less, then allow the shower to blend the water back down to user temperature, which will also probably give you a better shower.
  8. noidea678

    noidea678 New Member

    Just me, but I get at least one 10-15M shower every morning and work from home so constantly using the water all day so I think I go through a full tank easily before the water becomes stagnant

    Do you mean literally move the cylinder stat up? Like the thing the thermostat is connected to nearer the top of the tank?
  9. Bazza-spark

    Bazza-spark Screwfix Select

    Given that turnover of water you have no worries.

    Legionella loves the sort of temperatures you are operating at, but the water will need to be stagnant for days if not weeks for it to breed. Once you have it, it's difficult to shift without proper chemical use. Pipe deadlegs are also a big source.

    If you are really concerned, I used DTK Water TTC/TTC dipslides in our factory to monitor our water systems, 1 dipslice per point once a month. You will need to incubate the dipslide for 48 hours, but instructions are supplied with each box of 10 slides.


    Chlorinated water is pretty resilient to infection, but over time the chlorine will evaporate off. I monitored 26 points a month for abour 4 years and only a couple caused us problems, usually the ones that had metal cutting coolant in as they just recycled through the machine.

    In a domestic situation it is very rare to have an issue.
  10. noidea678

    noidea678 New Member

    Great, thanks for that :)

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