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Heat Interface Units and Legionella

Legionalla temperatures

HSE guidance states "It is a simple fact that the organism will colonise both large and small systems so both require risks to be managed effectively."

As manufacturers we take responsibility for the safe function of our equipment, and this extends to the potential for the to introduce a risk of Legionella.

Historically, hot water cylinders, stored at 55-65C, provide protection by sterilising the stored water, combination boilers go cold when not in use and then heat water to over 50C typically, and heat interfaces have generally maintained themselves around hot water setpoint temperatures, so Legionella has never been difficult to address.

Today however, the need for more efficient HIUs, running at lower temperatures, has introduced a new set of conditions - a plate heat exchanger held deliberately at temperatures low enough to for Legionella to breed, rather than die.

The lowest DHW setpoint we have been asked to ever set systems up to is 45C. In our view this is a sensible setpoint - too hot to shower in, but low enough to enable heat networks to run relatively cool and make use of low grade technologies such as heat pumps. This is not hot enough to kill Legionella that may exist, but without stored water there should still be little if any chance of problematic levels of Legionella. Certainly never any reported cases we have heard from, or have any water authorities we have talked to including Thames Water and WRAS. Even still, it is nice to have the option to pull in Legionella protection to be sure.

If however we maintain the heat exchanger at such a temperature indefinitely through the use of a keep-warm mode, how does this effect Legionella risk, and if the keep warm is set below the DHW supply temperature, then this may result in water held at temperatures within the optimal Legionalla growth range - so is this a potential problem ?

As standard we would typically deploy an anti-Legionella cycle that after a period of no use brings the plate heat exchanger to 60C for over an hour to kill of any existing Legionella. This is possible with electronic HIUs as they can incorporate timing functions - effectively raising keep warm temperatures for a period.

Mechanical HIUs however do not offer this timing functionality, and as such do not have the ability to sterilise periodically. Are we therefore at risk from basing HIU designs on mechanical controls with a reduced keep warm temperature ? For advice we turned to the LinkedIn Legionalla Group. One of the most concerning responses was as follows:

"HSG 274 discusses low volume/low risk systems. As soon as you put a shower into the system you obviously increase the potential risk due to aerosol. As for 38°C - you are asking for problems. If you can guarantee (which you can't) that the water will be flushed daily, then you could argue that the system throughput would help prevent proliferation. The trouble is that people go on holiday etc, therefore units may not be used for weeks, at 38°C you'd have a lovely biofilm! With regards to cases of LD - 50% are source unknown, a once-through systems that has stagnated over a holiday then been flushed clear whilst infecting someone is a perfect example of why the source would be unknown."

Further to this, the Plumbing Engineering Services Design Guide summarises that one should avoid water temperatures which may encourage the growth of Legionella and other micro-organisms.

With the DATA HIU going through WRAS approval, we were particularly keen to tick all the boxes that may relate to Legionella that WRAS were concerned with in order when using lower keep warm temperatures, to ensure prompt certification. Our investigations have determined that there are no requirements or tests currently in the WRAS approval mechanism regarding Legionella, and as such any Legionella policies are for the installer to put in place and the user or managing agents to maintain.


In summary, a plate heat exchanger may only hold small quantities of domestic water, but there stands a reasonable chance that if we maintain the contents of the plate at favorable bacteria temperatures for extended periods, that a biofilm will form and start adhering to the surfaces of plates. Once formed, such a film is difficult to remove, even with higher temperatures, and may act to accelerate bacterial growth by providing nutrition. Once a hot water outlet is run, the contents of the plate will be cleared, and would only provide a temporary risk of exposure if the first run was to a shower.

We conclude that the conditions required for risk to become present are:

  • keep warm temperature that maintains the contents of the PHE between 25C and 45C,
  • the HIU is left standing without use for a week or more,
  • their is no Legionella cycle in place,
  • outlets include showers

The risks are maximised by:

  • having a keep warm set at 38C
  • the HIU is left standing for longer periods
  • the HIU is left standing at regular intervals
  • their is low occupancy, so higher DHW flow rates are not experienced for flushing of biofilms from the heat exchanger
  • incoming cold mains water temperatures are over 20C

Our policy on Legionella is therefore as follows:

Where domestic water in an HIU is to be maintained at an elevated temperature by the use of a keep warm mode, should this temperature be below 48C then a Legionella sterilisation cycle should be deployed, or the keep warm disabled after 48 hours.  As a minimum, the sterilisation cycle should come into operation once water has remained stagnant in the HIU for 48 hours, and should raise the water temperature to at least 60°C for a minimum of one hour.

Combating Legionella Awards

All our electronic HIUs offer the option of Legionella protection as standard. This includes the DATA, DIGI, and SLIM HIUs. The anti-Legionella function on the DATA HIU was recognised by the industry last year when the system made it through as a finalist for the Combating Legionella Awards. The system is now fully WRAS approved and as well as been officially the most efficient HIU in the industry (as independently tested) it also offers the most advanced protection there is against Legionella.


From LinkedIn Legionella Risk Group

Questions from Ditrict Heating Group

Dear Legionella Risk Group,

In the District Heating Groups we've been having a lively debate about where the law stands on Legionella prevention in Heat Interface Units. It seems there is no real consensus.

They are the same as a combination boiler, except take heat from a network. A common feature is to keep the water in the plate heat exchanger at 38C when not used.

We would be grateful for any solid advice.

