At 3 kW of delivered coil power, a heat pump reheats a fully cold 200-litre cylinder from 10 °C to 50 °C in roughly 3 hours; at 8 kW, about 70 minutes. Energy in kWh = litres × temperature rise × 1.16 ÷ 1000; divide by transferred kW for hours. Much slower reheats almost always indicate an undersized cylinder coil.
How do you calculate cylinder reheat time?
With one line of physics. Heating water takes 1.16 watt-hours per litre per degree, so the energy required is: kWh = litres × temperature rise (K) × 1.16 ÷ 1000. Divide that energy by the power the heat pump actually transfers into the cylinder, and you have the reheat time in hours. A 200 L cylinder heated from 10 °C to 50 °C needs 200 × 40 × 1.16 ÷ 1000 = 9.3 kWh; a heat pump transferring 3 kW does that in 9.3 ÷ 3 ≈ 3.1 hours. The same formula works for any tank — including checking whether your cylinder is the right size for the household's morning routine.
What are typical reheat times for 200 L and 300 L cylinders?
The table assumes a full reheat from 10 °C to 50 °C (a 40 K rise) and shows the power actually delivered through the coil:
| Cylinder | Energy required | At 3 kW DHW power | At 8 kW DHW power |
|---|---|---|---|
| 200 L | 9.3 kWh | ~3 h 5 min | ~70 min |
| 300 L | 13.9 kWh | ~4 h 40 min | ~105 min |
Two practical notes. First, a full 40 K reheat is the worst case; after a normal shower draw only the bottom third of a well-stratified cylinder is cold, so everyday recovery is typically 30–60 minutes. Second, quoted heat pump capacity is not automatically delivered capacity — a nominal 8 kW unit may taper to 5–6 kW as the cylinder approaches 50 °C and the condensing temperature rises.
Why is my real reheat slower than the calculation?
Almost always because the coil, not the heat pump, is the bottleneck. A coil transfers heat in proportion to its surface area and the temperature difference between primary water and stored water; heat pumps run at 55–60 °C rather than a boiler's 70–80 °C, so that difference is small and the coil surface area must be much larger — roughly 0.2–0.3 m² per kW rather than the 0.05–0.1 m² typical of boiler-era cylinders. Push 8 kW at a 1 m² coil near the end of a reheat and it simply cannot absorb it: the heat pump raises its flow temperature, efficiency drops, and eventually the unit modulates down or trips. Other culprits: limescale coating the coil in hard-water areas, a flow temperature set too low to ever finish the reheat, and air or a lazy pump restricting primary flow. This is exactly what makes a cylinder heat pump ready — or not.
Why does the last part of the reheat take longest?
Because the driving temperature difference collapses. Early in a reheat the primary circuit at 55 °C meets stored water at 15 °C and heat pours across the coil; near the end the store is at 48 °C and the same coil transfers a fraction of the power. The heat pump is simultaneously working at its least efficient point — producing its highest flow temperature — which is why COP during DHW production is measurably lower than during space heating, a difference captured by the EN 16147 test standard for heat pump water heating performance.
What does chronically slow reheat indicate?
Work through the list in order of likelihood: an undersized or scaled coil (most common in reused boiler-era cylinders), delivered power far below nominal because the unit is also juggling space heating, primary flow restriction, or simply a target temperature the heat pump cannot efficiently reach. A reheat that has slowed gradually over months in a hard-water area points to scale; one that has been slow since commissioning points to the coil specification. And schedule reheats deliberately: heating domestic hot water in the warmest part of the afternoon can raise DHW COP noticeably compared with a 5 a.m. reheat, because source air is warmer and defrost interruptions are fewer.
Frequently asked questions
Does a bigger heat pump reheat hot water faster?
Only if the cylinder coil can transfer the extra power. Reheat speed is capped by coil surface area and the temperature difference across it; an 8 kW heat pump feeding an undersized coil delivers little more than a 5 kW one. Match coil area — roughly 0.2–0.3 m² per kW — before chasing capacity.
Is a 4-hour reheat too long?
For a full 300 L reheat at about 3 kW of delivered power, roughly 4.5 hours is exactly what physics predicts and is normal. For a 200 L cylinder or a partial top-up after a shower, 4 hours is far too long and points to a coil, scaling or flow problem.
How often should the cylinder reach 60 °C for legionella control?
Most European guidance calls for regularly achieving 60 °C in stored water — commonly via a weekly anti-legionella cycle, usually completed by an immersion heater because most heat pumps reach 50–55 °C efficiently. Follow your national health and safety guidance for the exact regime.
Why does my heat pump switch to hot water and back so often?
Most controls prioritise DHW: the unit interrupts space heating, reheats the cylinder, then returns. If it flips too frequently, widen the DHW hysteresis (start reheat at a lower stored temperature) so each reheat is a longer, more efficient run rather than many short top-ups.