Hot water cylinder (boiler)

The hot water cylinder stores heated drinking water. It should be as large as necessary and as small as possible to keep heat losses to a minimum.

In a hot water cylinder, usually called a "Boiler" in Austria, domestic water is heated and stored until it is drawn off. With a heat pump, it is heated via a heat exchanger inside the cylinder. Alternatively, there are solutions where the heat is stored in heating water and only transferred to the domestic water via a fresh water module when drawn.

The right size

• According to klimaaktiv, cylinders in detached homes are usually between 200 and 400 litres.
• As a rule of thumb, klimaaktiv suggests around 100 litres of cylinder volume per person.
• Per person, you can expect around 40 litres of hot water (60 °C) per day, depending on habits.

Cylinders that are too large have higher heat losses, and the water stands for longer, which is detrimental to hygiene.

Temperature and losses

For a heat pump, every degree less is a bonus. According to klimaaktiv, heat losses increase by an average of around 1.6 % per degree. A temperature of around 50 °C is normally sufficient for all applications. Insulation is also important: according to klimaaktiv, energy efficiency class B is the minimum energy standard for a detached home.

Practical tips

• A thermosiphon at the connection prevents hot water from rising unintentionally into the pipes.
• Insulate pipes according to the 3/3 rule: insulation thickness equal to the pipe diameter.
• Heat hot water at midday when solar power is available, see photovoltaic self-consumption.

For hygiene, read legionella protection.

All terms · Heat pump knowledge