How to size a heat pump hot water cylinder
Why heat pump domestic hot water sizing depends on recovery time and coil surface area, not stored volume alone, and how a slow reheat rate changes the calculation.
What this means
A heat pump reheats a cylinder far slower than a gas boiler or an immersion element, because its heat output per unit of coil area is lower at the temperatures a heat pump can deliver. Sizing therefore balances two things a boiler-era rule of thumb treats as one: stored volume, which covers a peak draw-off before recovery matters, and recovery rate, set by coil surface area and heat pump output, which covers what happens if draw-off continues after the initial store is used. Undersizing either one produces a cylinder that runs out of hot water mid-use even though its rated litre capacity looks adequate on paper.
Equipment and model context
- Indirect hot water cylinders heated by an air source or ground source heat pump
- Worked figures illustrate the method and are not a rating for any product
This explains the two variables sizing balances and why heat pump recovery differs from boiler recovery. It does not size a specific cylinder. That requires the actual heat pump's output at its design flow temperature, the cylinder manufacturer's coil performance data, and the household's actual draw-off pattern.
What this covers
- Why a cylinder sized correctly for a boiler can run out of hot water on a heat pump.
- How coil surface area, not stored volume alone, sets recovery rate.
- Why heat pump flow temperature choice trades against cylinder size.
- What a draw-off pattern actually requires from stored volume versus recovery.
What changes the result
- Copying a boiler-era cylinder size onto a heat pump installation without recalculating recovery time at the heat pump's actual output and flow temperature.
- Selecting cylinder volume alone without checking the coil's rated heat transfer area against the heat pump's output.
- Setting a lower flow temperature for heat pump efficiency without accounting for its effect on hot water recovery rate and legionella-control reheat.
- Sizing for average daily hot water use rather than the peak draw-off period that actually determines whether the household runs short.
Why stored volume alone does not answer the sizing question
Stored volume answers one question: how much hot water is available before the cylinder needs to reheat at all. It says nothing about what happens if draw-off continues, or resumes shortly after, the stored volume has been used. That second condition is exactly where a heat pump cylinder, sized the same as a boiler cylinder would have been, runs into trouble.
A gas boiler or an immersion element can push heat into a cylinder coil quickly, so a boiler-era cylinder recovers fast enough that undersizing the coil rarely mattered in practice; the stored volume alone covered most draw-off patterns. A heat pump's heat output at the coil is lower, so the same coil area takes materially longer to reheat the same volume, and a cylinder that looked adequately sized on stored litres alone can leave a household without hot water during a second draw-off later the same day.
What sets recovery rate
Recovery rate depends on the heat pump's actual heat output at its operating flow temperature, and on the coil's surface area, which governs how much of that heat can actually transfer into the stored water per unit time. A heat pump derated by a low flow temperature setting, chosen to protect efficiency, delivers less heat into the coil per hour than the same heat pump running a higher flow temperature would, directly slowing recovery.
This is why cylinder and heat pump selection are not independent decisions. A cylinder with generous coil area paired with a heat pump running an efficiency-optimised low flow temperature can still under-recover if the coil area was sized against a higher flow temperature assumption than the system actually runs at.
The flow temperature trade specific to hot water
Space heating and domestic hot water often want opposite things from flow temperature: space heating efficiency improves at a lower flow temperature, while hot water recovery and the reheat needed for legionella control both want a higher one. Many heat pump systems address this by running a higher flow temperature specifically during a hot water cycle, separate from the lower flow temperature used for space heating, which is a control strategy decision that interacts directly with cylinder sizing.
A cylinder sized assuming the system's hot water cycle runs at a higher, dedicated flow temperature will under-recover if the installed controls do not actually implement that separate cycle, which is a commissioning check as much as a sizing one.
Sizing against the actual draw-off pattern, not an average
Average daily hot water consumption is the wrong figure to size against, because a household does not draw hot water evenly across 24 hours. What matters is the peak period, a morning routine or an evening period with multiple showers or baths close together, and whether stored volume plus whatever recovers during that period covers it.
A cylinder that comfortably meets average daily demand on paper can still run cold mid-shower if its recovery rate cannot keep pace with a concentrated peak, which is the specific failure mode heat pump domestic hot water sizing exists to catch before installation rather than after a complaint.
An illustrative relationship for a 250 litre indirect cylinder reheated from 15 to 55 degrees Celsius, showing how long full recovery takes as coil surface area changes for one fixed heat pump heat output.
- A coil sized to a boiler-era standard, close to the marked point, recovers this cylinder in roughly two and a half hours at heat pump output, far slower than the same cylinder would recover from a gas boiler's higher heat input.
- Doubling coil area to 2.5 square metres cuts recovery time by less than half, because heat transfer area gives diminishing returns once the heat pump's own output, not the coil, becomes the limiting factor.
- Beyond a certain coil area the curve flattens, since heat pump output rather than coil surface becomes the constraint on how fast the cylinder can recover.
- A household with two consecutive peak draw-off periods, morning and evening, needs recovery time checked against the actual gap between them, not against an average daily figure.
| Symptom | Likely cause | What to check |
|---|---|---|
| Runs out mid-morning after a full cylinder overnight | Stored volume adequate but recovery too slow for the peak period | Recovery time at actual heat pump output and coil area |
| Adequate in the morning but short again by evening | Recovery has not caught up between the two peak periods | Gap between peak periods against calculated recovery time |
| Hot water temperature lower than the set point after reheat | Coil undersized relative to cylinder volume for this heat pump output | Coil surface area against the manufacturer's rating at the heat pump's flow temperature |
| Adequate volume and recovery but inconsistent day to day | Flow temperature not switching correctly for the hot water cycle | Control sequence for the dedicated hot water flow temperature |
Questions people ask about this
Is a bigger cylinder always the answer to a heat pump hot water shortfall?
Not necessarily. A larger stored volume helps if the shortfall is a single concentrated peak the existing cylinder cannot cover, but it does nothing for a recovery rate problem, where the cylinder runs out between two separated peak periods regardless of how much it held to begin with. Diagnosing which limit is binding, volume or recovery, decides whether a bigger cylinder actually fixes the complaint.
Does a higher-output heat pump always mean faster recovery?
Only if the coil can actually transfer that additional output into the stored water. A coil sized for a smaller heat pump becomes the limiting factor once heat pump output exceeds what the coil surface area can absorb, at which point a larger heat pump adds capacity the coil cannot use for hot water recovery specifically.
How does legionella control affect cylinder sizing?
Periodic thermal disinfection, where storage temperature runs below a level that reliably controls legionella growth on its own, needs a scheduled reheat to a higher temperature, which is an additional recovery event the sizing calculation should account for rather than treat as incidental to normal draw-off recovery.
Should an immersion element be sized as a backup for slow recovery?
An immersion element can cover an occasional peak the heat pump and coil cannot meet on their own, but relying on it routinely undermines the efficiency case for the heat pump in the first place, since resistance heating carries a materially lower coefficient of performance than the heat pump does. It is a backup for an exceptional draw, not a substitute for correct cylinder and coil sizing.
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