Hot Water Cylinder Selection: Direct, Indirect, Unvented & Sizing Guide

Summary

The hot water cylinder is the component most commonly under-specified in system and regular boiler installations. Undersizing leads to running out of hot water during peak demand; oversizing wastes energy heating water that sits unused and increases Legionella risk if reheat is infrequent. Both sizing and type selection require understanding the heat source, available water pressure, and household usage patterns.

The distinction between vented and unvented is fundamental. Vented (open-vent) cylinders rely on gravity-fed water from a cold water storage cistern — pressure at the hot tap equals the head of water above it. Unvented cylinders connect directly to the mains cold water supply, delivering mains-pressure hot water at every outlet. For households with adequate mains pressure (minimum 1.5 bar dynamic, 2.5 bar recommended), unvented is the preferred modern specification.

A third category — thermal stores — deserves mention. A thermal store is a mains-pressure system where the stored water is used as heating water (not hot water directly); domestic hot water passes through a heat exchanger coil within the store, achieving mains pressure without the full G3 regulatory burden of an unvented cylinder. Thermal stores are common with solid fuel stoves and heat pump systems.

Key Facts

• Direct cylinder — electrically heated (immersion heater only); no boiler connection; suitable for off-grid or as backup
• Indirect cylinder — heated by primary coil from boiler; one or two coils; standard with regular and system boilers
• Vented (open-vent) — gravity-fed from CWST in loft; pressure = head (typically 0.7–2.0 bar); BS 1566 applies
• Unvented (mains pressure) — direct mains connection; pressure equals mains (typically 2–5 bar); G3 Building Regulations
• G3 notification — unvented cylinder installation is a G3 notifiable building regulation; must be done by G3-qualified installer
• Thermal store — primary water stored; HW via plate heat exchanger or internal coil; mains pressure HW without full G3 cylinder
• Minimum sizing (BS 6700 guidance) — 30–50 litres per person per day peak demand
• Recovery time — time to reheat a fully cold cylinder; 2–3 hours for indirect at typical boiler output; faster with larger coil
• Standing heat loss — insulated cylinders lose 1–3 kWh/day; factory-insulated foam cylinders significantly better than lagged copper
• Solar thermal coil — separate lower coil for solar circuit; pre-heats water before boiler top-up; 200–300L cylinder typically required
• Heat pump coil — oversized coil required for heat pump (larger surface area to allow lower flow temperature transfer)
• Expansion vessel — unvented systems require a dedicated expansion vessel; sized to cylinder volume
• Pressure reducing valve (PRV) — unvented cylinders require incoming supply PRV set to manufacturer's specified pressure (typically 3 bar)
• Legionella management — stored hot water must reach 60°C throughout once daily; cylinder thermostat set to 60°C minimum
• TMV (thermostatic mixing valve) — required downstream of unvented cylinder at outlets; blend to 48°C for bathrooms, 43°C for care/healthcare (TMV3)

Quick Reference Table

Detailed Guidance

Indirect Vented Cylinders: Traditional Open-Vent Systems

The indirect vented copper cylinder (BS 1566) is the traditional UK domestic hot water vessel. Heated by a primary coil connected to the boiler circuit, with an immersion heater boss for backup or off-peak heating. Water is stored at atmospheric pressure (technically slightly above, due to the head from the CWST in the loft).

Standard sizes: 114L, 120L, 140L, 162L, 180L, 210L (BS 1566 nominal capacities)

Flow rate at the hot tap is determined by the head available — typically 10–15 litres/minute for a gravity system, insufficient for unassisted power showers. Shower pumps (single or twin impeller) can boost pressure to 1.5 bar or above, but require a minimum head of 1 metre above the pump inlet.

Heat-up time depends on boiler output and coil size. A standard 2–3m² coil at 80°C primary temperature will reheat a 180L cylinder in approximately 2.5–3 hours. "High recovery" cylinders have larger coils for faster reheating — important in high-demand households.

Unvented Cylinders: G3 Compliance

Unvented hot water cylinders deliver mains-pressure hot water — typically 2–4 bar — to all outlets. The regulatory framework is:

• Building Regulations Part G (G3) — requires installation by a G3-qualified person and notification to Building Control
• BS EN 12897 — unvented water heaters; performance and safety requirements
• G3 qualification — separate qualification module (ACS or NVQ) in addition to core plumbing; required for legal installation

Mandatory safety devices on unvented cylinders:

1. Pressure reducing valve (PRV) — set to manufacturer's specification (typically 3 bar)
2. Expansion vessel — accommodates water expansion during heating
3. Non-return valve — prevents backflow into mains
4. Temperature relief valve (Tundish) — discharges to tundish and D2 pipe if temperature exceeds 90–95°C
5. Pressure relief valve — separate from temperature relief in some systems
6. Thermostat — controls primary or element temperature; set to 60°C minimum
7. High limit (second stat) — thermal cut-out at approximately 80–85°C; manual reset

The tundish and D2 discharge pipe must terminate safely — air gap at tundish, then D2 pipe running at minimum 1:200 gradient to a safe, visible location. D2 pipe material and bore must comply with the manufacturer's guidance and Approved Document G3.

