Capacity is how much near-boiling water is stored; recovery is how fast the tank reheats after a draw. A large tank with slow recovery serves one big demand well and a series of them badly.

For a household filling a few mugs, a small tank with brisk recovery is ample. For an office kitchen at break time, or a household filling a pan for pasta, stored volume is what prevents the tap running merely hot.

Drawing beyond capacity does not stop the flow, which is the part that catches people out. The water simply arrives cooler, and the tap appears to have underperformed rather than been over-drawn.

A deliberate two-stage action on the handle: a push, a squeeze or a twist that has to be held while turning, so the tap cannot be opened by a single casual movement.

Many models add a visual indicator and a spring return, so the tap closes itself if released rather than being left running, and some offer a child-lock mode that disables boiling delivery entirely.

Outlet design contributes as well. A stream that is aerated or shaped to fall vertically into the sink rather than spreading reduces splashing, which is the most common way people are scalded by these taps.

It is held just below boiling in the tank, commonly in the high nineties Celsius, and it may briefly boil as pressure drops on delivery. It is hot enough for tea and for blanching, and hot enough to scald on contact.

Storing at a true rolling boil would vent steam continuously and waste energy, so tanks are held a few degrees below and vented safely.

Delivery temperature is adjustable on most units. Lowering it slightly extends element life and reduces standby energy, at the cost of brewing performance for some teas.

Practically always, and most systems include a filter as part of the package. The tank holds water at high temperature continuously, which precipitates hardness onto the element and the tank wall.

Scale reduces output, lengthens recovery and eventually causes element failure, and descaling a sealed under-counter tank is not a simple job.

The filter also improves taste, which matters more here than at a cold tap since the water goes straight into tea or coffee. Cartridge changes are the main routine maintenance these systems need.

A tank, a power supply and space around both. The unit needs clearance for ventilation and for service access, plus a filter housing with room to withdraw the cartridge.

A dedicated electrical supply is usual rather than a shared socket, and it needs to be positioned where it will not be splashed by the waste or a leaking trap.

Cabinet depth is the constraint people underestimate. The tank plus its connections often takes most of the depth of a standard base unit, which affects what else can live in that cupboard.

Multi-function models do, combining the boiling outlet with a conventional mixer, and increasingly with filtered ambient and chilled water as well.

The attraction is one tap and one hole. The trade is that a fault anywhere in a combined body tends to mean replacing the whole tap, and the internal complexity is considerable with several separated waterways in one casting.

Keeping the boiling tap separate from the main mixer costs a second hole and gives redundancy: a failure leaves the kitchen with a working sink.

Standby energy is the difference. A boiling tap keeps a small volume hot all day, so it uses energy when nobody is drawing water, which a kettle does not.

Against that, it heats only what is drawn rather than a kettle filled well beyond need, which is where most kettle energy is wasted. Insulation quality and tank size decide how the balance falls.

Usage pattern settles it. A household making frequent single cups usually comes out ahead; one that boils twice a day may not. Many units offer a scheduled standby mode for overnight and holidays, which removes most of the idle cost.