A cylinder is undersized when the last shower runs cold more than about once a week or the tank cannot recover between morning and evening peaks; it is oversized when the water is still hot after 24–36 hours of no use and standing loss dominates the bill. The right size is 30–50 L per person on a boiler, rising 30–50% with a heat pump or solar thermal.
What are the symptoms of an undersized cylinder?
Running out is the headline symptom: the third or fourth consecutive shower goes cool, a bathtub fill empties the tank, or hot water is rationed around laundry and dishwashing. The second symptom is recovery stress — the tank cannot rebuild temperature between the morning and evening peaks, so an electric boost heater cuts in at roughly three times the running cost of a heat pump. As the baseline, a household needs 30–50 litres of stored hot water per person per day, and the full sizing method lives in what size hot water tank do I need. If your routine includes bathtubs or a high-flow rain shower, size to the peak event, not the daily average.
What are the symptoms of an oversized cylinder?
An oversized tank fails silently, on the bill rather than in the shower. The signs: stored water still fully hot after a day or two away, reheat cycles that are rare but long, and standing loss — roughly 1.7–2.4 kWh per day for a typical 300 L class C tank — that rivals the energy in the water actually used. Slow turnover is also a hygiene concern, because stored volume should be exchanged regularly and pass anti-legionella cycles; a tank sized for a family of six serving a couple exchanges its volume every few days rather than daily. Persistent oversizing is worth fixing at replacement time rather than immediately — the daily kWh leak, quantified in how much heat a cylinder loses per day, rarely justifies scrapping a healthy tank.
Which symptom points to which problem?
| Symptom | Likely cause | First fix |
|---|---|---|
| Runs cold after 2–3 showers | Undersized volume — or collapsed stratification | Check sizing; check for mixing before buying bigger |
| Hot water arrives lukewarm from the start | Thermostat set low, mixing valve, or destroyed layering | Check set temperature and stratification |
| Reheat takes hours on a heat pump | Coil too small, not tank too big | See coil surface area requirements |
| Water hot after 36 h unused, high bills | Oversized or set too hot | Reduce set point; downsize at replacement |
| Gradually shrinking hot water amount | Scale build-up displacing volume and blanketing the coil | See descaling a cylinder |
When is the problem not the size at all?
Often. A tank that mixes instead of stratifying delivers lukewarm water while holding plenty of energy — high flow rates, a recirculation return piped to the wrong height or a failed diffuser are the usual culprits. A heat pump cylinder that reheats slowly usually has an undersized coil rather than the wrong volume; the physics of reheat time are laid out in how long a heat pump should take to reheat a cylinder. Thermostat position matters too: a sensor mounted low reads cold water and over-fires the heat source, while one mounted high declares victory with half the tank still cold. Rule out these free fixes before paying for litres.
When should I deliberately size up?
Three cases justify a bigger tank than the per-person rule suggests. First, heat pumps and solar thermal: both store cooler water than a boiler — heat pump efficiency falls roughly 2–3% per extra degree of storage temperature — so volume substitutes for temperature, and 200–300 L becomes the four-person norm. Second, time-shifted tariffs: charging the tank overnight or at midday solar peak needs the full day's demand stored at once. Third, planned expansion — a loft conversion or a solar retrofit later; a twin coil cylinder fitted now avoids replacing the tank twice. Solimpeks builds storage tanks from 50 to 1,000 litres precisely because the right answer spans that entire range; with water heating near 15% of household energy use (Eurostat), neither direction of sizing error is free.
Frequently asked questions
Is it worse to oversize or undersize a hot water cylinder?
Undersizing hurts daily comfort and forces expensive boost heating; oversizing costs quietly through standing loss and slow turnover. Between the two, modest oversizing is the lesser evil — but with a heat pump, deliberate oversizing by one step is often correct design.
Why does my hot water run out even though the cylinder is big enough?
Usually because stratification has collapsed and the tank is mixing to lukewarm, or scale has displaced volume and blanketed the coil. Check set temperature, flow rates and the recirculation return height before replacing the cylinder.
Should I get a bigger cylinder when switching to a heat pump?
Usually yes, by 30–50%. A heat pump stores water at 50–55 °C instead of 60–65 °C, so each litre carries less usable energy, and a family of four typically moves from 150–200 L to 200–300 L. The coil surface area matters as much as the volume.
How long should a cylinder stay hot without reheating?
A well-insulated modern cylinder loses heat slowly enough to stay usable for 24–36 hours. If yours is still fully hot after two days of absence, that is a sign of generous sizing; if it is cool by morning after an evening heat-up, suspect insulation or thermosiphoning losses rather than size.