A hygienic tank is a combination store that heats drinking water instantaneously as it flows through a corrugated stainless-steel coil immersed in a large volume of non-potable heating water. Because no potable water is stored, the 20–45 °C band in which legionella bacteria multiply contains no standing risk volume. One vessel serves heating buffer, solar storage and hot water duties at once.
How does a hygienic tank work?
A hygienic tank stores technical heating water — never drinking water — and heats potable water only at the moment a tap opens. Cold drinking water enters a corrugated stainless-steel coil near the bottom of the vessel, picks up heat from the surrounding stored volume as it spirals upward, and leaves at the top at tap temperature. The coil itself holds only a small potable content, typically around 5–30 litres even in large stores, and that water is completely exchanged within seconds of any draw-off.
The stored heating water around the coil can be charged by any combination of sources: solar collectors, a heat pump, a boiler or an electric element. In effect, a hygienic tank merges a buffer tank and an instantaneous water heater into one vessel — which is why it is also called a fresh water combination store.
Why is a hygienic tank better against legionella?
Legionella bacteria multiply in water stored between 20 and 45 °C — exactly the band found in the middle of a conventional cylinder for much of the day. ECDC surveillance has recorded more than 10,000 Legionnaires' disease cases per year across the EU/EEA in recent annual reports, and warm stored water is a recognised source. The hygienic tank removes the hazard at its root: there is no stored potable volume in which a bacterial population can establish itself, because the few litres inside the coil are flushed at every draw-off.
This is also why German DVGW Worksheet W551, which requires conventional potable stores above 400 litres to be held at 60 °C, treats small-content fresh-water systems far more leniently — the rules target stored drinking water volume, and a hygienic tank barely has any.
What does a hygienic tank mean for heat pump efficiency?
It allows lower storage temperatures. A conventional cylinder is commonly held at 60 °C for hygiene; a hygienic store can run at 50–55 °C because legionella control no longer depends on stored-water temperature. Each 1 °C reduction in the temperature a heat pump must produce improves its COP by roughly 2–3% (EHPA design guidance), so a 5–10 °C lower set point saves meaningful electricity across a season. Combined with the large coil surface transferring heat at a small temperature difference, this makes hygienic tanks the standard recommendation for heat pump hot water in much of central Europe.
Hygienic tank vs cylinder vs buffer: what is the difference?
| Feature | Hygienic tank | DHW cylinder | Buffer tank |
|---|---|---|---|
| Stored water | Heating (non-potable) | Drinking water | Heating (non-potable) |
| Hot water delivery | Instantaneous, through coil | From stored volume | None |
| Potable volume at risk | Coil content only (~5–30 L) | Full tank (150–500 L) | None |
| Corrosion protection | Not needed on shell | Enamel + anode, or stainless | Not needed |
| Typical set temperature | 50–60 °C | 60 °C | 30–60 °C |
Because the shell never touches oxygen-rich drinking water, it needs no enamel lining or magnesium anode — only the stainless coil is potable-grade.
When is a hygienic tank the right choice?
Choose a hygienic tank when several heat sources must share one store, when legionella compliance would otherwise force high storage temperatures, or when a heat pump provides domestic hot water. Good stratification is essential — the top of the store must stay hot enough for the final metres of coil to lift drinking water to tap temperature — so quality stores use tall geometry and inlet diffusers. For pure space-heating buffering, a plain buffer remains cheaper; for simple solar hot water in warm climates, a thermosiphon cylinder still wins on cost. Solimpeks manufactures hygienic combination tanks alongside its Solibuffer range at its Konya factory, with the internal coil heat exchanger sized to the intended draw profile.
Frequently asked questions
Does a hygienic tank need a magnesium anode?
No. The steel shell holds closed-circuit heating water, which is deoxygenated and non-corrosive, so neither enamel lining nor a sacrificial anode is required. Only the coil carries drinking water, and it is made of corrosion-resistant stainless steel.
How much hot water can a hygienic tank deliver?
Delivery is continuous rather than volume-limited: as long as the store holds temperature, the coil keeps heating fresh water. Practical flow rates depend on coil surface and store temperature, so store volume and coil are matched to the building's peak draw profile.
Can solar collectors and a heat pump charge the same hygienic tank?
Yes — that is the core use case. Solar charges the lower, cooler zone while the heat pump or boiler maintains the hot standby zone at the top, and both feed the same instantaneous hot water coil.
Is a hygienic tank the same as a fresh-water station?
The principle is identical: drinking water heated instantaneously from a heating-water store. A fresh-water station uses an external plate heat exchanger with a controlled pump, while a hygienic tank integrates the exchanger as an internal coil with no extra pump or electronics.