May I throw in two other factors?

1. Biology: The bacteria have to come from somewhere in the first place. Historically the culprit was a header-tank in the loft which supplies the DHW cylinder.

In these days of having DHW at mains pressure, the opportunity for Legionella pneumophila to enter the system is much reduced. Modern likely sources are hot-tubs.

Even so, let's note that the pneumonia which affects humans comes from breathing in water droplets, rather than through drinking the water.

This is perhaps why a Legionella infection catches us by surprise. You could use a shower for years without getting pneumonia even tho' there's Legionella in the water. And then one day you clean out the shower tray without first soaking the cloth in disinfectant and you breath in the bacteria which gets wafted into the air.

Perhaps the worst possible source of Legionella is the water which comes from flushing out an UFH pipe network, or getting close to the air-relief valve at the end of the manifold.

2. Stagnation: Those of us with thermal stores are less likely to provide a breeding ground for Legionella pneumophila.

The coil which picks up the heat energy for DHW has low volume and the water in it is frequently being replaced.

To have sufficient bacteria present for infection to occur requires them to replicate in water being held at 35°C - 45°C for long enough.
That occurs in a DHW cylinder [left side of diagram] where there is a larger volume of water which isn't getting used.

The human body is pretty good at fighting off small numbers of bacteria. We are susceptible to pneumonia only when the natural defenses are overwhelmed.

Heat Geek have a good article on this. I normally heat the DHW to only 42degC, but we have a high turnover. It gets plenty of extra temperature in these sunnier months as we make use of the abundant energy from the solar PV.

My take from the above is; If the DHW is never stored below 45c the a LHC to 60c is unnecessary?

So, is it cheaper to maintain a temp’ above 45c and ignore a LHC or lower and run a regular LHC?  Interesting. 

Perhaps I should also observe that bacteria can be eradicated by UV-C 'light', but I've not heard of that approach being used against Legionella.

UV radiation of 265nm wavelength is usually produced using a fluorescent tube, but you can now buy LED emitters at that wavelength. I have one of each type in my house, where they're used in the cold water supplies from my well.

UV-C destroys the RNA within the bacterial cell, which stops it reproducing.
DNA in protozoa is also denatured by UV-C, so that reduces any risk of amoebic dysentery too.

I'm only mentioning it here in case someone else comes up with a possible defence against Legionella.
At the moment I can't think how that radiation could be usefully deployed into DHW.

Posted by: @steve81uk

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Heat Geek have a good article on this. I normally heat the DHW to only 42degC, but we have a high turnover. It gets plenty of extra temperature in these sunnier months as we make use of the abundant energy from the solar PV.

This is from that article and i found it useful:

@transparent I believe D Trump had a cunning plan to fight a recent pandemic using fluorescent tubes 😀 

My take is I run my anti-Legionella cycle on Monday afternoons ending at 5pm, and the temperature probe (at the bottom end of the HW cylinder) says 58.5C; it's still above 55C at 2am on Tuesday, which ought to be ample for killing them all off. After that, I don't see how it would re-enter the system, but for 2 hours a week, it's peace of mind.

@mikefl

I do believe that D Trump also suggested injecting disinfectant. 🙄 

It is unfortunate that D Trump doesn't take advantage of discussion forums such as this one to allow such notions to be subjected to 'peer review'!

And I regret the awkward position in which he placed Dr Deborah Birx, White House coronavirus advisor, who was sitting close by. The press/media were less likely to follow her advice after that incident in which she was undermined.

Are we not in a similar position regarding the wider issues of energy strategy in this country?

This forum is widely populated with tales of incorrect installation and commissioning of heat-pumps.
Whilst most homeowners will notice inefficiencies and high bills, I doubt that they'd be aware of a failure to implement anti-legionella procedures.
How long will it be before we hear of the first case of pneumonia as a result?
... and what effect will that have on the uptake of heat-pumps to achieve net zero?!

Do you think MCS even has a suitably experienced scientific advisor on its staff to forewarn them of a possible legionella infection?

