Use an internal coil for a single household and a plate heat exchanger where hygiene or peak output matters more than simplicity. A coil stores 150–300 litres of drinking water and reheats it slowly; a fresh water station with a plate exchanger stores none — under three litres sits in the pipework, the threshold below which German rule W 551 treats a system as low legionella risk.
Coil or plate heat exchanger — which should I choose?
A coil is the right default for a house with one heat source and a normal draw-off pattern. A plate heat exchanger earns its place where drinking water must not be stored, where peak output has to exceed what a tank can deliver, or where one store serves several dwellings.
| Internal coil | Plate heat exchanger (fresh water station) | |
|---|---|---|
| Drinking water stored | 150–300 litres | Under 3 litres, in the pipework only |
| Peak output | Limited by stored volume | 30–60 kW continuous, tank-independent |
| Required store temperature | 50–55 °C works | 65–75 °C for comfortable output |
| Heat pump compatibility | Excellent | Poor |
| Moving parts | None | Modulating pump, flow sensor, controller |
| Behaviour in hard water | Scale on the coil, slow to matter | Scale on the plates, output falls quickly |
| Cost | Built into the tank | A separate module plus controls |
How does a fresh water station actually work?
It heats the drinking water as it flows. A modulating pump draws hot water from the top of a buffer or thermal store and passes it through one side of a brazed plate exchanger; the cold mains passes through the other side and leaves at tap temperature. A flow sensor detects the draw-off and the controller varies pump speed to hold the outlet steady within a degree or two.
Nothing is stored on the drinking-water side. That is the whole point, and it is also the limitation: the station can only deliver what the store can supply, so a 40 kW station on a store that has fallen to 55 °C will not produce a 45 °C shower at full flow.
Which is more hygienic?
The plate exchanger, unambiguously. Legionella multiplies in stored water between about 20 °C and 45 °C, and an instantaneous system stores no drinking water at all. German worksheet W 551 recognises this by treating domestic systems holding less than three litres of hot drinking water in the distribution as low risk, without a mandatory 60 °C storage regime.
A coil-based cylinder manages the same risk differently: store at 60 °C, or at 50–55 °C with a periodic pasteurisation cycle, and protect the outlets with a thermostatic mixing valve. Both approaches are accepted across Europe; the instantaneous route simply removes the reservoir instead of sterilising it. A hygienic combination tank sits between the two, using a corrugated stainless flow-through pipe inside the buffer to cut the stored drinking-water volume without a separate module.
Which is more efficient?
The coil, in a heat pump house, and by a wide margin. A plate exchanger needs roughly 10–15 K of approach temperature to deliver its rating, so a 45 °C shower demands a store above 60 °C and comfortable simultaneous outlets demand 70 °C. Every one of those degrees costs the heat source: a machine achieving a COP near 4.0 at 45 °C typically manages about 2.5 at 65 °C.
Standing loss follows the same logic, since it is proportional to the difference between store and room. A store held at 70 °C for a fresh water station loses roughly 1.7 times as much as the same vessel at 50 °C. Hard water tilts it further: scale settles on the high-flux plates of an exchanger faster than on a coil, and every millimetre of scale on a heat transfer surface costs around 7% more energy for the same output.
Which suits solar thermal and heat pumps?
For a heat pump feeding one household, use a coil in a properly sized cylinder — see what coil surface area a heat pump cylinder needs.
For solar thermal the answer depends on scale. A domestic array pairs naturally with a twin coil tank, the solar coil low and the backup high. Larger installations — apartment blocks, hotels, district schemes — favour a big buffer tank with one or more fresh water stations, because the solar loop then charges a store that never has to satisfy drinking-water hygiene rules, and peak output is decoupled from stored volume. Solimpeks supplies both routes: coil-based enamelled tanks for houses, and combination tanks where the stored drinking-water volume has to be minimised.
Frequently asked questions
Is a plate heat exchanger better than a coil for hot water?
It is more hygienic and gives higher peak output, but it needs the store held at 65–75 °C and adds a pump and controller. For a single house with a heat pump, a large coil in a well-insulated cylinder is cheaper to buy and significantly cheaper to run.
How much hot water does a fresh water station store?
Almost none. Only the water inside the plates and the pipework to the tap, typically under three litres in a domestic installation, which is the volume threshold German worksheet W 551 uses to classify a system as low legionella risk.
Do plate heat exchangers scale up in hard water?
Yes, faster than coils, because the plates run at high heat flux with a thin water film. Above roughly 200 mg/L of calcium carbonate, plan on periodic acid cleaning, phosphate dosing or softening — a 1 mm scale layer adds around 7% to the energy needed.
Can I use a fresh water station with a heat pump?
Technically yes, but it forces the heat pump to charge the store to 65–75 °C instead of 50–55 °C, cutting the COP by roughly a third or handing the last stretch to an immersion heater at a COP of 1. A coil cylinder is the better match.