Vented vs Unvented Cylinders: Which Is Right for Your Home?

A vented hot-water cylinder is supplied from a cold-water storage cistern, usually in the loft. An unvented cylinder takes its cold-water supply directly from the mains. The right choice depends on your water supply, hot-water demand, available space and installation requirements.

Neither option is automatically best for every home. An unvented cylinder can remove the need for the loft cistern, but it cannot make an inadequate incoming mains supply deliver unlimited water. A working vented system may remain a sensible choice where its performance meets your needs.

Vented vs unvented: the main differences

• Water supply: a vented cylinder is fed by a storage cistern; an unvented cylinder is mains-fed.

• Pressure: vented hot-water pressure depends on the height of the cistern above the outlet. Unvented performance depends on the incoming supply and the system's controls and pipework.

• Space: both need room for the cylinder. A vented arrangement also needs its supply cistern; an unvented installation needs suitable space for its safety and expansion arrangements.

• Installation: changing between systems can involve pipework and discharge-route changes, not just swapping the cylinder.

• Maintenance: unvented safety devices require regular professional checks. Include servicing in the ownership comparison.

Heatrae Sadia's explanation of unvented cylinders describes mains feeding, expansion arrangements and the need for suitably qualified installation.

What is a vented hot-water cylinder?

A vented cylinder stores water for your hot taps and showers. Water from the supply cistern replaces what you use, while an open vent provides part of the expansion and safety arrangement.

Gravity provides the driving pressure. As a useful physical guide, one metre of vertical water head is approximately 0.1 bar before losses through pipes and fittings. The relevant height is above the outlet, not the number of storeys in the house. This helps explain why an upstairs shower close to the cistern's water level may feel less powerful than a lower tap.

A suitable shower pump may be an option in a properly designed vented installation, but pump selection and pipework need assessment. A pump is not a universal fix for every low-flow problem.

When keeping vented can make sense: the existing arrangement works well, the cistern and pipework are in good condition, and a replacement can meet your needs without a larger system conversion. Ask an installer what the actual problem is before paying to change the system type.

What is an unvented hot-water cylinder?

An unvented cylinder stores hot water under pressure from a controlled mains-fed supply. It uses a designed combination of expansion provision, temperature controls and relief devices rather than an open vent to a cistern.

It can free up loft space and provide good performance when the incoming supply is suitable. However, the cylinder still needs an appropriate location, access for maintenance and a compliant safety-discharge arrangement. “No loft tank” does not mean “no installation constraints”.

Ask the installer to check the water supply under realistic demand. A static pressure reading with every tap closed does not show the flow available when several outlets are running. Heatrae Sadia's cylinder FAQs provide further manufacturer guidance on suitability and professional support.

Direct vs indirect is a separate choice

Direct describes water heated by an element in the cylinder, such as an electric immersion heater. Indirect describes heat transferred from another heat source through a coil or heat exchanger. An indirect cylinder may also have an immersion heater for backup.

These words explain how the water is heated. Vented and unvented explain the supply and pressure arrangement. For example, an indirect unvented cylinder is mains-fed and heats its stored water using a separate heat source. It is not a contradiction.

Heatrae Sadia's heating glossary explains these terms. If a quote simply says “indirect cylinder”, ask whether it is vented or unvented and which heat source it is designed to work with.

Which gives better showers?

The answer depends on the whole system. Ask about the measured flow and pressure available at the property, the shower's requirements and what happens when another outlet runs. The cylinder's brand or size in litres does not answer all of those questions.

For a household that uses two showers together, discuss simultaneous demand explicitly. A larger store of hot water helps with available volume, but it does not on its own increase the incoming mains flow. For a home with one modest-demand shower, a major conversion may offer less benefit than expected.

Describe the problem precisely: weak flow, water running cold, temperature fluctuations or too little stored hot water. Each points towards a different assessment. Our shower temperature guide covers combi-specific symptoms if you discover that your home does not use a cylinder.

What size cylinder do you need?

Start with how you use hot water, not a fixed litres-per-bedroom rule. Tell the installer the number of occupants, bath and shower use, whether outlets run together, the heat source and when you need hot water available.

The recommendation should explain both stored capacity and recovery time: how quickly the heat source can replenish hot water after use. A cylinder selected for a boiler may not be the right specification for a future heat pump. If you plan to change the heat source, ask about compatibility before ordering.

Also ask about standing heat loss, insulation and access. Buying the largest cylinder that fits can use more space and may not be the most appropriate solution to your household's demand.

How do installation costs compare?

A like-for-like cylinder replacement and a vented-to-unvented conversion are different jobs. A meaningful price needs the cylinder specification and the work included; a cylinder-only retail price should not be presented as a complete installation cost.

• Confirm the model, capacity, heat source and included controls.

• Ask whether removing a loft cistern, altering pipework or improving the water supply is included.

• Check the location, support, access, expansion components and safety-discharge work.

• Include commissioning, required documentation, disposal, applicable taxes and future servicing.

Get both options quoted for the same property. That shows whether retaining vented, converting to unvented or changing another part of the system offers the best value. Running costs also depend on the heat source, tariff, insulation and use; the vented/unvented label alone does not determine the bill.

Servicing and safety

Heatrae Sadia recommends annual servicing of its unvented direct and indirect cylinders, including checks of the safety controls and expansion arrangements. Follow the instructions for your model and keep records of the work. See its cylinder servicing guidance.

Use a professional competent in unvented hot-water systems for an unvented installation. Gas Safe registration is relevant for gas work but should not be treated as proof of every other qualification. Ask the installer to explain the certification and notification arrangements for your job.

Do not block a vent, safety outlet or tundish discharge, or adjust safety valves to stop water escaping. Persistent discharge, unusual overheating or leaks need professional attention. The tundish provides a visible indication of discharge; it is not an overflow to cap off.

Can you convert a vented cylinder to unvented?

You cannot turn an ordinary vented cylinder into a pressure-rated unvented one by closing the vent or connecting mains pressure. A conversion means selecting suitable equipment and redesigning the relevant installation, with the necessary safety components.

Before agreeing, ask: is the mains supply suitable, where will the discharge run, how much space is needed, what work is included and what improvement should I expect? Those answers are more useful than a general claim that unvented is always better.

Choosing the right system for your home

Keep a satisfactory vented arrangement on the shortlist; consider unvented when its space and performance benefits suit the property and the supply supports it. Compare direct and indirect heating separately, then choose capacity and controls for the household.

If the cylinder decision is part of replacing your boiler, iHeat can help you consider a suitable boiler package. Read our boiler type guide before starting a quote, and explain any planned cylinder changes during the assessment.