Domestic Hot Water Cylinder Sizing: Recovery Rate, Primary Coil, G3 Compliance and Selection by Property

Quick Answer: Size a domestic hot water cylinder on occupancy and bathroom count — roughly 35–45 litres of stored hot water per person for a boiler-fed system, more for heat pumps and high-demand homes. The cylinder must recover fast enough for the household, which depends on the primary coil surface area: a gas/oil boiler cylinder typically has a 1.0–1.5 m² coil, while a heat pump cylinder needs a much larger coil (2.5–3.0 m²+) to transfer heat at low flow temperatures. Unvented cylinders are notifiable under Building Regulations Part G3 and must be installed by a G3-qualified operative with the mandatory safety devices.

Summary

Get the cylinder wrong and the homeowner either runs out of hot water mid-shower or pays to heat a tank far bigger than they need. Cylinder sizing is two questions answered together: how much hot water do they store, and how fast does it recover? Stored volume covers the peak draw (back-to-back showers, filling a bath); recovery rate — driven by the heat source and the primary coil — determines how quickly the tank is hot again. This article covers sizing by property type, recovery and coil sizing (including the much larger coils heat pumps demand), reheat times, vented vs unvented, the G3 legal framework, and the discharge pipework that catches more installers out than any other detail.

The big shift in recent years is the heat pump. A traditional gas or oil boiler runs its primary circuit at 70–80°C, so a modest coil transfers plenty of heat into the cylinder. A heat pump runs the cylinder coil at perhaps 50–55°C, a far smaller temperature difference, so the coil has to be physically much bigger to move the same energy. Fitting a standard boiler cylinder on a heat pump gives glacial reheat times and poor performance — heat pump cylinders are a different specification, not just a different label. This is covered in depth in heat pump cylinders.

The most common misconception is that bigger is always safer. An oversized cylinder costs more to buy, takes longer to reheat, suffers more standing heat loss, and on a heat pump can hurt efficiency. Size to the household's real peak demand, then make sure recovery keeps up. The cylinder-selection fundamentals (direct/indirect, vented/unvented, thermal store) are in cylinder selection and unvented cylinders; this article concentrates on getting the size and coil right.

Key Facts

• Stored volume rule of thumb — approximately 35–45 litres of hot water per person for a boiler-fed indirect cylinder; scale up for high-demand homes and heat pumps
• Building Regulations Part G3 — unvented hot water storage systems are notifiable; installation requires a G3-qualified operative (e.g. holding the relevant unvented competency)
• Stored water temperature — store at ≥60°C to control Legionella; distribute via TMV where scald risk applies (bath outlet limited to 48°C under Part G); see legionella management
• Primary coil — boiler cylinder — typical surface area ~1.0–1.5 m²; recovers quickly at 70–80°C primary flow
• Primary coil — heat pump cylinder — typical surface area ~2.5–3.0 m²+ to transfer heat at 50–55°C low flow temperature; smaller coils give very slow reheat
• Recovery rate — the rate the cylinder reheats; depends on coil area, primary flow temperature, and boiler/heat pump output (kW)
• Reheat time — boiler cylinder reheats a full tank in roughly 20–40 minutes; heat pump cylinders take longer (often 1–2+ hours) by design
• Unvented mandatory safety devices — temperature relief / temperature & pressure relief valve, expansion vessel (or internal air gap), pressure-reducing valve, expansion relief valve, and a tundish on the discharge — all required under G3
• Tundish — a visible air break fitted in the discharge pipe within 500 mm of the safety valve(s)
• Discharge pipe (D1) — metal pipe from the relief valve to the tundish; D2 the larger pipe from the tundish to a safe discharge point
• D2 minimum size — at least one pipe size larger than D1, minimum 22 mm, sized for the run and number of elbows per G3 / manufacturer table
• Discharge termination — to a safe, visible location where hot water discharge cannot scald (low level to a gully, or specified termination); must be obvious if it operates
• Vented cylinder — open-vented, fed from a cold water storage cistern in the loft; low pressure; no G3 notification
• Unvented cylinder — sealed, mains-pressure hot water at every outlet; G3 notifiable; needs adequate mains flow and pressure
• Standing heat loss / ErP — cylinders are ErP energy-labelled (A–G); insulation quality drives the standing-loss figure (kWh/day); spray-foam-insulated modern cylinders lose far less than old felt-lagged tanks
• Mains requirement for unvented — needs sufficient incoming mains flow rate and pressure (commonly ~1.5–2 bar minimum and adequate litres/min) for satisfactory simultaneous outlet performance

Quick Reference Table

Heat pump installs typically step up one size and require a high-surface-area coil cylinder.

