Size from simultaneous and sequential peak draw-off, not from house size or occupancy alone. Coil surface area determines reheat rate and matters as much as volume, and a heat pump needs a substantially larger coil than a boiler.
Peak, not average
A household’s average daily hot water use is not a useful design figure. What determines whether a cylinder is adequate is the peak – the morning, when demand is concentrated into a short window.
The questions that actually matter:
- How many showers, and are any of them simultaneous? Two at the same time is a completely different requirement from two in sequence.
- What kind of showers? A high-flow mains-pressure shower can use several times what a gravity one does. A rain head with body jets is a different order again.
- Is there a bath, and is it used? A full bath is a single large draw-off.
- How long between the first and last draw-off? A cylinder reheats during the morning. Thirty minutes of reheat between showers changes the answer substantially.
- What is the heat source and how fast does it reheat? This is where the coil comes in.
- Is there a second peak? An evening bath routine with children, for instance.
The coil, which matters as much as the volume
This is the part most often overlooked, and it explains why two cylinders of identical capacity can perform completely differently.
The indirect coil is the heat exchanger between the boiler’s water and your stored water. Its surface area determines how quickly heat transfers – which determines the reheat rate. A cylinder with a small coil reheats slowly regardless of how powerful the boiler is, because the bottleneck is the coil rather than the heat source.
| Small coil | Large coil | |
|---|---|---|
| Reheat rate | Slow | Fast |
| Effective capacity over a morning | Roughly the stored volume | Stored volume plus what it recovers during the peak |
| Suits | Low demand, or storage-led design | High or concentrated demand |
| Cost | Lower | Higher |
| Heat pump suitability | Poor – cripples the heat pump | Essential |
A cylinder that reheats quickly effectively has more capacity over a busy morning than a larger one that reheats slowly. Before assuming you need a bigger cylinder, it is worth establishing whether the existing one is reheating as fast as it should – a scaled coil in hard water produces exactly this symptom.
Heat pumps change the arithmetic entirely
A boiler delivers water to the coil at a high temperature, so a modest coil transfers plenty of heat. A heat pump delivers at a much lower temperature, so the temperature difference driving heat through the coil is far smaller.
To transfer the same heat with a smaller temperature difference, you need substantially more surface area. That is why heat pump cylinders have very large coils – frequently several times the area of a standard boiler cylinder – and why fitting a heat pump to an existing boiler cylinder is one of the most common causes of a heat pump that disappoints.
If a heat pump is in prospect now or later, it is worth considering a heat-pump-rated cylinder at the point you replace, even while still running a boiler. It works perfectly well with a boiler and it removes a future obstacle. See cylinder replacement.
Other sizing factors
- Storage temperature. Storing hotter means more usable heat in the same volume, because it is blended down at the outlet. It also accelerates scale formation in hard water and requires a blending valve for safety. Store at the correct temperature and blend, rather than storing cool.
- Blending valves. A cylinder stored hot and blended down at the outlet effectively delivers more litres of usable water than its nominal capacity. This is a legitimate way to get more from a given cylinder and it is the standard modern approach.
- Standing losses. A larger cylinder loses more heat continuously. A well-insulated modern cylinder loses far less than an old one, but volume still costs something every hour of every day.
- Physical space. The airing cupboard sets a limit. Taller and narrower is frequently available where floor area is tight.
- Weight. A large cylinder full of water is a substantial load. On an upper floor this occasionally matters.
- Secondary circulation. In a large house with long runs, a secondary return keeps hot water at the outlets – convenient, and a continuous heat loss, and it has legionella implications.
- Solar or a second heat source, which needs a twin-coil cylinder – only worth it if the second source genuinely exists or is planned.
Getting it wrong in each direction
Undersized
Runs out during the morning peak. The household adapts by staggering showers, which is an ongoing quality-of-life cost nobody accounted for. Usually discovered in the first winter.
Oversized
Higher standing losses, continuously, whether or not you use the water. Takes longer to reheat from cold. Costs more and takes more space. Also a legionella consideration – a large cylinder in a low-demand household turns over slowly, and low turnover in stored water is precisely the condition that matters.
Oversizing is a milder fault than undersizing, and it is not free. The right answer is neither – it is a calculation from actual peak demand.
Running out of hot water? Check the cylinder thermostat and the reheat rate before concluding it is too small.
Questions we get asked about this
What size hot water cylinder do I need?
It depends on peak demand – how many showers, whether any are simultaneous, what type, and how fast the cylinder reheats between them. House size and occupancy alone are poor guides.
Why do we run out of hot water?
Either the cylinder is genuinely too small for your peak, the thermostat has drifted down, or the reheat rate is poor – which in hard water is frequently a scaled coil. Check the cheap possibilities first.
Does coil size really matter?
Considerably. A cylinder with a large coil reheats fast and therefore has more effective capacity over a busy morning than a bigger one that reheats slowly. It is the most overlooked factor in cylinder selection.
Do I need a different cylinder for a heat pump?
Almost certainly. Heat pumps deliver at a much lower temperature, so they need a far larger coil area to transfer the same heat. A standard boiler cylinder will cripple a heat pump.
Should I store hot water at a higher temperature?
Store at the correct temperature for legionella control and blend down at the outlets with a thermostatic mixing valve. That gives you more usable water and it is safe. Storing cool to avoid scalding is the wrong solution to the right concern.
Is a bigger cylinder always safer?
No. Oversizing means higher standing losses continuously, and in a low-demand household it means slow turnover in stored water, which is a legionella consideration. Correct sizing beats generous sizing.
Need this doing?
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