Hot Water Cylinder Types: Vented vs Unvented Guide

Summary

Every domestic hot water cylinder falls into one of a small number of fundamental types, and the type determines not just how the cylinder performs day-to-day but who is legally allowed to install it, what regulatory sign-off is required, and what can go wrong if it's specified incorrectly. Confusing "vented" and "unvented" with "direct" and "indirect" is the most common source of mis-specification: vented/unvented describes how the water is pressurised (gravity versus mains), while direct/indirect describes how the water is heated (immersion element only versus a coil fed by a boiler or heat pump). A cylinder can be any combination of the two — indirect vented, indirect unvented, direct vented, direct unvented — and each combination has a distinct place in UK domestic plumbing.

This article focuses specifically on the cylinder types themselves — their construction, materials, coil configuration, and the practical differences a tradesperson needs to explain to a customer choosing between them. For the regulatory detail of unvented cylinder installation (G3 qualification, mandatory safety devices, discharge pipework), see unvented cylinders. For sizing a cylinder to a household's demand, see domestic hot water cylinder sizing. For choosing between cylinder types as part of a wider boiler/heating system decision, see cylinder selection.

The core trade-off customers need to understand is this: vented systems are simpler, cheaper, and unregulated, but deliver hot water at whatever pressure gravity provides from the loft tank — often inadequate for a modern power shower without a booster pump. Unvented systems deliver strong, even mains-pressure hot water to every outlet simultaneously, which is what most customers actually want, but they are a genuinely dangerous appliance if the safety devices fail, which is precisely why the law restricts who can install and service them.

Key Facts

• Vented cylinder — stored water is open to atmosphere via a vent pipe running to a cold water storage cistern (CWST) in the loft; pressure at the tap equals the "head" (vertical height) of water above it, typically producing 0.1–0.5 bar in a domestic house (roughly 0.1 bar per metre of head)
• Unvented cylinder — sealed system connected directly to the mains cold supply; stores water at mains pressure (typically 2–5 bar), delivering strong flow at every outlet without a pump
• BS 1566 — the product standard for indirect vented copper cylinders; the traditional UK domestic hot water vessel
• BS EN 12897 — the product standard for unvented (closed) hot water storage systems
• G3 notifiable work — any unvented cylinder over 15 litres requires a G3-qualified installer and Building Control notification (Approved Document G, Section G3); a vented cylinder requires neither
• Direct cylinder — heated by an internal electric immersion element only; no coil, no boiler connection
• Indirect cylinder — heated by a coil (or coils) through which hot water from a boiler or heat pump primary circuit circulates; almost always fitted with a backup immersion element too
• Single coil vs twin coil — a single-coil cylinder has one heat source (typically the boiler); a twin-coil cylinder has a second, usually lower, coil for a secondary source such as a solar thermal array
• Cylinder material — copper (traditional, BS 1566, generally vented), duplex/marine-grade stainless steel or copper (common for unvented, pressure-rated), and glass-lined or enamelled mild steel (budget unvented and some heat pump cylinders)
• Cylinder insulation — factory-fitted foam insulation (modern standard, low standing heat loss) versus a fitted lagging jacket on an older bare copper cylinder (higher heat loss, retrofit option)
• Thermal store — a mains-pressure alternative to a true unvented cylinder; the stored water is primary heating water, and domestic hot water is drawn through a plate heat exchanger or internal coil, achieving mains pressure without full G3 status on some designs
• Combination (combi) boilers have no cylinder — hot water is heated instantaneously on demand; relevant to mention when a customer asks about "getting rid of the cylinder" as part of a boiler upgrade
• Heat pump cylinders — need a coil with significantly greater surface area (typically 2.0–3.5m²) than a standard gas-boiler cylinder, because heat pumps deliver heat at a lower flow temperature (45–55°C) than a gas boiler (70–80°C) and need more contact area to transfer the same heat output
• Expansion vessel — a mandatory component on unvented and sealed heating systems, absorbing the volume increase as water heats; not required on an open-vented system, where the loft cistern itself accommodates expansion
• Legionella control — applies to all cylinder types; stored water must be maintained at ≥60°C at all times (heat pump cylinders running a lower steady-state temperature are the specific exception, using a weekly ≥60°C pasteurisation cycle instead), regardless of vented/unvented status

Quick Reference Table — Cylinder Type Comparison

Detailed Guidance

Vented cylinders — construction and how the pressure actually works

A vented (open-vent) cylinder is connected to a cold water storage cistern (CWST), usually in the loft, via a cold feed pipe, and has a vent pipe that rises from the top of the cylinder and terminates open to atmosphere over the CWST. This vent pipe is the safety feature: if the water inside were ever to overheat and generate steam, the steam vents harmlessly up the open pipe rather than building dangerous pressure inside a sealed vessel — which is precisely why vented systems don't need the elaborate safety-device chain that unvented systems require.

