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 system, such as a combi boiler or a multipoint heater, has no cylinder or cold-water storage cistern: it heats mains water as it flows, only while a tap is open.
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 fixed maximum output, so two outlets running together share that output and each may run cooler. A combi shower that alternates warm, hot and cool while space heating works may instead be cycling on temperature: check whether outlet flow is too low or scale has restricted heat transfer in the exchanger.
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 heat pump's lower-temperature, steady output suits topping up a store over time rather than matching a combi's instant, high-temperature demand, which is why heat-pump systems usually rely on a generously sized cylinder rather than heating on demand.
Working out volume and heat-up time
Estimate a cylinder's stored volume by treating it as a plain cylinder: volume equals π × radius² × height, worked in metres, which gives cubic metres; multiply by 1,000 for litres. To find how long a heater takes to raise that volume to temperature, work out the energy needed (litres × 4.19 kJ, per litre, per °C of rise) and divide by the heater's power in kW to get the time in seconds. Real recovery is slower than this, because it ignores the heat lost while the water is warming.