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Hot water tanks, Temperature and Legionella


ProDave
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I have a 300 litre unvented water tank. Heated primarily from an air source heat pump but later we intend to install solar PV with excess power heating the tank via the immersion heater.

 

The reason for the large tank is to enable hot water to routinely be stored at the lowest usable temperature as heat pumps are not good at heating water very hot.

 

The sweet spot I have arrived at is a target water temperature of 48 degrees.  The heat pump has a built in hysteresis so does not start to re heat until the temperature drops 5 degrees, so in my case, it turns on again at 43 degrees.  At that temperature, the water is just hot enough for the hottest task of dishwashing, if you run hot water only with no cold added, so I cannot reduce the hot water temperature any lower.

 

So this is all working well, should not give the heat pump a problem, minimises standing losses, and still allows plenty of capacity later on for excess solar PV to heat the water a lot more.

 

So onto the question.  Legionella and what do I really need to do to be safe?

 

By default, the heat pump wants to heat the water once per week using the immersion heater to 70 degrees.  Does it really need to be that hot?  Wickepedia suggests 60 degrees is enough https://en.wikipedia.org/wiki/Legionnaires'_disease and does in need to be weekly or is a longer time still safe?

 

What do other heat pump users do?

 

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In commercial calorifiers they are set to run a pasteurisation cycles every so often. It usually depends on several factors, but for discussion sake and because I was involved with the design and O&M manual creation, a major bank's head office in the UK runs a pasteurisation cycle twice a month - the controls take the water up to 70° for about 45 minutes. Legionella dies, as you quotes at about 60° odd after 30 minutes, so for liability issues we go higher and longer and this also creates a tolerance in case of sensor faults etc.

 

Also, interestingly and this is known to me from my M&E background and from working with FM managers over the years, there has never been a recorded case of a legionella outbreak in stored hot water tanks in the UK. 

 

So, can you run a pasteurisation cycle with your controls - from what you have said it already does - leave well alone and don't worry too much to be honest as it sounds like the manufacturers have incorporated this cycle for this reason!

 

 

Edited by Carrerahill
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I can just stick with the controls the heat pump has (when I get around to actually connecting the immersion heater to it) or later on I can implement my own controls when I install the solar PV and the excess energy dump controller that will also have control of the immersion heater.

 

I guess I am trying to minimise cost of using resistance heating.  Currently what I am noticing (when doing a manual pasturisation) is from turning on the immersion heater, it probably takes half an hour before I actually see the temperature start to rise. That is probably stratification, and the immersion is lower down in the tank than the heat pump input coil reaches.  I guess that's a good thing as the immersion will be heating that pool of cool water sitting at the bottom of the tank.

 

Using the heat pump controls I can't go beyond weekly as it only knows "days of the week" and has no mechanism to say every other week, but I can set the target temperature and how long it wants to stay there.  That latter point is academic as having heated the tank it will stay at that temperature for many hours unless you are drawing a lot of hot water off.

 

I am likely when I install the PV to do the pasturisation from that, as it can be more intelligent, i.e. the controller can monitor the temperature and know for instance it already reached 70 degrees a couple of days ago when it was partucularly sunny, and reset the count down to the next cycle, something the heat pump controller cannot do.

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Oh dear.  I have hit a "problem"

 

The immersion heater thermostat that came with my Telford UVC cuts off at 54 degrees even on it's hottest setting. That's neither hot enough for a pasturisation cycle or to give a decent amount of solar PV storage.  Clearly the thermostat needs to be changed, that can't be right.

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1 minute ago, ProDave said:

Oh dear.  I have hit a "problem"

 

The immersion heater thermostat that came with my Telford UVC cuts off at 54 degrees even on it's hottest setting. That's neither hot enough for a pasturisation cycle or to give a decent amount of solar PV storage.  Clearly the thermostat needs to be changed, that can't be right.

Very strange. Normally they are user definable. Make / model of UVC?

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28 minutes ago, Nickfromwales said:

Very strange. Normally they are user definable. Make / model of UVC?

300L Telford unvented with heat pump input coil.

 

The immersion is a Thermowatt type, the ones where the thermostat plugs onto the element terminals with 2 spade terminals.

 

I have sent an email to Trevor, but it surely must be an "out of calibration" thermostat?  Surely it should be capable of 70 degrees?

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Perhaps a bit of a basic point, but this is a sealed system, isn't it?

 

If so, then there's no way any airborne bacteria, like legionella, will be able to get into the hot water tank.