The pressure delivered at any hot tap is purely a function of the vertical height ("head") between the CWST's water level and the tap. A loft cistern 3 metres above a ground-floor kitchen tap gives roughly 0.3 bar; the same cistern above a first-floor shower gives much less. This is why gravity-fed showers are traditionally weak without a shower pump, and why vented systems are increasingly seen as the budget or legacy option rather than the aspirational choice on a refit.

Standard vented cylinder sizes (BS 1566 nominal capacities): 114L, 120L, 140L, 162L, 180L, 210L. These are copper, factory-insulated (modern) or requiring a fitted lagging jacket (older bare-copper cylinders), and are almost universally indirect (boiler coil plus immersion backup) rather than direct in modern installations.

Unvented cylinders — construction and why they're regulated so tightly

An unvented cylinder has no vent pipe to atmosphere. It connects directly to the mains cold water supply and is, in effect, a sealed pressure vessel that heats stored water to 60–65°C under pressure. This is inherently more hazardous than a vented system: if the normal thermostat fails and nothing else intervenes, stored water heated well above 100°C under pressure and then suddenly released (for example, by a failed fitting) can flash to steam with explosive force. This is precisely why Approved Document G, Section G3 mandates three independent layers of protection — normal thermostatic control, a non-self-resetting high-limit cut-out, and a temperature/pressure relief valve as the final failsafe — and why only a G3-qualified person may legally install, commission or service one. Full detail on the safety device chain, discharge pipework sizing, and Building Control notification routes is covered in unvented cylinders.

Unvented cylinders are constructed from materials rated for pressure: duplex stainless steel, copper, or glass-lined/enamelled mild steel, each with different cost and corrosion-resistance profiles. Duplex stainless steel is now the dominant material for mid-to-premium domestic unvented cylinders due to its strength-to-weight ratio and corrosion resistance; glass-lined mild steel remains common at the budget end.

Why customers choose unvented over vented: mains pressure means every outlet in the house gets strong, even flow simultaneously — a shower doesn't weaken because someone flushes a toilet or runs a tap elsewhere, which is the classic complaint about poorly-designed gravity systems. It also frees up loft space by removing the CWST, and removes a common source of leaks and airlocks associated with an ageing loft tank and ball valve.

Direct vs indirect — a separate axis from vented/unvented

Direct and indirect describe the heat source, not the pressure system, and this distinction is often muddled by customers (and sometimes by installers quoting over the phone without a proper survey):

• Direct cylinders contain an electric immersion heater only — no coil, no boiler connection. They're used in properties without a central heating boiler (all-electric or off-grid), as a dedicated backup heat source alongside a primary system, or where an Economy 7/off-peak tariff makes overnight electric heating cost-effective. Running an immersion heater on standard-rate electricity costs roughly 4–5× as much per kWh delivered as gas boiler heating, so a direct cylinder as a primary heat source is a genuine ongoing cost consideration to flag to the customer, not just an installation choice.
• Indirect cylinders contain one or more coils through which hot primary water circulates, transferring heat from a boiler, heat pump, or stove without mixing the two water circuits. Virtually every indirect cylinder also has an immersion heater boss fitted as a backup or for boiler-off periods — this is standard practice, not an unusual specification.

Twin-coil and solar thermal cylinders

A twin-coil cylinder has two separate heat exchanger coils at different heights — typically a lower coil connected to a solar thermal array and an upper coil connected to the boiler or heat pump. The solar coil pre-heats the lower portion of the stored water using free solar energy; if the sun hasn't raised the temperature enough by the time hot water is drawn, the upper coil tops up the temperature to setpoint. This stratified approach — cooler water at the bottom near the solar coil, hotter at the top near the outlet — is what makes solar thermal worthwhile even in the UK's limited sunshine hours. Twin-coil cylinders for solar thermal are typically sized at 200–300L, larger than an equivalent single-source cylinder, to store enough solar-heated water to make a meaningful contribution to daily demand.

Heat pump cylinders — why they're not interchangeable with a standard cylinder

A standard indirect cylinder is designed around a gas boiler's flow temperature of 70–80°C, with a coil sized accordingly. A heat pump typically runs at a much lower flow temperature — 45–55°C for efficient operation — and transferring the same heat at a lower temperature difference requires substantially more coil surface area, typically 2.0–3.5m² rather than a standard gas-boiler coil. Fitting a heat pump to an undersized-coil legacy cylinder forces it to run at a higher, less efficient flow temperature just to get adequate heat transfer, undermining much of the efficiency benefit. Dedicated heat pump cylinders (Joule, Dimplex, Mitsubishi and others) are sized and coiled for the purpose and should always be specified on a retrofit rather than reusing an existing gas-spec cylinder. See heat pump cylinders for full technical detail.

Thermal stores — mains pressure without a full unvented cylinder

A thermal store inverts the usual relationship between stored water and heating water: the vessel stores primary heating water (heated by a boiler, stove, or heat pump), and domestic hot water is generated on demand by passing mains-pressure cold water through a plate heat exchanger or an internal coil immersed in the hot store. Because the domestic hot water itself is never stored under pressure inside a sealed vessel — it's generated instantaneously as it passes through the heat exchanger — some thermal store designs avoid the full G3 unvented cylinder classification, though this depends on the specific design and should always be confirmed against the manufacturer's certification rather than assumed. Thermal stores are particularly common where a solid fuel stove is part of the heating mix, since a stove's heat output can't be easily switched off the way a boiler can, and a large thermal mass of stored heating water buffers this. See thermal stores for the full comparison against a conventional unvented cylinder.

