A practical sizing guide for M&E engineers and specifiers working on hotels, schools, care homes and healthcare premises.

If you haven’t already, it’s worth starting with the complete guide to commercial hot water cylinders, this post digs into one specific piece of that picture, getting the size right first time.

Why Undersizing Is the Expensive Mistake

Undersized cylinders are one of the most common, and most expensive, mistakes on commercial projects. Not because the cylinder itself is cheap to fix, but because the fix usually means ripping out and replacing rather than topping up. A cylinder that’s a little too small doesn’t fail gracefully, it fails at exactly the worst moment, the middle of a hotel’s morning checkout rush or a school’s mid morning break, when every outlet wants hot water at once and there simply isn’t enough stored to give it to them.

Oversizing has its own cost, just a quieter one. A bigger cylinder than the building needs still has to be kept at temperature around the clock for legionella control, and that standing heat loss runs up the energy bill every single day the system sits idle between draws. Business electricity rates in the UK average 27.0 to 30.0p per kWh, while business gas rates sit at 9.0 to 9.5p per kWh (MoneySuperMarket, Bionic panel data, June 2026), so an oversized electric system in particular racks up wasted cost fast. Getting the size right first time avoids both failure modes.

The Three Numbers That Actually Matter

Proper sizing works from three figures, not one.

Peak demand is the maximum volume of hot water needed in the worst fifteen or thirty minute window of the day, not the daily total. A building can have a modest daily litre count and still fail badly at sizing if that demand arrives in one sharp spike rather than spread evenly across the day.

Recovery rate is how fast the heat source can reheat the stored volume once it’s been drawn down. This is where the heat source itself starts to matter as much as the cylinder. A boiler with a high output recovers quickly. A heat pump, delivering heat at a lower flow temperature, recovers more slowly for the same kW rating, which is exactly why heat pump fed cylinders need a different specification approach, more on that below.

Storage volume is the buffer that sits between peak demand and recovery rate, big enough that a spike in demand doesn’t outrun the system’s ability to keep up, but not so big that standing losses become the dominant cost.

Get all three talking to each other properly and the system disappears into the background. Get just one wrong and you either run out of hot water or pay to keep too much of it hot.

Peak Demand Looks Different in Every Sector

This is where generic per person litre figures fall apart, because commercial demand patterns vary enormously by building type.

Hotels concentrate demand into two sharp windows, a heavy one in the morning as guests shower before checkout and a smaller one in the evening. England’s hotel occupancy held stable across the whole of 2025 at 79%, on par with 2024 (VisitEngland, England Hotel Occupancy Survey), which gives a reasonably steady basis for demand modelling, but the peak itself is still driven by how many of those rooms are occupied on any given night and how tightly guest departures cluster around checkout time. Hot water accounts for roughly 30 to 35% of total energy use in a full service hotel, rising further where laundry and spa facilities are on site (nzero), so getting the peak demand figure right has a direct and sizeable impact on the energy bill, not just on guest comfort.

Schools compress demand into short, sharp windows too, but for a different reason, form time, break, and lunch, when hundreds of pupils wash their hands within a few minutes of each other. Average infant class size in England fell to 26.2 pupils in 2024/25, the lowest figure since 2009 (GOV.UK, Schools, pupils and their characteristics 2024/25), and multiplying that across a primary school’s intake gives a genuinely useful starting point for estimating peak simultaneous draw at the sinks nearest each classroom block, rather than relying on a single school wide average that smooths the peaks away.

Care homes and healthcare premises look different again, demand stays fairly constant across a long day rather than spiking sharply, because residents and patients are being washed and cared for on a rolling basis rather than all at once. That steadier profile actually makes recovery rate more important relative to peak buffer than it is in a hotel or school, because the system needs to keep pace with continuous draw rather than surviving one short, brutal peak.

How the Heat Source Changes the Sizing Calculation

The heat source isn’t just a separate decision from sizing, it directly changes the numbers. UK heat pump sales hit a record 125,037 units in 2025, up 27% on 2024, with domestic hot water heat pump sales alone growing 36% year on year (Heat Pump Association), and every one of those installations needs a cylinder sized differently to how it would be for a boiler.

