Recovery rate is the speed at which a hot water cylinder reheats, set by the power its coil can transfer — not by tank size. Each kilowatt of coil power heats about 21 litres of water through 40 °C per hour, so a 200-litre cylinder recovers in roughly one hour on a 9 kW coil but over three hours on 3 kW. For busy households, recovery often matters more than volume.
What exactly is cylinder recovery rate?
Recovery rate is the thermal power, in kilowatts, that a cylinder's coil heat exchanger transfers from the heat source into the stored water — it determines how quickly a drained tank becomes usable again. It is not a fixed property of the cylinder alone: the same coil transfers more power from an 80 °C boiler primary than from a 50 °C heat pump primary, because heat flow scales with the temperature difference between the coil and the surrounding water. That is why a cylinder datasheet quotes coil ratings at a stated primary temperature, and why the same tank that reheats in 40 minutes on a boiler can take two hours on a heat pump. The coil's surface area is the lever manufacturers control — the subject of what coil surface area a heat pump cylinder needs.
How do you calculate reheat time from recovery rate?
Reheat time in hours = litres × temperature rise (°C) × 1.163 ÷ (coil power in W). The constant 1.163 Wh per litre-kelvin is just water's specific heat converted to electrical units. In practice: one kilowatt-hour lifts about 21.5 litres through 40 °C, so each kilowatt of sustained coil power restores roughly 21 litres of fully reheated water per hour:
| Coil power | Water restored per hour (ΔT 40 °C) | 200 L full reheat | 300 L full reheat |
|---|---|---|---|
| 3 kW | ≈ 65 L | ≈ 3 h 6 min | ≈ 4 h 39 min |
| 6 kW | ≈ 130 L | ≈ 1 h 33 min | ≈ 2 h 19 min |
| 9 kW | ≈ 195 L | ≈ 1 h 2 min | ≈ 1 h 33 min |
| 15 kW | ≈ 320 L | ≈ 37 min | ≈ 56 min |
Worked examples with real heat pump outputs, and what an abnormally slow reheat tells you about the system, are in how long a heat pump should take to reheat a hot water cylinder.
Does recovery rate matter more than volume?
For households with repeated draws, yes — a smaller tank with a powerful coil often outperforms a bigger tank with a weak one. Volume answers only one question: how much hot water is available in a single burst. Recovery answers the more common one: how soon the next shower is possible. A 200 L cylinder on a 9 kW coil is ready again in about an hour, while a 300 L cylinder on 3 kW makes the fourth consecutive shower wait most of the afternoon — despite holding 50% more water. Volume still rules for simultaneous peak draws, which is why hotels and multi-bathroom homes size differently — see what size cylinder a multi-bathroom property needs — and remember that not all stored litres are usable anyway, as how much usable hot water a cylinder gives explains.
Why do heat pump cylinders need bigger coils to recover fast?
Because a heat pump primary arrives at only 50–55 °C, the temperature difference driving heat into 45 °C stored water is a few kelvin instead of the 30–40 K a boiler enjoys — so the coil needs several times the surface area to move the same power. Heat-pump-ready cylinders therefore carry coils of around 3 m² or more, versus roughly 1 m² in older boiler cylinders. Fitting a heat pump to an old small-coil cylinder produces the classic complaint: hours-long reheats and a compressor cycling against a coil that cannot absorb its output. As a tank manufacturer, Solimpeks enlarges the coil on its heat pump cylinders for exactly this reason — the TSE-VS HP range carries 2.1 m² at 200 L and 3.1 m² at 300 L — because the coil, not the compressor, is usually the bottleneck. What else distinguishes a compatible tank is covered in what makes a cylinder heat pump ready.
What recovery rate should I look for when buying?
Match the coil's rated power at your real primary temperature to your heat source and household rhythm: the coil rating should at least equal the boiler or heat pump output, or the source will short-cycle. Under EU Ecodesign rules (Regulation 814/2013) water heaters and tanks carry declared performance data, so ask the manufacturer for the coil rating at heat pump temperatures — not just the headline boiler figure. As a rule of thumb, a family relying on back-to-back evening showers is better served by 150–200 L with a high-power coil than by 300 L with a slow one; only sustained simultaneous demand justifies chasing volume first.
Frequently asked questions
What is a good recovery rate for a hot water cylinder?
The coil should absorb at least the full output of your heat source — commonly 3–9 kW for heat pumps and more for boilers. Each kilowatt restores about 21 litres per hour through a 40 °C rise, so match the rate to how quickly your household needs the tank back.
Is a bigger hot water tank better than a faster one?
Only when many outlets draw at the same time. For repeated sequential use — the normal family pattern — a smaller cylinder with a high-power coil is ready again sooner and loses less standing heat than an oversized slow tank.
Why does my cylinder reheat so slowly on a heat pump?
Almost always because the coil is too small for low-temperature primaries. A heat pump feeds the coil at 50–55 °C, leaving only a few kelvin of driving temperature difference, so a boiler-era 1 m² coil transfers a fraction of its rated power. Heat-pump-ready cylinders use around 3 m² or more.
How do I calculate how long my cylinder takes to reheat?
Multiply litres by the temperature rise and by 1.163, then divide by the coil power in watts to get hours. Example: 200 L × 40 °C × 1.163 ÷ 6,000 W ≈ 1.55 hours, or about 93 minutes.
Sources & further reading
- EUR-Lex — Regulation (EU) No 814/2013 (Ecodesign: water heaters and hot water storage tanks)
- Energy Saving Trust — Hot water advice