Managing the risk of legionella in domestic hot water systems
What is legionella?
Legionella pneumophilia are the bacteria which can cause Legionnaire’s and other pneumonia-like diseases. These bacteria can thrive in stagnant or slow-flowing water at the right temperature and conditions. Insufficient flow, coupled with the right temperature range (20-45°C), could promote Legionella bacteria in any system. Stratification can provide Legionella bacteria with the ideal temperature range but can be guarded against with the right equipment and processes.
Stratification means that temperature is not even throughout a cylinder or other vessel and distinct layers of heat and cold are present. A poorly stratified vessel will have gradually mixed hot and cold creating patches of lukewarm water.
What is a legionella protection cycle?
A legionella protection cycle is a regular process to guard against stratification in your hot water system and to prevent Legionella bacteria from forming. This cycle involves raising the temperature in the water heater to a minimum of 60oC and distributing the water throughout the hot water system for at least one hour a day to ensure that Legionella is unable to survive.
How does this work with a direct-fired water heater?
The hot water cylinder should be fitted with a top-to-bottom recirculation - or destratification - pump to ensure even mixing of the water within the cylinder that will be used during the legionella protection cycle. Thermostatic mixing valves should be used at all outlets to manage the risk of scalding.
How often should the Legionella Protection Cycle be carried out?
The cycle should be carried out when there is no hot water demand (often during the night) and is best done on a daily basis. It is particularly important to run the cycle when the system hasn’t been used for more than one week as this can have created ideal conditions for legionella to thrive (when water has not been cycling through the system, and temperatures have remained low).
How can I guard against Legionella in water heaters day-to-day?
You can manage the risk of Legionella in commercial system water heaters by installing a pump to each cylinder as part of a destratification kit (or top-to-bottom recirculation kit) which helps distribute hot water evenly around the vessel. This can help maintain a high average water storage temperature above 60oC which will kill off Legionella bacteria within 2 minutes.
What products do Hamworthy offer for managing a regular Legionella Protection Cycle?
- Our Dorchester DR-FC Evo condensing water heater can be ordered with a destratification kit to decrease the risk of Legionella building up. The integrated controls incorporate a specific anti-legionella safety function which can be set to perform a weekly anti-legionella purge cycle. This works by running the water heater for a period at a high temperature (default 65°C for one hour) to prevent the risk of legionella bacteria forming in the vessel.
- The Powerstock calorifier can be ordered with a destratification kit to decrease the risk of Legionella building up.
- Halstock stainless steel calorifiers can be ordered with a de-stratification/top to bottom recirculation pump kit. This is factory fitted and must be specified at the same time as ordering the calorifier
- Our Dorchester DR-CC condensing water heater does not require a top-to-bottom re-circulation kit (destratification kit) due to the heat exchanger coil positioned in the centre of the unit for even heat distribution and reduced likelihood of stratification.
This video shows how the legionella protection cycle process in a domestic hot water system using a Hamworthy Dorchester direct fired water heater fitted with a recirculation pump.