A heat pump delivers heat at a lower flow temperature than a gas or oil boiler, which means the coil surface area needs to be larger to transfer the same amount of heat in the same amount of time, and the overall stored volume typically needs to be bigger to compensate for the slower recovery rate. Specify a heat pump against a cylinder that was sized for boiler style recovery and the system will technically work, it just won’t keep up with demand the way the building needs it to. This is one of the most common questions we get asked directly, and it’s worth getting an accurate coil sizing calculation before committing to a heat pump project, not after the cylinder has already been ordered.

Legionella control adds a constraint that applies regardless of heat source. HSE guidance requires stored hot water to be kept at 60°C or above, and to reach outlets at 50°C, or 55°C in healthcare premises, within one minute (HSE, Hot and cold water systems). A heat pump struggling to reach and hold 60°C storage temperature isn’t just an efficiency problem, it’s a compliance problem, so the sizing calculation and the legionella strategy need to be worked out together, not bolted together afterwards.

Making Sense of Litre Sizes

Commercial cylinders are commonly specified in round litre figures, 500, 600, 1000, 1500 litres and upward, and it helps to know where the regulatory thresholds sit against those numbers. Building Regulations Approved Document G3 requires any unvented hot water storage system over 15 litres to be installed, commissioned and notified by a competent, qualified person (GOV.UK, Approved Document G), so essentially every commercial cylinder in this size range falls under that requirement, there’s no meaningful size of commercial system that sits below it.

At the other end of the scale, unvented systems over 500 litres capacity are usually bespoke designs, and are not appropriate for a standard third party accredited product conformity certification scheme (GOV.UK/Defra, Approved Document G3 technical guidance). In practice that means a 500 litre cylinder or smaller can typically be specified as an off the shelf, third party certified product, while anything above that starts to move into bespoke engineered territory, designed for the specific building rather than pulled from a standard range. That’s worth knowing early in a project, because bespoke systems carry longer lead times and need earlier engagement with a specifier or manufacturer’s design team.

It’s also worth keeping an eye on where the demand assumptions behind these figures are heading. A government consultation running from September to December 2025 proposes tightening the Part G water efficiency standard for new buildings from 125 litres per person per day to 105 litres per person per day (GOV.UK/Defra, Water Efficiency Standards consultation). That’s a domestic dwelling figure rather than a commercial one directly, but it signals the direction of travel on design demand assumptions generally, and commercial guidance tends to follow domestic regulation with a lag. Sizing calculations done today should account for the fact that per person demand assumptions are trending down, not up.

A Practical Decision Matrix

Rather than working from a single per person figure, work through these in order for any commercial project.

Identify the demand pattern. Is it sharp and peaked, like a hotel or school, or steady and continuous, like a care home? This decides whether you’re optimising for buffer volume or for recovery rate.

Calculate peak draw, not daily total. Model the worst fifteen to thirty minute window specifically, using occupancy or pupil numbers relevant to the building, not a generic per person average.

Match recovery rate to heat source. A boiler fed system can lean more on recovery rate and less on storage volume. A heat pump fed system needs more storage and more coil surface area to compensate for slower recovery.

Check the regulatory thresholds. Anything over 15 litres needs G3 qualified installation. Anything approaching or exceeding 500 litres is likely to be a bespoke design rather than an off the shelf product, and needs to be programmed into the project timeline accordingly.

Confirm legionella compliance is achievable at the sizing and heat source combination you’ve landed on, not as an afterthought once the cylinder has been chosen.

England and Wales’ non domestic building stock covered by government energy data runs to 1,755,000 buildings (GOV.UK/DESNZ, ND-NEED 2024), and no two of them have quite the same demand profile. That’s the whole reason a decision matrix beats a lookup table, the right answer depends on how the building actually behaves, not on which category it falls into on a spec sheet.

Get It Right Once

Sizing a commercial hot water cylinder properly means resisting the temptation to reach for a single number and move on. Peak demand, recovery rate and storage volume all pull against each other, and the heat source changes the relationship between them. Get the calculation right at the start and the system quietly does its job for the next fifteen or twenty years. Get it wrong and you’re either replacing an undersized cylinder within a couple of years or paying every day for standing losses on one that never needed to be that big in the first place. It’s worth the extra hour at the specification stage.