What's the consensus on legionella and dhw storage temps

[SimonD]

Interesting to see there's been a bit of a change to the narrative about this, especially in the last few months, but I've been surprised to hear a number of people suggest that people can turn down their dhw cylinders to 'just above 50.' I've even seen a vid from Heat Geek and a couple of others on youtube making this suggestion. Now, I know that over 50 kills the bacteria over a few hours but always seen the plus 60 recommendation from the like of HSE etc.

 

What's the consensus here, if there even is one? Would you be happy to keep your dhw at just above 50 without a regular 60+ cycle?

 

 

[joe90]

Yes, from many threads on this forum it’s been established that there have been no deaths from legionella in a domestic situation in this country, I keep my DHW at 48’ purely as that is most efficient fir my ASHP and hot enough for every use within the house. I have a larger DHW tank as I have to blend less for showers and baths. I believe others here do the same.

[TonyT]

If you are in town water no issue as it’s chlorinated and hopefully the system has been designed with no dead legs, has good turnover.

in my opinion it’s not a risk in a domestic situation.

 

if it’s a business well you have to comply with L8 regulations 

 

I look after legionella at my place of work

[SteamyTea]

Don't think my cylinder has been over 50°C for over 15 years.

 

Just having the bacteria in the system is not enough to cause the illness, the water needs to be atomised and then breathed in by a vulnerable person.

 

Showers do not atomise the water. 

[nod]

I think there is more chance of a loose brick dropping on your head and killing you than catching Legionnaires 

[Radian]

Doesn't it depend on vented vs unvented systems? There can be all sorts of things going on in water storage tanks in the loft. The thing that promotes bacteria growth is slowly changing temperature gradients - particularly ones that pass through human body temperature. Like your fridge is full of bacteria but they only thrive at low temps, a long way from body temperature. And that rough 30 degree delta is held similarly for hot water storage at the recommended over 60 degrees.

[Conor]

I absolutely would for a vented system. Ever since finding a dead bird in dad's attic tank anyway. Chlorine has a finite life and gets used up. A few animal remains and a few days brewing, you've potential issue, not just legionnaires.

 

Unvented, no point at all.

[SteamyTea]

17 minutes ago, Radian said:

Doesn't it depend on vented vs unvented systems?

I have always had vented systems except in one house.

 

 

[MJNewton]

Heat Geek did a video on this recently which may be of interest/help:

 

https://youtu.be/oJeyc_cGIMU

 

(Sorry SimonD, just reread your post and see you’ve seen this already!)

Edited October 23, 2022 by MJNewton

[Radian]

I'm dredging this topic up again due to recent revelations about the amount of stratification in my own open-vent DHW cylinder.

 

On 23/10/2022 at 14:37, SteamyTea said:

I have always had vented systems except in one house.

 

 

It's possible that the majority of UK households still have vented systems. Regardless, they're common enough to consider the lack of reports about lethal infection as an indication that this is a non-issue. However, I know understand that the nature of Legionella infections are on a spectrum with the majority of exposures only resulting in non fatal flu-like symptoms. Maybe there is an issue after all?

 

To quote hse.gov.uk:

Quote

People contract  Legionnaires' disease by inhaling small droplets of water (aerosols), suspended in the air, containing the bacteria. Certain conditions increase the risk from legionella if:

the water temperature in all or some parts of the system may be between 20-45 °C, which is suitable for growth

 

Likewise, most sources state that water temperatures should either be below 20^oC at cold water outlets or higher than 50^oC at hot water outlets. But I can identify plenty of situations where this isn't going to be the case. Last summer for instance, the cold water direct from the rising main here was 24^oC around the days we hit 40^oC, Freak event for sure, but maybe to be repeated much more often.

 

But what does occur on a regular basis is the stratification of water temperature in a DHW cylinder. I'm left pondering the 80 liters of water that regularly sits at between 20^oC and 40^oC in the bottom half of my cylinder. Sure it gets heated to over 55^oC and collects in the top half from where it's drawn - until the second long shower in succession runs cold. And what about the pipework between the mixing valve and hot tap in the bathroom - this transports and holds water at no more than 40^oC by design.

 

Clearly there are numerous cases where the recommended temperatures are not achieved in practice and I wonder if this might give rise to less serious health issues that don't make the headlines. Mysterious cold-like symptoms that could be mistaken for hay fever perhaps?

[SteamyTea]

15 minutes ago, Radian said:

Clearly there are numerous cases where the recommended temperatures are not achieved in practice and I wonder if this might give rise to less serious health issues that don't make the headlines. Mysterious cold-like symptoms that could be mistaken for hay fever perhaps

You need to atomise the infected water as well.

