Legionella protection with heat pumps

Friends, I am loving our new Vaillant Arotherm+ heat pump.

Click image for larger version: Vaillant Arotherm+ Air Source Heat Pump has replaced our gas boiler. It provides up to 5 kW of heating but uses only (about) 1.5 kW electricity! It can either supply hot water – at up to 70 °C – to a cylinder, or circulate hot water through our radiators.

At the moment it is just heating our domestic hot water (DHW) but in the next couple of weeks I expect that it will begin to circulate water through our radiators in order to warm the house.

One aspect of using a heat pump for DHW at first sounds very alarming: it is a requirement to run an ‘Anti-Legionella’ cycle once a week or so.

‘Legionella’ is the name of an amoebic bacterium which lives naturally in water. It can breed in ‘warm’ water but is killed in ‘hot’ water. Very roughly it thrives in temperatures between 20 °C and 50 °C.

The bacteria are harmless to people when in the water – it’s fine to wash in the water. But if ultra-fine droplets are breathed directly into the lungs, they can cause a potentially lethal pneumonia called Legionnaire’s disease.

Even the droplets from a shower are unlikely to be capable of causing infection – they are too large.

But the disease presents an interesting risk profile.

  • IF the disease is caught (which is unlikely) it is potentially fatal to the elderly,
  • BUT prevention is trivial – just heat stored DHW to 60 °C or above.

What’s this got to do with heat pumps?

Most gas boilers are ‘Combi’ boilers which instantly heat the water as required, and don’t store hot water at all – so they offer no risk of harbouring Legionella bacteria.

So-called ‘system’ gas boilers which store water in a DHW tank typically heat the water to 70 °C – and so kill any legionella bacteria present.

But heat pumps typically struggle to heat water above 50 °C, and typically store water at 50 °C – which is easily hot enough for normal domestic purposes – but just opens the window to potentially allow Legionella to thrive in a DHW tank.

For that reason, modern heat pumps typically run an ‘Anti-Legionella’ cycle once a week which heats the water to above 60 °C. For older heat pumps this can involve the use of a direct electric immersion heater, but more modern heat pumps (such as mine ;-)) can heat water to 60 °C or even 70 °C with no problem.

No problem? So why am I writing this?

When we first started using the heat pump, I noticed that occasionally the water from the taps was extremely hot. It felt dangerously hot.

So I began to measure the temperature of the tap water at three outlets in the house: the kitchen, and two of the bathrooms. The temperatures were typically within ±0.5 °C of each other.

I found that on the morning after the anti-legionella cycle, the tap temperatures were just a bit less than 70 °C. It felt to me like it would be just a matter of time before someone was hurt by this.

Looking on-line (1, 2) I saw that the time to ‘scald’ was less than one second at these temperatures.

Click image for larger version: Graph showing the immersion time before ‘scalding’ for water at various temperatures. For flowing water, these times would be reduced. Notice that the vertical scale is logarithmic.

Safety Action Step#1

After about 1 month of use, I realised why the water was getting so hot. The anti-legionella cycle had been timed to run exactly after the daily heating cycle. It was this ‘double-heating’ which was producing the high water temperatures.

So cancelling the daily DHW heating cycle for that day (Wednesday) meant that the water coming out of the taps was now only about 60 °C – still ‘too hot’ in my opinion.

Safety Action Step#2

A little research on line showed that thermostatic devices existed which could prevent excessive temperatures reaching the taps. They were called ‘blending valves’ or ‘anti-scald’ valves.

These devices act like the valves in thermostatic showers and blend cold water with the hot water to maintain a set temperature – set-able between 45 °C and 65 °C.

The wonderful Twickenham Green Plumbers installed such a device on the top of the DHW cylinder and now my concern about scalding  is a thing of the past. The hot water temperature at the taps is now a consistent 47 ± 1 °C independent of the day of the week.

Click image for larger version: Graph the temperatures of the water emerging from 3 outlets in the house versus time. The anti-legionella cycle takes place early on Wednesday mornings. The installation of a blending valve on the cylinder means that now the tap temperatures does not vary from day-to-day, and does not reach potentially harmful temperatures.

Coefficient of Performance

The practical miracle of heat pumps is that they extract heat from the environment in order to warm our houses, and so provide more heating energy than the electrical energy used to operate them.

