Recovery rate is the speed at which a hot water cylinder reheats after a draw-off, set by the heat input in kW rather than by tank volume. The physics is fixed: water needs 1.16 Wh per litre per degree, so a 15 kW boiler coil reheats 200 litres by 45 °C in about 42 minutes, while a 3 kW immersion element needs around 3.5 hours.
What is cylinder recovery rate?
Recovery rate describes how quickly a cylinder returns to temperature once hot water has been drawn off — effectively the kilowatts the heat source can push into the stored water. Two cylinders of identical volume can behave completely differently: one hanging off a 20 kW boiler coil recovers a shower's worth of hot water in minutes, while the same tank on a 3 kW immersion heater needs the best part of an hour for the same recovery. Volume tells you how much hot water you start with; recovery rate tells you how soon you get more.
How do you calculate cylinder reheat time?
Reheat time follows one formula: energy needed equals litres × temperature rise × 1.16 Wh, and time equals that energy divided by the input power. Reheating 200 litres from 10 °C to 55 °C therefore takes 200 × 45 × 1.16 ≈ 10.4 kWh regardless of the heat source — only the clock time changes:
| Heat source | Typical input | Time to reheat 200 L by 45 K |
|---|---|---|
| Gas or oil boiler via coil | 15–20 kW | 31–42 minutes |
| Heat pump via large coil | 5–9 kW | 1.2–2.1 hours |
| Immersion element | 3 kW | ~3.5 hours |
| Solar thermal coil | 0–7 kW (irradiance-dependent) | Weather-dependent |
Real times run slightly longer because heat exchange slows as the tank approaches target temperature. What counts as a normal heat pump reheat is examined in how long should a heat pump take to reheat hot water.
Why does recovery rate often matter more than litres?
Because households run out of hot water at moments of clustered demand, and fast recovery shrinks the gap between one demand and the next. A 150-litre cylinder recovering at 18 kW can serve back-to-back showers almost indefinitely — it regenerates a 10-minute shower's heat in roughly the time the shower takes — while a 300-litre tank on slow recovery becomes a once-a-day resource. Sizing on volume alone therefore overbuilds storage and its standing losses; sizing on the combination of stored litres and recovery kW matches the real peak draw profile. The trade-off is central to what is cylinder recovery rate and to cylinder sizing generally: EN 12831-3, the European standard for DHW heat load, dimensions systems around draw-off profiles over time, not a single volume figure.
What limits the kW a cylinder coil can transfer?
Coil surface area and the temperature difference across it. Heat flows in proportion to both, so a small coil fed at high temperature can move the same kW as a large coil fed at low temperature. That is why heat-pump-ready cylinders carry conspicuously large coil heat exchangers — often 2–3 m² and more of surface — to absorb useful power from flow at only 50–60 °C, where a boiler coil half the size works fine at 80 °C. A cylinder's quoted recovery rate is therefore only valid at its rated primary flow temperature and flow rate; connect a low-temperature heat pump to a legacy boiler coil and the deliverable kW collapses. The sizing rules are set out in what coil surface area does a heat pump cylinder need.
Frequently asked questions
What is a good recovery rate for a family home?
With a boiler, coil inputs of 15–20 kW recover a 200-litre cylinder in 30–45 minutes, which comfortably covers morning peaks. With a heat pump, 5–9 kW into a large-coil cylinder is typical, so recovery takes one to two hours — usually planned outside peak draw windows by the controller.
How do I calculate how long my cylinder takes to reheat?
Multiply litres by the temperature rise in kelvin and by 1.16 to get watt-hours, then divide by the heat input in watts. Example: 150 L × 40 K × 1.16 = 6.96 kWh; at 3 kW that is about 2.3 hours, at 15 kW about 28 minutes.
Does a bigger cylinder mean better hot water performance?
Not by itself. More litres extend the first uninterrupted draw but slow the reheat and raise standing losses. A moderately sized cylinder with strong recovery often outperforms a large, slowly recovering one — the right answer depends on how bunched your household's demand is.
Why is my heat pump's recovery slower than my old boiler's?
A heat pump delivers less power into the coil — typically 5–9 kW versus 15–20 kW — and at lower flow temperature, so heat transfer is gentler. That is normal and efficient; problems only arise if the cylinder's coil is too small for low-temperature primary flow.