A shower does not atomise like a cooling system.

[Radian]

12 hours ago, SteamyTea said:

A shower does not atomise like a cooling system.

I really do find that hard to believe. What's that mist of tiny water droplets I see in the beam of the shower light/fan as they're drawn in then? The point about atomisation is that being breathed in - this being a pulmonary condition.

[TonyT]

Give the shower head a clean with shower head plus  every 3 months or similar to kill anything in this area, personally I would do a risk assessment on the property if you had any concerns 

[Temp]

Our system has a thermal store that you can dose with chemicals. The DHW is heated by a heat exchanger. So none of the water that comes out of taps is stored for very long.

[SteamyTea]

9 minutes ago, Radian said:

What's that mist of tiny water droplets I see in the beam of the shower light/fan as they're drawn in then

Just that, droplets.

I cannot think of one case of a legionnaire outbreak that has not been caused by a cooling system, and a badly neglected cooling system at that.

There was an outbreak in Aylesbury 20+years ago. Unmaintained A/C unit on the shopping centre.

I used to stand by it during fag breaks.

[Radian]

1 hour ago, TonyT said:

Give the shower head a clean with shower head plus  every 3 months or similar to kill anything in this area, personally I would do a risk assessment on the property if you had any concerns 

 

In addition to cleaning I actually replace them about once a year as the plastics degrade and become difficult to get properly clean. They're cheap enough on Amazon! An informal risk assessment is really what I'm talking about now.

 

1 hour ago, SteamyTea said:

Just that, droplets.

 

 

But if they're small enough to be suspended in the air, then they're breathable. I can see how I might be coming across as a unnecessary worrier, however it's more like I can see lot's of dots that don't seem to be joined up.

 

1 hour ago, SteamyTea said:

I cannot think of one case of a legionnaire outbreak that has not been caused by a cooling system, and a badly neglected cooling system at that.

 

 

I know there are (thankfully) very few reported cases of outbreaks - what I refer to as headline cases. But you may have noticed that I'm talking about lower level health issues that may be going unreported.

 

What I'm making little headway with is the mechanics of the issue. The recommendations are clear enough but don't address the technicalities. Just recommending temperatures in excess of 60^oC doesn't acknowledge the reality of stratification for example. And avoiding 'dead legs' isn't really referencing distant hot taps that have generally not reached full temperature by the time they're turned off. Or been fed by mixer valves to restrict their maximum temperature.

 

In another recent topic, an MCS compliance document was posted up regarding ASHP installations that had some interesting points about Legionella, revealing that "very little specific advice is available for domestic applications"

 

"ACoP L8 is supported by HSG274 Part 2 which cites temperature as the most common method of Legionella control: “It is recommended that hot water should be stored at 60°C and distributed so that it reaches a temperature of 50°C within one minute at outlets.” However, this guidance is aimed primarily at commercial hot water systems with either secondary returns (a pumped circulation pipe loop returning hot water to the cylinder for continuous reheating), or trace heating (an electrical resistance tape attached to the pipe where reheat is achieved by current flow in response to change in electrical resistance due to any drop in temperature). It therefore should be noted that all HSG & ACoP’s refer to non-domestic situations and very little specific advice is available for domestic applications."

 

IMO the conclusions were fluffed stating that "between ⅓ and ¼ of the sites examined show regular patterns of immersion." which was used as a proxy to demonstrate that anti-Legionella measures had been implemented. It did not monitor actual DHW storage temperatures nor make clear those systems that achieved high temperature flows. 

[SteamyTea]

16 minutes ago, Radian said:

I can see how I might be coming across as a unnecessary worrier, however it's more like I can see lot's of dots that don't seem to be joined up.

Maybe the dots don't join up.

A negative result is just a valid as a positive one, more o if you are looking for a positive one.

[Radian]

From the report on MCS compliance page 51, my emphasis in bold:

 

"Sterilisation or pasteurisation is achieved by raising the cylinder water to over 60°C. HSG274 (which refers to commercial installations) states that: “Arrangements should therefore be made to heat the whole water content of the calorifier, including that at the base, to a temperature of 60°C for one hour each day.” However, a review of heat pump manufacturers’ literature identifies varying guidance on the sterilisation process with temperatures ranging from 65°C to 73°C for between 30 and 90 minutes either weekly or fortnightly."

