You heat to 75 so you can shower at 38

A number that does not add up

Your conventional water heater is probably set to 65 or 75 degrees. Your shower runs at around 38. That is a difference of almost double, and it costs you money every single day. Not because anyone made a mistake, but because a tank that stores hot water has to cheat with the temperature.

Why the tank has to overstate the heat

You would think a conventional water heater kept the water at the temperature you actually use it. It does not, for two reasons:

Legionella. The bacterium thrives in lukewarm, standing water. A tank full of 38-degree water would be an ideal growth environment, so the heater keeps the water hot enough to prevent growthFolkehelseinstituttet. We have written more about that in Legionella: the problem flow-through does not have.

Capacity. The hotter the water in the tank, the further those 200 litres of hot water will stretch, because you mix in more cold water at the tap. A tank at 45 degrees would run out too quickly.

The solution to both problems is the same: store the water hotter than you need it, and mix it down with cold water at the tap. Practical, but it comes at a price.

The bill for the excess temperature

Here is the catch: heat loss increases with the temperature difference between the water and the room around it. The hotter you keep the tank, the faster the heat seeps out, and the harder the element has to work to keep up.

So you pay extra to heat the water to 70 degrees, when all you were going to do was use it at 38. The difference between «the temperature you store at» and «the temperature you use» is pure excess temperature, and it leaks away as standby loss around the clock.

The right temperature depends on the task:

Note the gap between the top four rows and the bottom one. Everything you do with hot water happens below 45 degrees. The tank still sits at 70.

What flow-through does differently

A flow-through heater stores nothing, so it does not need to cheat with the temperature. It heats the water directly to the temperature you are going to use it at, at the moment it runs through.

If you are showering at 38, it heats to 38. Not to 70 and then mixed down. That means:

• No excess temperature to pay for. You heat to the temperature of use, not to a storage temperature.
• No heating for legionella reasons. Without a store there is no growth environment, so there is no hygiene reason to keep the water extra hot.
• Stepless control. The water comes out at the temperature you set, regardless of how cold the incoming water is.

It is a small difference in principle, but it removes the entire gap between storage temperature and temperature of use, and that gap is something you pay for every day with a conventional water heater.

In short

A conventional water heater has to store water hotter than you use it, for reasons of hygiene and capacity, and the excess temperature leaks away as heat loss. A flow-through heater heats the water to the temperature of use and not a degree more, because it stores nothing.

If you want to know how much hot water you actually use, and at what temperature, see How much hot water do you actually use or work out your own consumption in the calculator.