Detailed Guidance

Sizing by occupancy and bathrooms

Start with peak simultaneous demand, not average use. A four-person family with one bathroom rarely draws hot water simultaneously; the same four people across two bathrooms can run two showers at once. Sizing logic:

• Stored volume — budget around 35–45 litres of hot water per person for a boiler system, then sanity-check against the peak event. A full bath is roughly 80–100 litres of blended water (so ~50–60 litres of stored hot at 60°C mixed down). Two back-to-back showers at 9–12 litres/min for 8 minutes is a serious draw on a small tank.
• Bathrooms drive the peak. Each additional bathroom raises the chance of simultaneous draw. Two-bathroom homes generally want 210 L+, three-bathroom homes 250–300 L or twin/larger cylinders.
• High-flow showers and baths — power showers and large baths can empty a modestly sized cylinder; size up where the spec includes them.
• Recovery offsets storage. A fast-recovering boiler cylinder can be a little smaller because it reheats between events; a slow-recovering heat pump cylinder must be sized larger because it cannot catch up quickly within a busy morning.

Recovery rate and primary coil surface area

Recovery is where sizing gets technical. The coil is a heat exchanger; the heat transferred depends on its surface area and the temperature difference between the primary water and the stored water:

• A boiler pushes 70–80°C primary water through a ~1.0–1.5 m² coil. The big temperature difference means a small coil moves a lot of heat — fast reheat.
• A heat pump runs the primary at 50–55°C. The temperature difference to ~55–60°C stored water is small, so the same coil would move far less heat. To compensate, heat pump cylinders use a much larger coil — commonly 2.5–3.0 m² or more — so they can still transfer the heat pump's output into the tank at low flow temperature.

Fitting a standard boiler cylinder on a heat pump is a classic error: the coil is too small, the heat pump short-cycles or runs flat out for hours, reheat is painfully slow, and efficiency suffers. Always specify a heat-pump-rated cylinder for a heat pump install. See heat pump cylinders and low temperature design.

Reheat time — what to tell the customer

   Reheat expectation by heat source
   ---------------------------------
   Boiler cylinder (1.0-1.5 m2 coil, 70-80C primary)
     full tank cold -> hot:  ~20-40 minutes
   Heat pump cylinder (2.5-3.0 m2 coil, 50-55C primary)
     full tank cold -> hot:  ~1-2+ hours (by design)

   Implication:
     boiler -> can run smaller tank, reheats between draws
     heat pump -> size tank larger, schedule reheat for
                  off-peak / before peak demand

Heat pump owners need to understand the cylinder reheats slowly and is best scheduled (e.g. a timed reheat before the morning peak, often on a cheaper electricity tariff). This is normal, not a fault — managing expectations at handover prevents callbacks.

Vented vs unvented

• Vented (open-vented) cylinder — fed by gravity from a cold water storage cistern in the loft, with an open vent pipe. Low pressure at the taps (driven by the head of water from the loft tank). Simple, cheap, no G3 notification, no mains-pressure dependency. The trade-off is weaker shower/tap pressure unless a pump is fitted.
• Unvented cylinder — a sealed, mains-pressure stored hot water system. Delivers full mains pressure and flow to every hot outlet, no loft tank needed. The trade-off: it is notifiable under G3, requires the full set of safety devices and discharge pipework, and needs adequate incoming mains flow and pressure to perform. If the mains is weak, an unvented cylinder will disappoint — check the incoming supply before specifying one. Detail in unvented cylinders and expansion vessels.

G3 compliance, safety devices and discharge pipework

Unvented hot water storage is controlled by Building Regulations Approved Document G3 (part g water). Key requirements:

• Notifiable work — installation must be notified to Building Control, normally via a competent-person scheme, and carried out by a G3-qualified operative.
• Two-tier safety — the system must have both a non-self-resetting energy cut-out (thermostat) and a temperature relief valve (or combined temperature & pressure relief valve), plus the pressure controls (PRV and expansion relief). These are factory-fitted on packaged units; do not remove or defeat them.
• Expansion — heated water expands; an expansion vessel or an internal air gap (bubble-top) absorbs it. Check the vessel charge pressure against the MI.
• The discharge pipe is where installs fail. The relief valve(s) discharge through a tundish (a visible air break) fitted within 500 mm of the valve. The pipe before the tundish is D1; the pipe after is D2. D2 must be at least one size larger than D1, minimum 22 mm, and sized for the actual run length and number of bends per the G3 / manufacturer table — long runs with many elbows need upsizing again. It must run continuously downhill to a safe, visible discharge point where scalding water cannot harm anyone and where operation is obvious (so a fault gets noticed). Discharging into a concealed location or upsizing incorrectly is a common G3 failure.