Insulation and standing heat loss

Modern cylinders (vented and unvented alike) are factory-insulated with rigid foam, typically achieving standing heat losses of 1–2 kWh per day — a significant improvement over older bare-copper cylinders fitted with a loose lagging jacket, which commonly lose 3 kWh per day or more. When replacing an older cylinder, factory insulation quality is worth quoting as a genuine efficiency upgrade, not just a like-for-like swap.

Converting from vented to unvented — what actually changes

Replacing a vented cylinder with an unvented one is more than a cylinder swap. The loft CWST is decommissioned since the unvented cylinder connects directly to the mains; the cold feed changes from gravity to a mains connection via a pressure-reducing valve; and all downstream pipework now carries mains pressure, which can expose previously-hidden weaknesses (weeping gate valves, undersized pipework, poor joints) that never caused a problem under low gravity pressure. The full G3 safety device chain, discharge pipework and Building Control notification apply regardless of what was there before.

Always survey the mains supply pressure and flow rate before quoting a conversion — an unvented cylinder needs adequate incoming pressure and flow (typically at least around 1.5 bar dynamic, more for larger cylinders or multiple simultaneous outlets) to perform as the customer expects.

Frequently Asked Questions

Which type of cylinder should I recommend for a typical semi-detached house refit?

For a property with a boiler already in place and reasonable incoming mains pressure and flow (survey this, don't assume it), an indirect unvented cylinder is generally the preferred modern specification — it gives strong, even hot water at every outlet and removes the loft cistern. Where mains pressure or flow is poor, or the customer has budget constraints, an indirect vented cylinder paired with a shower pump for the bathroom remains a perfectly valid and much cheaper specification. The decision should be driven by the mains supply survey and budget, not by an assumption that unvented is always "better" — it's better for flow performance, not universally better for every property.

Can I install an unvented cylinder if I only have a standard plumbing qualification?

No. G3 is a separate qualification from general plumbing competency, gas qualifications, or NVQ plumbing certificates. It must be held specifically and renewed every 5 years. This applies to new installations, like-for-like replacements, and servicing alike — see unvented cylinders for the full qualification and notification detail.

What's the difference between a thermal store and an unvented cylinder for the customer, in practical terms?

From the customer's tap, both deliver mains-pressure hot water with no loft tank needed. The differences are mostly regulatory (a thermal store may not always require full G3 notification, depending on its design) and in how well each integrates with a solid fuel stove or multiple heat sources, where thermal stores are traditionally stronger. For a straightforward single-boiler or single-heat-pump property, an unvented cylinder is the more common specification; thermal stores are a specialist choice where solid fuel or multi-source heating is part of the picture.

Does the cylinder type affect Legionella risk?

Not fundamentally — Legionella control (maintaining ≥60°C storage temperature at all times, with a weekly ≥60°C pasteurisation cycle as the specific exception for lower-temperature heat pump systems; avoiding "dead legs" of stagnant pipework; and using thermostatic mixing valves for scald protection rather than simply turning the stored temperature down) applies equally to vented and unvented systems, direct and indirect. The specific safety devices differ (unvented cylinders have additional overheat protection layers unrelated to Legionella control), but the Legionella management principles themselves are the same across all cylinder types. See legionella risk for the full detail.

Regulations & Standards

• Building Regulations Approved Document G (Part G3) — hot water supply and systems; unvented cylinder notification, qualified installer requirement and mandatory safety devices

• BS 1566 — copper indirect cylinders; capacity, construction and performance requirements for vented cylinders

• BS EN 12897 — unvented water heaters; requirements and test methods

• BS 8558:2015 — guide to the design, installation, testing and maintenance of services supplying water for domestic use within buildings

• Water Supply (Water Fittings) Regulations 1999 — WRAS approval requirements for cylinder materials and fittings, backflow prevention on mains-connected systems

• HSE L8 / ACoP and HSG274 — Legionella risk management in building hot and cold water systems

• Approved Document G (2024 amendments) — HM Government

• CIPHE — Unvented Cylinder Guidance — professional guidance on G3 compliance and cylinder specification

• Heatrae Sadia — Megaflo Technical Manual — unvented cylinder installation and sizing guidance

• WRAS — Water Storage Systems — approved materials for water storage and fittings

• Joule — Heat Pump Cylinder Range technical guidance

• unvented cylinders — full G3 compliance, mandatory safety devices, and discharge pipework requirements

• cylinder selection — choosing a cylinder as part of a wider heating system decision, with sizing guidance

• domestic hot water cylinder sizing — sizing a cylinder to occupancy and heat source

• heat pump cylinders — coil sizing and specification for heat pump compatibility

• thermal stores — thermal store construction and comparison with unvented cylinders