 

It's different for a vented hot water tank, as it's possible for legionella to get in via the fairly open cold water tank that fills it, which is why you need to periodically heat up the hot water tank to kill anything off.  I can't see the point with running an anti-bacterial cycle on a sealed system.  It's a bit like our 300 litre sealed pressure vessels.  The incoming water is disinfected and bacteria free, so nothing grows in them, even though they could get up to outside air temperature in warm weather, and that's inside the temperature range where legionella can grow.

 

Legionella is primarily an airborne pathogen, so if no part of the system is open to air then there isn't really any risk.

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Thanks for that reply @JSHarris

 

Yes it's a sealed unvented tank, mains water supply. The water comes from Loch Glass about 6 miles away and treated by Scottish Water in a relatively new treatment plant. I don't know what treatment it gets there, but unlike some other parts of the country, it arrives to us a pure water with no taste, smell or even hint of chlorine.

 

But we are to assume it arrives as tested potable water, so you are saying there is no risk and I should not bother?

 

I still need to sort out the thermostat so solar PV has the ability to store more unwanted power in the tank.

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@ProDave I assume it's Assynt WTW (above Evanton) if you're on Loch Glass? In which case it has an ultrafiltration membrane for primary treatment. The incoming water is screened, pH adjusted before filtration and as far as I'm aware it's  dosed with sodium hypochlorite. As it's a newer works the chemical dosing is a bit more refined, hence it doesn't taste as bad as some other places. Sorry it doesn't answer your question, but you might be interested all the same!

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20 minutes ago, jamieled said:

@ProDave I assume it's Assynt WTW (above Evanton) if you're on Loch Glass? In which case it has an ultrafiltration membrane for primary treatment. The incoming water is screened, pH adjusted before filtration and as far as I'm aware it's  dosed with sodium hypochlorite. As it's a newer works the chemical dosing is a bit more refined, hence it doesn't taste as bad as some other places. Sorry it doesn't answer your question, but you might be interested all the same!

Yes that's the one.

 

When we first moved here, before the new treatment plant was built, when there was a strong SW wind it wold stir up the silt near the outtake and the water came out of the taps with a slight brown tint.  That has gone with the new works, but the water still tastes pure.  I really notice when I go south now how much chlorine is in a lot of the water.

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7 hours ago, JSHarris said:

There's no risk unless air can get at the water before the hot tank.  The legionella problem is caused primarily by airborne legionella bacteria getting into a water supply post-treatment and then multiplying.

Primarily perhaps, but not exclusively.  My understanding is, however, that domestic systems present minimal risk unless there is a storage system or dead leg that sits between 20 and 45C and allows multiplication. The HSE's website has a load of stuff on it if you fancy some bedtime reading. I suspect the threshold for control is a function of temperature & time, so you might be ok with >50C for an extended period.

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The bottom line is that water treatment (by water companies) allows for either enough residual disinfection to ensure that there is no risk of microbial growth in pipes etc with no flow for some period of time (i.e. when people go on holiday in summer and all the pipes in their roof space sit at relatively warm temperatures for a week or two) or they disinfect the water to the point where pathogens are eliminated. 

 

Legionella is an aerobic bacteria.  It cannot survive, let alone multiply, in anaerobic conditions.  All known cases of legionella effecting humans have come from the bacteria being breathed in, as a consequence of it being released in a mist or spray of water where the concentration of bacteria in the aerosol was sufficient to overcome the bodies defences.  The vast majority of legionella cases have resulted from open spray evaporation water cooling or humidification systems, often associated with office building or hotel air handling systems, where the cooling water was not properly disinfected.

 

The bacteria is pretty low risk, as it cannot form spores, therefore cannot bypass water treatment disinfection systems.  Ultrafiltration in a water treatment plant will very easily take out Legionella Pneumophila.  This means that a closed system, containing disinfected water, cannot have any bacteria within it, therefore there can be no risk that they will multiply and cause an infection risk from using a shower (the means of infection is always breathing in an aerosol that contains a sufficiently high bacterial load as to cause this form of pneumonia).

 

To put this into perspective, like other forms of pneumonia, infection can be spread from one infected individual to another when the infected individual coughs out an aerosol containing the bacteria.  Out of the ~300 cases per year of pneumonia in the whole of the UK that can be positively identified as having been caused by Legionella Pneumophila, around 120 of those were definitely people who acquired the infection on holiday overseas.  I cannot find a single case where the source was identified as a domestic hot water system - not one.  Far and away the highest risk comes from evaporative air cooling systems, where the water feeding the evaporative cooler has not been adequately treated.

