Unvented Cylinder Sizes Guide UK (90L–300L+) – What Size Do You Need?

Unvented Cylinder Sizes Guide

The definitive engineering resource for choosing the correct unvented hot water cylinder size. Compare capacities from 90L to 400L+, understand recovery rates, dimensions, and match cylinder capacity to your household's exact hot water demand.

Calculate Cylinder Size View Size Chart Hot Water Usage Guide Cylinder Dimensions

Interactive Cylinder Size Calculator

Enter your household details below to receive an engineering-based cylinder size recommendation, including recovery time estimates and simultaneous demand analysis.

Recommended Cylinder Size
210 L
Estimated Daily Demand
175 L
Recovery Time (Ξ”T=45Β°C)
22 min
Peak Simultaneous Showers
2
Standing Heat Loss /24h
1.4 kWh

Based on 45Β°C mixed water temperature, 10L/min shower flow rate. Storage factor applied: 1.3Γ—. Cylinder sized for morning peak demand with 70% usable capacity.

Unvented Cylinder Size Chart (90L – 400L+)

Comprehensive sizing reference table matching cylinder capacity to occupancy, bathroom count, property type, and expected hot water performance. Use this chart to quickly identify the correct cylinder size for your installation.

Cylinder Size Occupants Bathrooms Showers (Simul.) Property Type Typical Reheat Heat Loss/24h
90L 1–2 1 1 Flat / Apartment 14–18 min 0.9–1.1 kWh
120L 2–3 1 1 Small House 16–20 min 1.0–1.3 kWh
150L 3–4 1–2 1–2 2–3 Bed House 18–24 min 1.1–1.5 kWh
180L 3–4 2 2 3 Bed House 20–26 min 1.2–1.6 kWh
210L 4–5 2 2 3–4 Bed House 22–28 min 1.3–1.7 kWh
250L 4–6 2–3 2–3 4 Bed House 24–32 min 1.4–1.9 kWh
300L 5–7 3+ 3 5 Bed House 28–36 min 1.6–2.2 kWh
400L+ 7+ 4+ 3+ Large / Commercial 35–50 min 1.9–2.8 kWh

Reheat times based on indirect coil with 20kW boiler input, Ξ”T=45Β°C. Heat loss values for ERP-rated cylinders with 50mm+ insulation. Actual performance varies by manufacturer and installation conditions.

Household Hot Water Demand Guide

Understanding daily hot water usage per person and per appliance is essential for accurate cylinder sizing. Below are the engineering-standard demand figures used across the UK plumbing industry.

Per-Person Daily Demand

Usage TypeLitres (Mixed 40Β°C)
Shower (5–8 min)50–80 L
Bath (full)100–150 L
Washbasin use8–15 L
Kitchen sink15–30 L
Dishwasher10–15 L
Washing machine10–20 L
Total per person/day35–50 L

Simultaneous Demand Factors

When multiple outlets run simultaneously, peak demand can be 2–3Γ— the average. A well-sized cylinder accounts for the morning peak when showers, basins, and kitchen taps may all be in use within a 30–60 minute window.

Storage Factor: Multiply average daily demand by 1.2–1.5 to determine minimum cylinder capacity. For homes with 2+ bathrooms, use a factor of 1.4–1.6.

Engineering Tip Always size for the busiest morning, not the average day.

Cylinder Dimensions Guide (Typical UK Models)

Physical dimensions vary by manufacturer, but the table below shows typical height Γ— diameter for common unvented cylinder capacities. Always check manufacturer specifications for exact measurements and required installation clearances.

Capacity Typical Height Typical Diameter Weight (Empty) Weight (Full) Min. Cupboard Depth
90L850–950 mm450–500 mm22–28 kg112–118 kg550 mm
120L950–1100 mm450–520 mm26–34 kg146–154 kg570 mm
150L1050–1200 mm500–550 mm30–40 kg180–190 kg600 mm
180L1150–1350 mm520–580 mm35–46 kg215–226 kg630 mm
210L1300–1500 mm550–600 mm40–52 kg250–262 kg650 mm
250L1450–1650 mm550–620 mm46–60 kg296–310 kg670 mm
300L1600–1850 mm580–650 mm55–72 kg355–372 kg700 mm
400L1800–2100 mm650–750 mm70–95 kg470–495 kg800 mm

Dimensions are indicative for stainless steel unvented cylinders commonly available in the UK market. Always allow 100mm+ clearance above and 50mm+ around the cylinder for installation and maintenance access.

Direct vs Indirect Unvented Cylinders

Understanding the difference between direct and indirect heating methods is critical for selecting the right cylinder and ensuring compatibility with your heat source.

