Hot water and Legionella: what your BMS should be doing
The hot water temperatures UK guidance expects, how the BMS keeps calorifiers and circulation in line, which alarms to set, and energy savings that can backfire.
Most of what a BMS does is about comfort and energy. Hot water is different, because it’s a health and safety job. Legionella bacteria thrive in lukewarm water, and in the UK the main way of keeping them under control is temperature. A calorifier allowed to run a bit cooler “to save gas” can create a genuine risk.
Every site with hot and cold water systems needs a Legionella risk assessment and a written control scheme drawn up by a competent person. Whatever the BMS does has to follow that scheme. This article explains the principles; it isn’t a replacement for the scheme.
The temperatures that matter
The HSE’s technical guidance, HSG274 Part 2, boils down to keeping hot water hot and cold water cold. The figures you’ll hear quoted are:
| Where | Target |
|---|---|
| Hot water leaving the calorifier | 60 °C or more |
| Hot water at the outlets | At least 50 °C within a minute of running (55 °C in healthcare) |
| Hot water coming back on circulation loops | 50 °C or more (55 °C in healthcare) |
| Cold water at the outlets | Below 20 °C within two minutes |
Legionella grows best somewhere between 20 °C and 45 °C, which is why both the hot and the cold systems need to stay out of that range.
Of course, water that hot can scald. Outlets used by vulnerable people usually have thermostatic mixing valves (TMVs) close to the tap. That way the system can run hot enough for Legionella control while the water that comes out of the tap is safe.
What the BMS actually does
A typical indirect calorifier is heated by LTHW from the boilers, through a coil or a plate heat exchanger. The BMS is usually controlling a few things:
- The primary side. It works the heating valve or pump to hold the stored water at temperature. To keep water leaving the vessel at 60 °C or more, the setpoint normally sits a few degrees above that.
- The secondary circulation pump. This keeps hot water moving round the building, so the taps run hot quickly and the return stays above 50 °C.
- A destratification or shunt pump, if there is one. Large vessels can end up with cooler water sitting at the bottom. For bigger calorifiers, the guidance recommends the whole vessel, base included, reaches 60 °C for at least an hour each day. That’s usually done with a timed shunt pump or a pasteurisation cycle.
- Monitoring and alarms on flow, return and vessel temperatures.
One thing that catches sites out: the calorifier is often heated by the same boilers that do the space heating. Make sure the boiler strategy still delivers heat to the calorifier when the heating’s off, in summer and overnight. A summer shutdown that switches the boilers off and quietly lets the calorifier go cold is a surprisingly common fault.
Alarms worth having
At a minimum, think about alarming on:
- calorifier flow below 60 °C for longer than a set time
- return below 50 °C (55 °C in healthcare), which usually points to a circulation or balancing problem somewhere in the building
- a circulation pump that’s been told to run but isn’t proven running
- a calorifier running well above its normal setpoint, which suggests a stuck valve and a scalding risk
- a cold water tank above 20 °C
Put sensible time delays on them so a short dip during a busy draw-off doesn’t flood the alarm list. And make sure somebody actually gets these alarms and does something about them.
Trend logs help, but they aren’t the whole story
BMS trends of calorifier flow and return temperatures are useful evidence. They show the vessel stayed hot and help when you’re investigating a problem. What they don’t do is replace the checks in the written scheme, such as monthly temperatures at the sentinel outlets (the nearest and furthest taps on each system). A sensor on the return header can’t tell you whether a tap on the fourth floor gets to 50 °C inside a minute.
If you do rely on BMS data, check every so often that its sensors agree with a calibrated thermometer. A sensor reading 3 °C high can hide a calorifier that’s actually running below 60.
Savings that can backfire
Hot water is tempting when someone’s looking for energy savings, but these changes can all cause trouble:
- dropping the calorifier setpoint below what the scheme requires
- timing the circulation pump off at night or weekends without the water hygiene team agreeing
- switching the calorifier off over the holidays and back on without a proper recommissioning plan
- turning off the shunt pump or pasteurisation cycle because it “uses too much gas”
Anything that touches hot water control should be agreed with whoever is responsible for Legionella on the site. That isn’t red tape. It’s the person who’ll be answering questions if something goes wrong.
If you’re the FM, ask your BMS contractor
- Do the BMS setpoints match our written scheme?
- Which hot water alarms do we have, and who receives them?
- Will the boilers definitely serve the calorifier in summer?
- When were the calorifier sensors last checked against a reference?
- Can we export the temperature trends for our records?
For how the heating plant feeding the calorifier is controlled, see how a BMS runs heating plant.