Sizing an unvented cylinder?
Sizing an unvented cylinder correctly is important because the cylinder must provide enough stored hot water for the property or building without being unnecessarily large, expensive to heat, or wasteful to run. An unvented hot water cylinder is connected directly to the mains water supply and provides mains pressure hot water to outlets such as showers, baths, basins and kitchen taps. Unlike a vented cylinder, it does not require a cold water storage tank in the loft.
Hot water demand
The first consideration when sizing an unvented cylinder is hot water demand. This means looking at how many people live in the property, how many bathrooms there are, whether there are baths or showers, and whether multiple outlets are likely to be used at the same time. A one-bedroom flat with one shower will need a much smaller cylinder than a large family house with several bathrooms, a large bath and high morning hot water use.
Unvented Cylinder Sizing Chart
What size unvented cylinder do I need?
Indirect unvented cylinders have a heat exchanger coil, which will be powered by a boiler or heat pump (they also have an immersion heater fitted for backup water heating)
Direct unvented cylinders are immersion heater powered, so these are electrically powered only domestic hot water heaters
| Property / Application | Typical Occupancy | Typical Hot Water Demand | Indirect Unvented Cylinder Size | Direct Unvented Cylinder Size |
|---|---|---|---|---|
| Studio flat / small office washroom | 1 person | Basin, kitchen sink, occasional shower | 80–100 litres | 80–120 litres |
| 1-bedroom flat | 1–2 people | 1 shower room, kitchen sink | 100–120 litres | 120–150 litres |
| 2-bedroom flat / small house | 2–3 people | 1 bathroom or shower room | 120–150 litres | 150–180 litres |
| 3-bedroom house | 3–4 people | 1 bathroom, regular shower/bath use | 150–180 litres | 180–210 litres |
| 3-bedroom house with higher demand | 3–4 people | Bath, shower, possible simultaneous use | 180–210 litres | 210–250 litres |
| 4-bedroom house | 4–5 people | 2 bathrooms or en-suite use | 210–250 litres | 250–300 litres |
| 5-bedroom house | 5–6 people | Multiple bathrooms, higher peak demand | 250–300 litres | 300–400 litres |
| Large house / high-demand property | 6+ people | Multiple showers, baths, en-suites | 300–500+ litres | 400–500+ litres |
| Small commercial washroom | Variable | Basins, light hot water use | 100–200 litres | 150–250 litres |
| Salon, small changing room, sports use | Variable | Repeated hot water draw-off | 250–500+ litres | 350 litres to larger |
For commercial hot water heating sizing, we highly recommend you read this webpage, which takes you through the sizing process to ensure you have the correctly sized hot water heater – How to size a commercial water heater
Our range of 80- to 500-litre indirect unvented cylinders from Flexiheat UK are detailed on this dedicated webpage Unvented indirect hot water cylinders
We also supply an indirect high recovery cylinder range for even better DHW heating performance; these are detailed here High recovery cylinder range
or our direct unvented cylinders, ranging from 80 to 500 litres, are detailed on this webpage direct unvented hot water cylinders
Common mistakes to watch out for
A common mistake is to size the cylinder only by the number of bedrooms. This can give a rough starting point, but it is not always accurate. The user or installer should also consider the real usage pattern. For example, two people who mainly shower may need less stored hot water than a household that regularly fills large baths. Properties with power showers, rainfall showers, large baths or multiple en-suites will normally need a larger cylinder and a good incoming mains water supply.
The good “rules of thumb” as such for hot water cylinders are
In a typical home, the average hot water consumption is 30 to 50 litres of warm water per person per day.
A low-consumption household will require 20–30 litres of warm water per person per day.
In a high-consumption residence, 50–70 litres of warm water per person per day is recommended.
Why direct cylinders are often sized larger
A direct unvented cylinder is heated by electric immersion heaters. Because immersion heaters usually have a lower heat input than a boiler coil, recovery time is often slower. For this reason, a direct cylinder may need a larger stored volume to cope with peak hot water periods.
Why indirect cylinders can sometimes be smaller
An indirect unvented cylinder is heated by a boiler, heat pump, electric boiler or other external heat source through a coil inside the cylinder. If the coil rating and heat source output are large enough in output, the cylinder can recover more quickly, so a slightly smaller storage volume may be suitable.
Incoming mains cold water supply
The incoming mains pressure and flow rate are critical. An unvented cylinder can only deliver strong hot water performance if the cold mains supply is sufficiently adequate. If the incoming flow rate is poor, fitting a larger cylinder will not automatically solve the problem. The cylinder may store enough hot water, but the system may still struggle to supply multiple outlets at a good flow rate. This is why a proper flow and pressure test should be carried out before specifying the cylinder.
Recovery time
The next factor is recovery time. Recovery time is how long the cylinder takes to reheat after hot water has been used. An indirect unvented cylinder is heated by a boiler, heat pump or other external heat source through a coil inside the cylinder. A direct unvented cylinder is heated by one or more electric immersion heaters. If the heat source is powerful and the cylinder coil is correctly rated, a smaller cylinder may recover quickly enough for the application. If the heat input is low, a larger volume of stored water may be required to avoid running out.
The type of heat source also affects sizing. A gas or oil boiler may be able to reheat a cylinder relatively quickly. An electric immersion heater will usually recover more slowly, depending on the immersion heater rating. A heat pump cylinder may need a larger coil or dedicated heat pump cylinder design because heat pumps usually operate at lower flow temperatures than traditional boilers.
Stored domestic hot water temperatures
Stored water temperature is another important design point. In UK hot water systems, legionella control must be considered. HSE guidance states that hot water should be stored at least at 60°C and distributed so that it reaches 50°C at outlets within one minute, or 55°C in healthcare premises. This does not mean every outlet should discharge at unsafe temperatures; thermostatic mixing valves will be required, especially for baths, showers, care settings or vulnerable users.
Hot water cylinder installation factors
The physical location of the cylinder also matters. The chosen cylinder must fit in the available space and allow access for servicing, immersion heaters, controls, expansion vessel, safety valves, tundish and discharge pipework. A full unvented cylinder is very heavy, so the floor structure must be suitable, especially in lofts or upstairs cupboards. Approved Document G covers standards for cold water supply, hot water supply and hot water safety in England, so the installation must be designed and installed correctly.
Efficiency
Efficiency should also be considered. Oversizing the cylinder can increase standing heat loss because more stored water is kept hot than the property actually needs. Modern cylinders are usually factory insulated, but correct pipe insulation, controls and time scheduling are still important.
Conclusion
As a general guide, smaller flats may only need a compact cylinder, while family homes with two bathrooms may need a medium-sized cylinder, and larger homes or commercial properties may require a much larger capacity or multiple cylinders. However, the correct size should always be based on hot water usage, incoming mains performance, heat source output, recovery rate and available installation space.
In simple terms, sizing an unvented cylinder is about balancing stored hot water capacity, recovery time, mains water flow, safety requirements and energy efficiency. The best cylinder is not always the largest one, but the one that reliably meets the property’s hot water demand without wasting space, heat or running costs.
