Domestic hot water (DHW) is the potable heated water drawn at taps and showers — legally and technically separate from the closed heating-circuit water. A European household member uses roughly 35–45 litres at 45 °C per day, about 1.5–2 kWh, and water heating represents around 15% of household energy use (Eurostat). DHW is stored at 60 °C or protected by scheduled anti-legionella cycles.
How is DHW different from heating water?
Domestic hot water is drinking water that happens to be warm — it leaves the system through taps, so it falls under potable water hygiene rules and must be heated in corrosion-protected vessels: enamelled steel with a magnesium anode, or stainless steel. Heating water, by contrast, circulates forever in a closed technical loop, is never drawn off, and may contain inhibitors and glycol.
The two must never mix. That is why a buffer tank holding heating water cannot supply a shower directly, and why combination stores put a hygienic separation — an internal tank or a stainless coil — between the two waters.
How much hot water does a household use?
Roughly 35–45 litres at 45 °C per person per day, equivalent to 1.5–2 kWh — with wide lifestyle spread. Across the EU, water heating accounts for around 15% of household energy consumption (Eurostat), second only to space heating.
| Household | Daily draw at 45 °C | Typical cylinder (heat pump / solar system) |
|---|---|---|
| 1–2 persons | 70–90 L | 150–200 L |
| 3–4 persons | 120–180 L | 200–300 L |
| 5–6 persons | 180–270 L | 300–400 L |
Cylinders for heat pumps and solar systems are sized above the bare daily draw because they store at moderate temperatures and must bridge generation gaps — a solar system needs to carry an evening and a cloudy morning, a heat pump prefers few, long charging runs.
What temperature should DHW be stored at?
The classic rule is 60 °C in storage and at least 55 °C through circulation lines, because legionella bacteria multiply between 25 and 45 °C and die quickly above 60 °C. Heat pump systems commonly store at 50–55 °C for efficiency and run a scheduled thermal disinfection cycle instead — an accepted approach when the whole system, including dead legs, is properly designed.
At the outlets the requirement reverses: water above 50 °C scalds skin in seconds, so a thermostatic mixing valve blends the stored water down to a safe 45–48 °C at the point of use. Storage hot, delivery tempered — both rules serve safety.
How do heat pumps and solar thermal prepare DHW?
A heat pump charges the cylinder through a large-surface coil — low-temperature generators need generous exchanger area to transfer their power at small temperature differences — and its DHW performance is tested to EN 16147. Solar thermal feeds a lower coil in the same cylinder; a correctly sized system covers 60–70% of annual DHW demand in Central Europe, and more in the Mediterranean.
Where stored potable volume itself is unwanted, a hygienic tank inverts the layout: the vessel stores heating water, and drinking water is heated instantaneously through an internal corrugated stainless coil as it flows — fresh-water comfort with minimal legionella-prone volume. Solimpeks manufactures the full spread of these options, from enamelled cylinders to hygienic combination stores and TSM thermosiphon systems of 120–350 litres.
Why does DHW dominate modern building design?
Because space heating is shrinking and hot water is not. As insulation standards improve, the space heating share of demand falls while showers stay showers — in new low-energy buildings DHW can approach half of the total heat demand, so cylinder losses, circulation losses and generation efficiency move from footnote to headline. This is also why DHW is the natural first target for solar: demand is constant year-round and peaks exactly in the season when collectors yield most.
Frequently asked questions
What temperature should my hot water cylinder be set to?
60 °C is the classic safe storage setting. Heat pump systems may store at 50–55 °C with a scheduled anti-legionella cycle, trading a little hygiene margin for meaningfully better efficiency — every 5 K of storage temperature saved is felt directly in compressor work.
How much hot water does a shower use?
A typical shower delivers 8–10 litres of mixed water per minute, so a five-minute shower draws roughly 40–50 litres at about 40 °C — of which around two-thirds comes from the cylinder, the rest from the cold line at the mixer.
Can a heat pump make water hot enough for DHW?
Yes. Standard units charge cylinders to 50–60 °C, and modern refrigerants extend the range further. The efficiency price of the last degrees is real, which is why heat pump cylinders use large coil surfaces and why storage setpoints above 55 °C should have a reason.
Is stored hot water safe from legionella?
Yes, when designed for it: either 60 °C storage with warm circulation returns, or moderate storage plus scheduled thermal disinfection, or a fresh-water principle that avoids storing potable water altogether. The risk lives in lukewarm, stagnant volumes — dead legs and oversized, cool cylinders.