Legionella: the problem flow-through does not have

The bacterium that thrives in lukewarm water

Legionella is a bacterium found naturally in small quantities in water. Usually it is harmless. The problem arises when it gets the chance to multiply, and it multiplies best under very specific conditions: stagnant water in the temperature range of roughly 20 to 45 degrees. If the water gets colder, the bacterium lies dormant. If it gets hot enough, it dies.

Infection does not come from drinking the water, but from breathing in tiny droplets, aerosols, for example in a shower. That is why the hot water system, and the shower in particular, is where the issue belongs.

This is not meant to alarm. For the vast majority, the risk is low, and Norwegian water heating systems are safe when they are correctly sized and used. But it is worth understanding why some solutions have the challenge and others do not, because it is directly bound up with how the hot water is produced.

Why the tank has to stay hot

A hot water tank stores 200 litres of water and keeps it hot over time. That is precisely «stagnant water in a temperature range» made systematic, and that is why hot water tanks are designed with legionella in mind.

The solution is to keep the water hot enough. The Norwegian Institute of Public Health recommends that the thermostat on the hot water tank is set to at least 70 degrees, so that the entire volume is hot enough to keep the bacterium at bayFolkehelseinstituttet. That is also the reason your hot water tank is kept significantly hotter than the temperature you actually shower at.

There are a few typical weak points even with a hot tank:

• The mixing zone at the bottom of the tank, where incoming cold water meets the hot water, can sit in the critical range.
• Rarely used tapping points (guest bathroom, outside tap, a shower that goes unused) end up with stagnant, lukewarm water in the pipes.
• Conventional water heaters that are turned down to save electricity, or that sit half-cold at a cabin between visits, can land right in the growth zone.

That is precisely why many smart hot water tanks have a dedicated legionella programme: at regular intervals they heat the whole tank to around 70 degrees for a period to kill off any growth. It works, but it costs extra electricity, and it tells you something: the challenge is built into the act of storing hot water.

The problem flow-through does not have

A flow-through heater does not store water. It heats the water at the moment it flows through, to the temperature you are going to use it at, and the rest of the time it is empty and cold.

That changes the premise completely:

• No tank of stagnant water. There is no volume sitting around staying lukewarm for hours.
• The pipes up to the heater carry cold water. Cold water is outside the growth zone, so the distribution network itself is not a growth environment in the way hot water pipes from a conventional water heater can be.
• Fresh water every time. The water you shower in became hot water seconds ago, not hours or days ago.

That does not mean flow-through makes water sterile or replaces good water from the waterworks. But the one mechanism legionella depends on, a store of lukewarm, stagnant water, simply does not exist in a decentralised system.

Particularly relevant for the cabin

At home, the conventional water heater is usually in daily use and kept hot. At the cabin the picture is different. A conventional water heater that sits half-cold between visits, or that is turned down to save electricity between trips, can end up in the growth zone, and then the first shower after a few weeks is precisely an aerosol from water that has been standing lukewarm.

That is one of the reasons the advice for longer absences is either to keep the tank hot enough or to drain it completely. Both come at a cost, in electricity or in heating time, which we have worked through in the story of Per and Kari and in the article on standby loss.

A flow-through heater at the cabin avoids the dilemma entirely: no tank that has to be kept hot for hygiene reasons, nothing sitting lukewarm while the cabin is empty, and fresh hot water from the first minute when you arrive.

In short

Legionella thrives in stagnant, lukewarm water. A conventional water heater is by definition a store of water that has to be kept hot enough, and that has its weak points. A flow-through heater does not remove the bacterium from nature, but it removes the condition it lives on: the lukewarm store.

If you have questions about water heating for a home or cabin, do get in contact, or see which models suit you. If you want to understand more about why hot water is best produced where it is used, read Centralised or decentralised hot water.