Standing loss, ErP labelling and insulation

Modern cylinders carry an ErP energy label (A–G) reflecting their standing heat loss — the energy lost from the stored hot water over 24 hours. A well-insulated cylinder (factory-applied rigid foam) holds heat far better than an old felt-lagged copper tank. For heat pump installs especially, low standing loss matters because the heat pump generates expensive low-temperature heat; choose a high-rated, well-insulated cylinder and keep pipe runs short and lagged (pipe lagging).

Frequently Asked Questions

How big a cylinder for a 4-bed, 2-bathroom house on a gas boiler?

Around 210–250 litres for a boiler-fed indirect cylinder is the usual sweet spot — enough stored hot water for two bathrooms with some simultaneous draw, and a fast-recovering coil to refill between events. If the household includes power showers, a large bath, or runs hot water heavily at one time, step up toward 250–300 L. On a heat pump, size up a notch and specify a heat-pump-rated cylinder with a large coil.

Why does the heat pump cylinder have such a big coil?

Because a heat pump runs the primary water far cooler than a boiler — about 50–55°C versus 70–80°C. The smaller temperature difference between the primary water and the stored water means each square metre of coil transfers less heat, so the coil must be physically much larger (typically 2.5–3.0 m²+) to move the heat pump's output into the tank at a useful rate. A standard boiler cylinder fitted to a heat pump reheats far too slowly.

Is an unvented cylinder always better than a vented one?

No. Unvented gives mains pressure at every outlet and removes the loft tank, which is excellent where the incoming mains is strong. But it is G3-notifiable, needs the full safety-device set and correct discharge pipework, and depends entirely on a good mains flow and pressure. On a weak mains supply, a vented cylinder (optionally with a pump) can deliver better real-world performance. Always check the incoming mains before recommending unvented.

Does the cylinder need to store water at 60°C?

Yes, for Legionella control the stored water should reach ≥60°C (and the pasteurisation/immersion cycle on heat pump systems is set to reach this periodically). To prevent scalding, hot water delivered to a bath is limited to 48°C under Part G via a thermostatic mixing valve. Store hot, deliver safe — see legionella management and thermostatic mixing valves.

What size does the discharge (D2) pipe need to be?

D2 must be at least one pipe size larger than the relief-valve discharge (D1) and a minimum of 22 mm, then upsized further according to the run length and number of bends using the G3 / manufacturer's table. A long discharge route with several elbows can require 28 mm or larger. It must fall continuously to a safe, visible termination. Undersizing or running it uphill is a G3 non-compliance and a safety risk.

Regulations & Standards

• Building Regulations Approved Document G3 — Hot water supply and systems; unvented storage notifiable work, safety devices, discharge pipework, G3 operative competency

• Building Regulations Approved Document G (Part G) — water efficiency, hot water safety (bath outlet ≤48°C via TMV)

• BS EN 12897 — Specification for indirectly heated unvented (closed) storage water heaters

• BS 6700 / BS EN 806 — Design and installation of water supply systems within buildings

• L8 / HSG274 (HSE) — Legionella control in hot and cold water systems (store ≥60°C)

• ErP / Energy Labelling Regulations — cylinder energy labelling and standing heat loss declaration

• GOV.UK — Approved Document G (Sanitation, hot water safety and water efficiency) — G3 unvented requirements and discharge pipework

• HSE — Legionnaires' disease (L8/HSG274) — hot water storage temperature control

• HHIC — Hot water cylinder guidance — sizing and heat pump cylinder specification

• BSI — BS EN 12897 — unvented storage water heater specification

• cylinder selection — direct/indirect/unvented/thermal store selection fundamentals

• heat pump cylinders — oversized coil requirements and heat pump reheat strategy

• unvented cylinders — G3 qualification, safety devices and discharge pipework in detail

• legionella management — storage temperature and scald-control requirements