I've ditched the weekly 1hr Legionella cycle using the 3kW immersion heater fitted to a Grant 250L pre-plumbed cylinder. I was finding it ineffective due to the location of the immersion and sensitivity of the in-built immersion control thermostat. The tank was never recording a higher temperature than 55degC and the immersion was just cycling on/off. It was creating a small pocket of hot water, but not effectively heating the cylinder contents.

I've proved this by noting how much quicker the DHW ran cool when heated with the 3KW immersion heater compared to the ASHP primary coil. The ASHP provides a much more thorough heating. *

Instead I have a daily ASHP DHW generation to 55degC. The ASHP happily supplies water in the primary side at 60degC to facilitate this. The tank thermostat is set to 55degC. The primary coil sits lower down in the cylinder and has a much greater surface area than the immersion heater element. It does a much better job of heating the cylinder up to 55degC. The DHW charge time using the ASHP is a maximum of 75 minutes scheduled at 1pm each day. The cylinder contents sit at 55degC for several hours in the afternoon, during which time almost no hot water is used, and that's sufficient a Legionella safeguard. I've not noticed any increase in running costs having the DHW set at 55degC than, say, 50degC. The standing losses from well insulated cylinder tanks are very low - the tank loses about a degree of indicated storage temperature every 3 hours or so. Crucially, her who shall be obeyed wants hot water at 50degC for pot washing!

* Most domestic small tanks use stratification intentionally - drawing off hot water from the top and replacing with cold water at the bottom. Gradually, the % of cold water in the tank increases as hot water is drawn off. The boundary layer between hot and cold contents will always sit around the tepid 'Legionella risk' temperature until the cylinder is recharged and reheated. There is no mixing and blending of incoming cold water with hot water, which is much more common with industrial calorifiers and hot water systems. Smaller DHW cylinders have their volumetric content quite quickly replenished based on an average daily consumption of 35-40 litres per occupant.

Posted by: @allyfish

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The boundary layer between hot and cold contents will always sit around the tepid 'Legionella risk' temperature [...]

There is no mixing and blending of incoming cold water with hot water...

I had suspected that might be the case.

I've just notified my County Councillor about the risks of legionella with the lower temperatures which occur with heat-pumps.
It took her less than 5-mins to forward my email to Environmental Health (Borough Council) and Public Health (County Council) officers.

I suspect that the wider population with heat-pumps are remaining 'protected' from legionaires disease because the bacteria aren't finding their way into the DHW cylinder in the first place, rather than because installers have ensured that an effective anti-legionella process is operational.

That could be readily verified.
Here's the relevant section of the Building Regs, Approved Documents Part-L:

A householder merely needs to look at the Commissioning Plan for their house...
... run their finger down the list until they arrive at the section on Anti-Legionella
... read what the system test-sequence involved
... and check that it's been ticked as 'completed'.

What's the chances of that having occurred? 🤔 

Two further points:

1: Legionella  is a Notifiable Disease to UK Health Security Agency. Any infection is subject to an investigation which must identify the source.

I've posed the question as to why a potential risk isn't notifiable.
Prevention would seem to be the better option!

2: The DESNZ Public Consultation on Delivering a smart and secure electricity system (closing date is tomorrow 21st June) contains a number of questions in Section-1 on their proposals to include Heat-Pumps within the corpus of devices subject to DSR controls (Demand Side Response).

In short that means they intend to legislate to allow Service Providers to switch off your heat-pump during times of high grid demand. This would work in similar fashion to smart EV chargers being controlled by your electricity supplier.

That comes with the risk that your heat-pump might've been switched off during a period when the anti-legionella cycle would normally have run. It also means that the DHW tank temperature is more likely to have dropped to a temperature where growth of legionella bacteria would be increased.

If that were so, how would a homeowner know that the anti-legionella cycle hadn't occurred?
Is it reported somewhere?
Can a heat-pump be requested to report the last time the cycle had run?

Given the dearth of site-specific information and instruction given to homeowners, how many are likely to know how to start an anti-legionella cycle manually, even if they realised that it hadn't occurred to a programmed schedule?