See the dedicated unvented cylinders article for full detailed installation guidance.

Sizing Cylinders: Demand Calculation

A practical sizing method based on peak daily demand:

• Shower: 40–60 litres per shower (8 litres/minute × 5–7 minutes)
• Bath: 150–200 litres (filling and topping up)
• Washing up: 10–15 litres
• Miscellaneous: 10–20 litres per person

Minimum recommended sizes by household:

• 1–2 persons: 120–150 litres
• 3 persons: 150–180 litres
• 4 persons: 180–210 litres
• 5 persons: 210–250 litres
• 6+ persons or frequent bathing: 250–300 litres

These figures assume a single reheat period per day. If the household has unpredictable hot water demand (shift workers, work-from-home with variable occupancy), size up by one category.

For solar thermal systems, the cylinder should be sized to store the full expected daily solar gain — typically 50–70 litres per m² of collector area. A 3m² solar thermal collector typically requires a 200–250L cylinder.

Heat Pump Cylinders

Heat pump-compatible cylinders differ from standard cylinders in two ways:

1. Larger primary coil — the coil must have sufficient surface area to transfer heat at the lower flow temperatures typical of ASHPs (45–55°C). Standard cylinders have coils designed for boiler flow at 70–80°C and are inefficient at heat pump temperatures. Look for cylinders with a coil surface area of 2.0–3.5m² for ASHP use.
2. High volume — larger volume improves COP by allowing the heat pump to run longer, uninterrupted cycles. 200–300L is typical even for smaller households.

Manufacturers such as Joule, Dimplex, and Mitsubishi supply dedicated heat pump cylinders. The MCS heat pump installation standard includes guidance on cylinder sizing for heat pump systems.

Direct vs Indirect: When to Choose Direct

A direct cylinder (immersion heater only, no boiler coil) is used:

• In properties without a central heating boiler (all-electric, off-grid)
• As a dedicated backup in systems with a primary heat source (immersion as secondary)
• In some new-build situations where future-proofing for heat pumps is intended
• With Economy 7 or off-peak tariffs where overnight electric heating is economical

Running an immersion heater at peak-rate electricity is significantly more expensive than gas boiler heating — approximately 4–5× the fuel cost per kWh. Off-peak E7 rate can reduce this substantially (typically 50% cheaper than day rate).

Frequently Asked Questions

Can I replace a vented cylinder with an unvented without a full system change?

Yes, in many cases. An unvented cylinder replaces the vented cylinder and the cold water storage cistern (CWST) is decommissioned (or removed). The hot water distribution pipework remains, though flow rates will increase significantly with mains pressure, which may expose inadequate pipe sizing or weeping gate valves. The cold feed to the cylinder now comes directly from the mains supply. G3 notification is required, and a G3-qualified installer must complete the work.

How often should a cylinder thermostat be set to 60°C?

The cylinder thermostat should maintain stored water at 60°C continuously (or at least achieve 60°C once per day) to prevent Legionella growth. Water at 60°C kills Legionella within 2 minutes; at 50°C it takes 80–124 minutes. In practice, the storage temperature should not drop below 55°C between heat-up cycles. Holiday mode on some cylinders (drop to 45°C) should only be used for short periods — if away for more than a week, a Legionella safety run at 60°C should be programmed.

Does an unvented cylinder require annual servicing?

The manufacturer's instructions typically specify annual inspection and servicing of the safety devices — particularly the temperature/pressure relief valve, which should be manually tested to confirm it operates. The expansion vessel pre-charge pressure should be checked annually (usually 3 bar). Many homeowners are unaware that unvented cylinders require servicing, so this is worth raising at the point of installation.

What size expansion vessel do I need?

The expansion vessel is sized based on the cylinder volume and incoming pressure. A rough guide: expansion vessel capacity should be approximately 10–12% of the cylinder volume. A 210L cylinder typically needs a 24L expansion vessel. However, this varies with system pressure and PRV setting — refer to the cylinder manufacturer's sizing nomogram or calculation table for the specific installation.

Regulations & Standards

• Approved Document G (Part G3) — Unvented hot water storage systems; notification and qualified installer requirement

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

• BS 1566 — Copper indirect cylinders; capacity and performance requirements

• Water Supply (Water Fittings) Regulations 1999 — WRAS approval for cylinder materials and fittings

• L8 (CIBSE/HSE) — Legionella risk management in building water systems

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

• Joule — Heat Pump Cylinder Range — Heat pump compatible cylinder specifications

• CIPHE — Unvented Cylinder Guidance — Professional guidance on G3 compliance

• WRAS — Water Storage Systems — Approved materials for water storage

• unvented cylinders — Full G3 compliance, safety devices, and discharge pipe requirements

• hot water systems — Combi vs system vs regular boiler comparison

• boiler selection — Boiler sizing with system boilers

• thermostatic mixing valves — TMV requirements downstream of unvented cylinders