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

Quick Answer: Cylinder selection depends on heat source, mains pressure, and hot water demand. Indirect vented cylinders suit gravity-fed (open-vent) systems with regular boilers; unvented cylinders (G3 compliant) suit mains-pressure hot water and are mandatory when high-flow showers without booster pumps are required. Size by peak demand: 50 litres per person as a minimum (150L for 3-person household, 210L for 4-person). All unvented cylinders require Building Control notification and G3-qualified installation.

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

Quick Reference Table

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Type Heat Source Pressure Notification Typical Size Best For
Vented indirect Regular boiler Gravity (~1 bar) No 120–250L Older systems, gravity-fed
Vented direct Immersion only Gravity (~1 bar) No 120–210L No boiler, off-peak tariff
Unvented indirect System boiler Mains (2–4 bar) G3 required 150–300L Modern homes, high flow showers
Unvented direct Immersion only Mains (2–4 bar) G3 required 150–250L New build, no boiler
Thermal store Boiler/stove/HP Mains (via HX) No (some types) 200–400L Multi-source heating, solid fuel
Solar thermal Solar + boiler Vented or unvented G3 if unvented 200–300L Solar pre-heat + boiler top-up
Heat pump cylinder Heat pump Unvented usually G3 if unvented 200–300L ASHP/GSHP compatible

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:

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:

Minimum recommended sizes by household:

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:

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