Wednesday 3rd December 2025
How to Choose the Right Hot Water Cylinder Size
A correctly sized hot water cylinder gives reliable hot water when you need it while keeping running costs low. This guide explains the difference between vented and unvented cylinders, how to estimate capacity, placement and space considerations, and how sizing impacts energy use.
Why cylinder size matters
Choose a cylinder that matches your household’s demand. A cylinder that’s too small risks cold showers during peak times; a cylinder that’s too large wastes energy heating water you don’t use. The right size keeps hot water available, improves comfort and helps reduce energy bills.
Vented vs unvented cylinders
Vented cylinders
Also called gravity-fed systems.
- Fed from a cold water storage tank (usually in the loft).
- Water pressure depends on gravity — pressure can be lower than mains pressure.
- Common in older or conventional central heating systems; can include an immersion heater as backup.
Unvented cylinders
Connected directly to the mains water supply.
- Deliver hot water at mains pressure — better shower performance without extra pumps.
- No loft cold water tank required, saving space.
- Require pressure-rated components and safety controls because of higher system pressure.
How to estimate the correct capacity
Start by estimating household hot water demand. A commonly used rule of thumb is 35–45 litres of hot water per person for typical domestic use. Use the guidance below to refine that figure.
Typical sizing guide
| Household | Typical cylinder size | Notes |
|---|---|---|
| 1 person / 1 bedroom | 100–125 L | Light usage: fewer showers, short baths. |
| 2 people / 2 bedrooms | 125–160 L | Two-person household with regular showers. |
| 3–4 people (family) | 180–300 L | Peak morning/evening use; consider the higher end if you have a bath + showers. |
| Large households / multiple bathrooms | 300 L+ | Consider multiple cylinders or high-output unvented systems for simultaneous use. |
Refining the estimate
- Per person: use 35–45 L per person as a baseline and adjust for bathing habits.
- Showers vs baths: baths use much more hot water — add 30–80 L for each bath during peak times.
- Appliances and underfloor heating: increase demand; factor them in.
- System efficiency: high-performance boilers and indirect cylinders may allow slightly smaller capacities.
Location and installation considerations
Where you place the cylinder affects pipe runs, heat loss and pressure. Think about access for maintenance and whether a loft tank is needed for vented systems.
Placement tips
- Vented cylinder: must sit below the cold water storage tank, so loft location matters.
- Unvented cylinder: more flexible — can be installed almost anywhere with suitable support and access.
- Proximity to the boiler: indirect cylinders benefit from being near the boiler to reduce pipework and heat loss.
- Service access: allow space for valves, expansion vessels and annual servicing.
Space and layout checklist
- Measure available floor-to-ceiling height and floor area where the cylinder will go.
- For loft installations, ensure there's room for the cold water tank and safe access.
- Allow clearance for servicing and replacement of safety components.
Energy efficiency & cost implications
Match cylinder capacity to actual demand to avoid unnecessary heating. Good insulation, correct controls and an energy-efficient model all reduce running costs.
What to look for
- Insulation: thicker jackets and low heat-loss designs keep water hotter for longer.
- Controls: timers, thermostats and smart controls reduce energy use.
- Energy ratings: choose high-rated models where available.
- Safety features: pressure relief valves and correctly sized expansion vessels are essential on unvented systems.
Tip: A 100-litre oversize can raise standby heat losses and running costs. Size conservatively and improve insulation if needed.
Common questions (FAQ)
Can a smaller unvented cylinder replace a larger vented one?
Sometimes — unvented cylinders deliver mains pressure and often higher flow rates, so you may get similar shower performance from a smaller unit. Always check peak demand and consult an installer.
How often should a cylinder be serviced?
Unvented cylinders require annual inspections of safety devices; vented systems should be checked periodically as advised by the installer or manufacturer.
Is an immersion heater enough?
An immersion heater is useful as backup but is generally less efficient than using the boiler for large-volume heating. Use the immersion as emergency or occasional top-up.
Making the final choice
Decide using three steps:
- Estimate household demand (35–45 L per person).
- Choose between vented (loft tank) and unvented (mains pressure).
- Factor in plumbing layout, insulation, controls and safety requirements.
If you’re unsure, an installer can measure your peak demand, inspect your heating system and recommend the most efficient cylinder for your home.
Need help choosing or installing?
Choosing the right cylinder depends on habits, plumbing layout and the heating system — all things our Tucker French specialists handle every day. For an accurate recommendation and professional installation, speak to a qualified installer.