 

The total risk of infection in the UK is currently a bit over 5 cases per 1,000,000, with most of those having either contracted the disease overseas or having been infected by someone that has been infected overseas.  Most infections from overseas are in people returning from Spain, most probably because evaporative air coolers or humidifiers are still commonly used in the air handling systems of hotels, office buildings etc.  I was amazed that there wasn't a massive outbreak following the Barcelona Olympic Games, as they used massive open air evaporative coolers to try and reduce the ground level air temperatures, and these drew water from open lagoons in many areas. 

 

Given your position, @ProDave I'd not even bother thinking about legionella, for several reasons.  Firstly, it's unlikely that your untreated source water is infected - it will be too cold and the pH will probably be on the low side.  The source water will also be poorly oxygenated, I suspect, given where it comes from.  Secondly, you have the advantage of an ultrafiltration water treatment plant, which is easily going to remove bacteria as large as Legionella Pneumophila.  Finally, you have a sealed hot water system where there is no possibility at all of airborne bacteria entering before the hot water storage tank.

 

 

 

 

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7 hours ago, Roundtuit said:

Primarily perhaps, but not exclusively.  My understanding is, however, that domestic systems present minimal risk unless there is a storage system or dead leg that sits between 20 and 45C and allows multiplication. The HSE's website has a load of stuff on it if you fancy some bedtime reading. I suspect the threshold for control is a function of temperature & time, so you might be ok with >50C for an extended period.

My thoughts, and practices are in line with that thinking tbh. Better safe than sorry. 

Mitigate against dead legs, eg I routinely promote a HRC if the customer has PV as its 'free' to run and when experienced by the punter they say they never go back to not having a hot return , and I make sure there is a means of 'boiling' the tank either routinely or by purge program. Its very easy to do so needs no speciality plumbing or controls, just an immersion and a cheap ass 7-day time-clock.  

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I do, temporarily have one hot water dead leg, the feed to the utility room that as yet has no sink or tap on the end of it.  But if there is nothing there to grow and multiply anyway.....

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But given that there has never been a case of pneumonia in the UK that has been traced to a domestic hot water system, that contracting pneumonia from Legionella Pneumophila is extremely rare, that the vast majority of cases of pneumonia in the UK are caused by streptococcus pneumoniae, with the rest being caused by other bacteria, viral, or even fungal, infections, then my personal view is that the risk of legionella within a domestic hot water system is already very, very low. 

 

The risk from a sealed, anaerobic, hot water system, fed with treated water from an ultrafiltration plant, which in turn is fed by a source that is, cold, slightly acidic and has a low oxygen concentration is so close to zero as to not be worth worrying about.

 

You are far more likely to catch pneumonia caused by Legionella Pneumophila from someone in your local supermarket checkout queue that's just come back from a  holiday in Spain...

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1 hour ago, JSHarris said:

But given that there has never been a case of pneumonia in the UK that has been traced to a domestic hot water system, that contracting pneumonia from Legionella Pneumophila is extremely rare, that the vast majority of cases of pneumonia in the UK are caused by streptococcus pneumoniae, with the rest being caused by other bacteria, viral, or even fungal, infections, then my personal view is that the risk of legionella within a domestic hot water system is already very, very low. 

 

The risk from a sealed, anaerobic, hot water system, fed with treated water from an ultrafiltration plant, which in turn is fed by a source that is, cold, slightly acidic and has a low oxygen concentration is so close to zero as to not be worth worrying about.

 

You are far more likely to catch pneumonia caused by Legionella Pneumophila from someone in your local supermarket checkout queue that's just come back from a  holiday in Spain...

That's good to hear. Since we moved in we have had our hot water tank set at 45C. It reduces standing losses and the EASHP seems to heat the water very efficiently to that temperature. There is ample hot water for the two of us.

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7 hours ago, JSHarris said:

But given that there has never been a case of pneumonia in the UK that has been traced to a domestic hot water system, that contracting pneumonia from Legionella Pneumophila is extremely rare, that the vast majority of cases of pneumonia in the UK are caused by streptococcus pneumoniae, with the rest being caused by other bacteria, viral, or even fungal, infections, then my personal view is that the risk of legionella within a domestic hot water system is already very, very low. 

 

The risk from a sealed, anaerobic, hot water system, fed with treated water from an ultrafiltration plant, which in turn is fed by a source that is, cold, slightly acidic and has a low oxygen concentration is so close to zero as to not be worth worrying about.

 

You are far more likely to catch pneumonia caused by Legionella Pneumophila from someone in your local supermarket checkout queue that's just come back from a  holiday in Spain...

The Scottish Government are bringing in a new rule that rented properties need a Legionela risk assessment.  I think I will be basing mine on that analysis.

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