Is it a legal requirement to maintain HIUs at temperatures over 55C to kill Legionella? How long for an how often? Has there ever been a recorded case of Legionella from a combi-boiler, or an HIU? Does a keep warm between 30C and 40C make Legionella more of an issue ? If taps are fairly regularly drawn so systems are flushed with temperatures over 50C, can we forget about sterilisation? Given a Legionella cycle just sterilises the HIU, whats the point given shower heads and warm mains supplies are the most likely sources for bacterial growth.

Does anyone have any tables on growth rates of Legionella against temperature? If keeping an instantaneous water heater at 38C for a few weeks forms a biofilm, would keeping it at 26 have a similar effect ?

JOHN SPENCE Bsc (Eng) The regulations and growth rates are not linear ie. it depends on many internal and outside factors I presume by instantaneous water heaters you mean POU heat on demand unit such as ###, the risk from these depends on inlet temperature and type of outlet - aerosol or not. So if your inline heater is fed by mains water below 20 deg C and has a plain tap outlet then being set at 38 deg C will be very low risk and very low growth , however if you fed the heater with a tank fed source at say 26 deg in summer and only set it to 38 deg C with a spray outlet it could become a high risk item very quickly. The safest way to check is to monitor the feed temps to such heaters and check the type of outlets and remember the golden rule keep it moving - so that means weekly flushing of little used outlets and log them in your log book. I do hope this helps, If you need more information just ask. At Watercare in Middlesbrough UK we believe in not assuming anything !!

David Tamblyn HSG 274 discusses low volume/low risk systems. As soon as you put a shower into the system you obviously increase the potential risk due to aerosol. As for 38°C - you are asking for problems. If you can guarantee (which you can't) that the water will be flushed daily, then you could argue that the system throughput would help prevent proliferation. The trouble is that people go on holiday etc, therefore units may not be used for weeks, at 38°C you'd have a lovely biofilm! With regards to cases of LD - 50% are source unknown, a once-through systems that has stagnated over a holiday then been flushed clear whilst infecting someone is a perfect example of why the source would be unknown.

Wilco van der Lugt " In Holland its law to have a daily sterilisation cycle." This is not in the Dutch law. "how often you need to sterilise to 60C " the ESGLI guidelines will give you a direction

Dr Pamela Simpson Richard refer to HSE L8 and also their technical guidance docs HSG274 part 2 hot and cold water systems. These are available on their website to download. The optima for growth of legionella is 37C but they survive extremes. If you send me your email address I can send you a graph of how long they survive at upper and lower temperatures. Send it to info@whitewatertech.co.uk

Paul Higgins Richard what we have be advised to do is to treat the stored water to 55C once a day and have it at 55C for one hour before using any showers and flush the shower for a min of 3mins before letting anybody use them, you could look at the user profile, this usually helps when you are doing the risk assessments.

Richard Hanson-Graville Thanks so much for help, but it feels like somethings missing. We are not talking stored water - its instantaneous (POU). However, unlike an electric heater, or gas heater, when no water is running, it doesn't go cold - it keeps the litre or so in the heat exchanger at between 26 and 55C - different manufacturers have different temperatures. This specifically relates to district heating, which is only just taking off properly - so not surprised no one in the health world has addressed the technology, or even looked at it seriously. I guessing that will happen when (if) there is enough cases of Legionella to tie them to a common (unidentifiable) cause. Also, you are talking blocks of 100 or more flats, with mix of social and private ownership. Insisting showers are run for 3 minutes, or any sort of regular checks, are simply impossible.

Fraser Inglis Hi Richard, I understand the principal of district heating, and I think I am picking up how the HIU are functioning during downtime. Temperature should be the primary control ideally to prevent legionella growth and therefore exposure, so where the units settings can be adjusted and it is reasonably practicable this 1litte remaining would be best held at >50c out of the growth zone. This is most effective way, I am unsure what implications this adjustment would have on consumption of the circulating heat medium in the community. Would there be enough to sustain this for all properties? If not, you could then look at flushing or sterilisation routines to control. Unfortunately, as others have identified, while the volumes are small, the HIU fitted with this system would be considered a risk system by virtue of the exposure hazards at downstream outlets. If these district heating projects are going to be more regular in the this really should be something that is highlighted to the industry, you would hope there would be some other leads from abroad on this. Hope this helps. Fraser

Jim Gott Alternatively address the issue using a continuous biocide treatment system. It makes sense to look at this option with renewable technologies and district heating where high temperatures are not practical or efficient.

Richard Hanson-Graville Keeping above 55C all the time has a massive effect on heat losses, and ruins the efficiency of many heat generators such as heat pumps. Taking units to 57-60C for an hour a day is our approach to protection. DH is set to increase significantly. Its part of the London Plan, and if house-building promises are to be believed, that's a lot of systems. Finding the legal position is a first step. What do you need to do to comply to UK law.

Note on Biofilm


Biofilm forms when bacteria adhere to surfaces in aqueous environments and begin to secrete a slimy, glue-like substance that can anchor them to many materials such as metals and plastics. A biofilm can be formed by a single bacterial species, but more often biofilms consist of many species of bacteria as well as fungi, algae, protozoa, debris and corrosion prod ucts. Essentially, biofilm may form on any wetted surface exposed to bacteria. Biofilm develops where the temperature is right for growth and where there is a nutrient source. Nutrients can be scale, sediment, corrosion products, or trapped organic and inorganic molecules supplied by the flowing water.