Why storage and instantaneous systems behave differently under demand, what standing loss is, and how to size and time a reheat.
Storage or instantaneous: what decides how much hot water you get
A stored system heats water ahead of time and holds it in a cylinder, ready to draw off. An instantaneous combi or multipoint heater heats mains water during a draw-off without a separate cylinder. Some combis include an integrated store, so first identify the installed layout.
That difference decides how each behaves under demand. A storage cylinder can supply several outlets at once from its reservoir, because the water is already hot and waiting. An instantaneous heater has a maximum heat output, so simultaneous outlets share its capacity. Within that capacity it can hold the set temperature; beyond it, the water runs cooler or the appliance limits the flow, depending on its controls. On an instantaneous plate-exchanger combi, a shower that alternates warm, hot and cool may be cycling on temperature: measure the flow and compare the appliance and shower outlet temperatures before blaming scale or the mixer.
Standing loss and why insulation matters
Stored hot water loses heat to its surroundings even when nobody draws any off: this is called standing, or standby, loss. A storage cylinder has standing loss because it keeps a reservoir hot around the clock; an instantaneous heater avoids most of it because it holds barely any water. Good cylinder insulation cuts standing loss, so less energy is wasted reheating water that has simply cooled while waiting, and it should always be put back after any work on the cylinder.
Sizing and recovery
A cylinder holds a fixed volume and reheats, or recovers, at a limited rate. If peak demand draws hot water faster than the stored volume plus the recovery rate can replace it, the delivered water turns cool, even though nothing is actually faulty. A typical domestic heat pump reheats a store between uses, while the stored energy supplies short draw-off peaks above the heat pump’s available output. Size the cylinder’s usable volume, coil and recovery duty for the actual demand and heat source; a bigger cylinder is not always the right choice. The Hot Water Association's rule of thumb for a typical household is 35 to 45 litres of cylinder for each occupant, adjusted for how they actually use hot water: a bath takes about 100 litres at 40 °C, roughly 60 litres from a 60 °C store, and a shower about 18 litres a minute at 40 °C.
Working out volume and heat-up time
For a rough geometric estimate, treat the water space as a plain cylinder: volume = π × radius² × height. Use the water-space dimensions in metres, then multiply cubic metres by 1,000 for litres. The insulated casing dimensions do not give the stored-water volume. Select an actual cylinder from its declared storage capacity and hot-water performance data.
For an ideal whole-store heat-up, energy in kJ = litres × 4.19 × temperature rise in °C. Divide that energy by the useful heating power entering the water, in kW, to obtain seconds. Boiler combustion input is not the same as useful heat transferred to the store. This calculation assumes steady power and ignores losses.
Actual recovery also depends on changing coil duty, heat loss, stratification and the usable hot-water volume needed. Use the manufacturer's recovery figures at their stated conditions: casing dimensions or an appliance rating cannot tell you how much hot water you will actually get.