The ratio of the heating effect of a heat pump to its electrical energy consumption is called the coefficient of performance or COP.

The graph below shows the COP for the DHW heating cycles (in which the water was warmed to 56 °C) and the anti-legionella cycles in which the water was warmed to either a nominal 70 °C or 60 °C (in both cases these temperatures were exceeded by about 5 °C).

Click image for larger version: The graph shows the COP for the DHW heating cycles in which the water was warmed to a nominal 50 °C and the anti-legionella cycles in which the water was warmed to either a nominal 70 °C or 60 °C. In all cases actual temperatures were exceeded nominal temperatures by about 5 °C.

For the normal DHW cycle, the average COP is 3.3; for the very high temperature combined DHW and anti-legionella cycle, the COP fell to 2.5; but for a normal anti-legionella cycle the average COP is 2.9.


The idea that preparing domestic hot water could potentially create a life-threating hazard is at first alarming.

But in fact the anti-legionella heating cycle – when programmed correctly! – is very simple and reduces COP by only a small amount.

Adding a blending valve to the DHW cylinder output maintains a safe temperature at the taps and has one additional benefit: it allows the DHW cylinder to store extra thermal energy.

Assuming the water is heated from 15 °C, a tank of water at 60 °C contains 28% more thermal energy than a similar tank at 50 °C. If hot water demand were high – e.g. visitors! – the tank could supply 30% more water at a safe discharge temperature of 47 °C.

2 Responses to “Legionella protection with heat pumps”

  1. Dan Grey Says:

    Just posted a comment on this topic but on the wrong post — hadn’t seen this!

    Basically, I’ve never seen a hot water tank heated to more than 50 °C by a gas boiler as such temperatures would be dangerous — they’d scald. Nor have I ever seen a gas boiler controller with anti-Legionella cycle. So why is it such a thing with heat pumps? Regulations?

    • protonsforbreakfast Says:


      Hi. Mmmm. I have a partial answer. Most modern gas boilers are combi-boilers and so don’t store water. They have a high enough power – greater than 20 kW – that they can instantly heat a few litres per minute to 50 °C. I think this is the maximum temperature you would really want on a non-thermostatic hot tap.

      But for system boilers that heat water for a DHW tank I think you have a point. The HSE have a page which states “What a Landlord must do” and this suggests storage at more than 60 °C – my emphasis in the quote below.

      But I think it’s a case where a new technology has to be more thorough in its safety assessments than existing technologies to avoid negative scrutiny.

      Anyway – you make a good point. I had just assumed these systems stored water at high temperatures capable of scalding just like the system did at home when I was a child!

      Best wishes



      The practical and proportionate application of health and safety law to landlords of domestic rental properties is that whilst there is a duty to assess the risk from exposure to Legionella to ensure the safety of their tenants, this does not require an in-depth, detailed assessment. The risks from hot and cold water systems in most residential settings are generally considered to be low owing to regular water usage and turnover. A typical ‘low risk’ example may be found in a small building (eg housing unit) with small domestic-type water systems, where daily water usage is inevitable and sufficient to turn over the entire system; where cold water is directly from a wholesome mains supply (no stored water tanks); where hot water is fed from instantaneous heaters or low volume water heaters (supplying outlets at 50 °C); and where the only outlets are toilets and wash hand basins.

      A simple assessment may show that there are no real risks and are being properly managed and no further action is needed. It is important to review the assessment in case anything changes in the system.

      Implementing simple, proportionate and appropriate control measures will ensure the risk remains low. For most domestic hot and cold water systems, temperature is the most reliable way of ensuring the risk of exposure to Legionella bacteria is minimised ie keep the hot water hot, cold water cold and keep it moving. Other simple control measures to help control the risk of exposure to Legionella include:

      * flushing out the system prior to letting the property
      * avoiding debris getting into the system (eg ensure the cold water tanks, where fitted, have a tight fitting lid)
      * setting control parameters (eg

        setting the temperature of the hot water cylinder (calorifier) to ensure water is stored at 60°C

      * make sure any redundant pipework identified is removed.

      The risk is further lowered where instantaneous water heaters (for example combi boilers and electric showers) are installed because there is no water storage.

      P.S. I deleted the comment from the other blog article

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