 

It refers to HSG274 which is directed towards commercial installations and seems to be the only in-depth HSE guidance available. Then its goes on to show a diagram with a fairly typical looking immersion mounted in the top of a cylinder:

 

 

 

Even if scheduled to heat at 60^oC for one hour each day, this will fail to heat the water below the immersion without additional destratification measures. I appreciate that most new DHW cylinders installed alongside ASHP's will be unventilated types probably with bottom mounted immersions but with the widespread installation of top mounted units, it's clear that in many households it's been common practice to only partially heat the water when relying solely on the immersion to achieve the temperatures required for sterilisation.

 

The lack of clarity in all this continues to bug me. I'm trying to emulate switching to an ASHP by modulating down my gas boiler but don't want to do something stupidly dangerous in the process. The plan was to use my solar PV diverter to heat the DHW via an immersion - hopefully exclusively so during the summer. I had already successfully reduced the flow temperature from the boiler to 55^oC for radiators but all but given up on heating the HW cylinder with anything less than 70^oC. I was hoping to eliminate this with the immersion but now I'm unsure. Obviously I can play it safe and run the boiler like this once a day but it's tempting not to. Hence wanting to get a thorough understanding of the issues.

[markocosic]

The shower valve and pipe are purged at relatively high velocities and scoured of boifilms.

 

Cylinders are not. 

 

60C heating (at the 1/3rd mark) is for quickly pasteurising the gunky water at the bottom of the cylinder.

 

It's never 60C at the base of old cylinders. Guidance that refers to this is bunk. Impossible in practice for most designs. It's more like setpoint 60C using a reheat stat 1/3rd of the way up.

 

New unvented cylinder with a big coil that runs to the base is fine at 50C.

 

Old junk ought to get junked along with the boiler and replaced with something clean with an appropriate coil in it and low standing losses etc IMO.

 

Cylinder replacement ahead of boiler replacement is not problematic.

[Marvin]

Our ASHP is set at 45°C. The PV diverter is set at 75°C at the moment. I ensure that the temp goes up to 60 every few weeks in the winter by overriding the diverter.

[dpmiller]

how long does it take the protective chlorine in mains water to dissipate?

[Conor]

33 minutes ago, dpmiller said:

how long does it take the protective chlorine in mains water to dissipate?

There's a cascade, chlorine breakdowns / reacts to another form, it's effective for a period of time, then there is another step. All in, I think five stages of decay, exponential and highly dependent on temperature and organic content in the water. We prefer water to get to the customer with 24hrs, any longer and you can't guarantee that there's enough active chlorine left to be effective. But in reality 48-72hours is not unusual.

[ProDave]

I thought we discussed before, the treated chlorinated mains water just means you can be pretty sure there is nothing nasty entering your HW tank.  Assuming it is an unvented cylinder then nothing else can enter, so once the clean water is there, it does not matter if the effect of the chlorine diminishes. It has no further function.

[Radian]

16 hours ago, markocosic said:

Old junk ought to get junked along with the boiler and replaced with something clean with an appropriate coil in it and low standing losses etc IMO.

 

 

I'm talking about a vented stainless steel cylinder with 23kW coil that I installed two years ago. I considered getting an unvented cylinder at the time but didn't think it was a DIY option. Still not sure if it is or not.

 

1 hour ago, ProDave said:

I thought we discussed before, the treated chlorinated mains water just means you can be pretty sure there is nothing nasty entering your HW tank.  Assuming it is an unvented cylinder then nothing else can enter, so once the clean water is there, it does not matter if the effect of the chlorine diminishes. It has no further function.

 

I'd agree with that but still do a regular sterilisation program to be sure. Maybe once a fortnight.

 

Cold storage tanks in lofts for vented cylinders like mine are just looking more and more of a liability though. Mine were even doubled-up on the recommendation of the plumber because we had power showers installed when the house was built so goodness knows how stale the water gets. And the loft regularly gets heat soaked in the summer. No amount of lagging will keep all the water at ground temperature (which I said above can be in the mid-twenties then).

 

There are also hot taps like the one in our downstairs loo that barely run warm by the time you've finished washing your hands (sentinel taps in the HSE doc - used for worst case measurements). Realistically these are going to be found in large numbers of homes and rarely ever see sterilisation levels of temperature. Perhaps this is mitigated by the fact that the drawn off water at the start of the pipe-run does remain at high temperature, along with any other T'd in outlets that might introduce greater high temperature flows. So the water coming out for the next use is sterile. HSE certainly wouldn't countenance this but it seems to work at an informal level. We seem to get away with all this. But it's all a bit vague. 

[markocosic]

Taps are not really a risk.

 

Only the shower can produce aerosols.

 

Cold water from tanks in summer is riskier then hot water.

 

People don't die.

 

If your tank is new (sorry haven't kept track) then fire away heating to 50C.

 

Just don't breathe in any lukewarm water from the cold pipework.