Hot Water Runs Out Quickly: Cylinder Causes and Tests

Is my cylinder too small, or is something faulty?

The first question is whether anything has changed. A cylinder that always ran out after one bath is a capacity problem. A cylinder that used to last the whole evening and now gives out halfway through is a fault, and the date it began is useful information. Ask what else changed at about the same time: a new shower, an extra person in the house, a replaced boiler, a change to the heating timer, or a new mixer tap.

Capacity is easy to underestimate because of how mixing works. Stored water is held hot, at least 60°C according to HSE guidance, and you blend it with cold at the tap or shower. If the stored water is at 60°C, the cold supply is about 10°C and you want 40°C at the outlet, then about three fifths of what you run is hot. A litre of stored hot water therefore gives roughly one and two thirds litres of usable mixed water. Turn the thermostat down, or let the cylinder heat only partly, and that gain shrinks quickly. A cylinder that only reaches 45°C gives very little spare.

So a cylinder that is the correct size on paper can behave as if it is much smaller when it is not reaching temperature. That is a fault, not a sizing issue, and it is why the test below comes before any talk of a bigger cylinder. If the household has genuinely grown, cylinder sizing for flats and houses covers what to look for.

How do I test how long my cylinder takes to recover?

The recovery test needs a clip-on pipe thermometer or a simple kitchen thermometer, a clock and a quiet evening. Work through it in this order.

• Let the cylinder heat fully with nobody drawing water, and note how long the heat source runs before it switches off
• Check the dial or display on the cylinder thermostat if it has one and write down the setting
• Measure the temperature at the hot tap nearest the cylinder after running it for a minute, and write that down too
• Run a bath at a normal temperature, timing how long until the water starts to cool, and note how many taps or showers were used
• Leave the cylinder to reheat and note how long it takes to be hot again at that same tap

What are the cylinder faults that make hot water run out early?

Once you know the time and the temperature, the likely causes separate out. A tap temperature well under what the thermostat says points to the heat source. A cylinder that reaches temperature but still runs out early points to the water inside it or the way it mixes.

A failed element is the common electrical cause. Many electric and unvented cylinders have two: an upper boost element and a lower element that heats the larger volume. If the lower element has failed, the cylinder still delivers hot water from the upper portion, which then runs out after a short time and is slow to recover. This catches people out because the cylinder is not cold, it just gives only a fraction of its capacity. Immersion heater not working: what to test gives the test order for elements and thermostats.

A thermostat that is set low, or that switches off early, produces the same pattern. So does a programmer that gives the cylinder too short a window. Heat times that were set for a previous household or a previous boiler are common culprits and are worth checking before assuming anything has broken.

On an indirect cylinder, the coil takes its heat from the boiler. A coil that is scaled, airlocked or fed by a stuck two-port valve transfers less heat, and the cylinder heats slowly or only partly. See cylinder heating coil fault and two-port valve and no hot water. Hard water compounds it, because scale on the coil acts as an insulator. Cylinder water hardness and scale explains the build-up.

Sediment settles at the base of the cylinder and takes up room, which cuts the usable volume and can smother the lower element. Some cylinder designs also have a baffle or diffuser at the cold inlet so that incoming water does not mix with the hot layer above it. If that part is damaged or missing, cold water mixes in as you draw off, and the water cools much sooner than it should. That is a diagnosis for an engineer to make, because the part sits inside the cylinder.

Could the problem be at the tap or shower rather than the cylinder?

Sometimes the cylinder is fine and the cold is getting into the hot side elsewhere. A mixer shower with a failed non-return valve, a thermostatic mixing valve that is stuck, or a mixer tap with a worn cartridge can let cold water cross over to the hot pipe. The symptom is hot water that is lukewarm at one outlet and normal at another.

This is the reason the test uses a tap close to the cylinder. If that tap is hot while the shower is not, the cylinder is doing its job, and the fault is downstream. Compare two or three outlets before drawing a conclusion. A shower that has lost its pressure or temperature stability, but only at that outlet, is a shower fault.

When should I ask an engineer, and does it mean a new cylinder?

Ask for a diagnosis when the test shows the cylinder is not reaching temperature, when it runs out much sooner than it used to, or when the element, the thermostat or the cut-out keeps tripping. If you have an unvented cylinder, anything beyond checking timer settings and the thermostat dial belongs to an engineer, because the controls and safety valves are not householder work.

A diagnosis first looks at the heat source, then the stored volume, then the mixing. Replacing an element, a thermostat, a two-port valve or a coil is a repair. A new cylinder is worth considering when the cylinder is the right type but is genuinely too small for the household, or when it has several failing parts. Repair or replace sets out the decision.

The hot water cylinder diagnosis service follows the order above, and hot water cylinder not heating up is the companion page when the cylinder stays cold.