Feature Direct Cylinder Indirect Cylinder
Heat SourceElectric immersion heater(s)Gas/oil boiler via heating coil
Recovery SpeedSlower (3–6 hrs from cold)Fast (20–35 min reheat)
Running CostHigher (electricity rates)Lower (gas/oil rates)
Installation CostLower (no boiler connection)Higher (requires boiler circuit)
Best ForFlats, no gas supplyHouses with gas boilers
Renewable CompatibleLimitedYes (twin-coil for solar/HP)
ERP Efficiency RatingC–D rating typicalA–B rating typical
Lifespan15–25 years15–25 years

Twin-Coil Cylinders for Renewable Heating

Twin-coil indirect cylinders feature two separate heating coils: one for the boiler and a second (typically lower-positioned, larger surface area) coil for solar thermal or heat pump integration. These are essential for renewable-compatible hot water systems and allow stratified heating for maximum efficiency.

Heat Pump & Solar Thermal Cylinder Sizing

Air Source Heat Pump Cylinders

Heat pumps operate at lower flow temperatures (typically 45–55Β°C) compared to boilers. This requires:

  • Oversized cylinders – typically 25–50% larger than boiler-fed equivalents
  • Larger coil surface area – 3.0–4.5 mΒ² vs standard 0.8–1.5 mΒ²
  • Slower recovery – allow 45–90 minutes for full reheat
  • Higher storage temperature may be needed with anti-legionella cycles

Recommendation For a 4-person home with an ASHP, specify a 250–300L cylinder with a dedicated heat pump coil.

Solar Thermal Cylinder Sizing

Solar thermal systems feed into a dedicated lower coil in twin-coil cylinders. Sizing considerations include:

  • Solar collector area – typically 2–4 mΒ² per household
  • Cylinder capacity should be 50–80L per mΒ² of collector
  • Stratification is key – taller, slimmer cylinders perform better
  • Summer oversupply – consider a heat-dump or larger cylinder

Recommendation For a 3 mΒ² solar array, specify a 180–250L twin-coil cylinder.

Engineering Formulas for Cylinder Sizing

These are the core thermal engineering equations used by building services engineers and plumbing designers to specify hot water storage systems. Understanding these formulas enables precise sizing and performance prediction.

1. Hot Water Demand Formula

Cylinder Size (L) = Daily Hot Water Demand Γ— Storage Factor

// Daily Hot Water Demand (litres at 60Β°C stored) D_daily = (N Γ— Q_person) + (B Γ— Q_bath) + (S Γ— Q_shower) // Where: N = number of occupants Q_person = 35–50 L/person/day B = baths per day Q_bath = 100–150 L/bath S = showers per day Q_shower = 50–80 L/shower // Storage Factor: 1.2–1.6 depending on peak simultaneity Cylinder Volume = D_daily Γ— SF

2. Recovery Rate Formula

Recovery Rate (L/h) = (Boiler Output kW Γ— 860) Γ· Ξ”T

// Recovery rate in litres per hour RR = (P_boiler Γ— 860) / Ξ”T // Where: P_boiler = boiler power dedicated to cylinder (kW) 860 = conversion factor (kJ to litre-Β°C) Ξ”T = temperature rise required (Β°C), typically 45–50Β°C // Example: 20kW boiler, Ξ”T=45Β°C RR = (20 Γ— 860) / 45 = 382 L/h // Time to reheat 210L cylinder (70% drawdown = 147L): t_reheat = 147 / 382 = 0.385 hours β‰ˆ 23 minutes

3. Stored Thermal Energy Formula

Q = m Γ— c Γ— Ξ”T

// Stored energy in kilojoules (kJ) Q = m Γ— c Γ— Ξ”T // Where: m = mass of water (kg) β€” 1 litre β‰ˆ 1 kg c = specific heat capacity of water = 4.186 kJ/kgΒ·K Ξ”T = temperature difference (K or Β°C) // Example: 210L cylinder heated from 15Β°C to 60Β°C Q = 210 Γ— 4.186 Γ— (60 - 15) Q = 210 Γ— 4.186 Γ— 45 Q = 39,558 kJ β‰ˆ 11.0 kWh stored thermal energy

Worked Engineering Examples

Real-world sizing scenarios demonstrating the engineering methodology for determining unvented cylinder capacity.

Example 1: 3-Bedroom House, 4 Occupants, 1 Bathroom

Assumptions: 4 people, 3 showers/day, 2 baths/week, gas boiler (indirect).

Daily demand calculation:

  • Occupant demand: 4 Γ— 40L = 160L
  • Shower demand: 3 Γ— 60L = 180L (some overlap with occupant baseline)
  • Bath demand: (2/7) Γ— 120L β‰ˆ 34L/day average
  • Net daily demand: ~175L (taking the higher of occupant vs fixture demand)

Storage factor: 1.3Γ— for single-bathroom morning peak β†’ Recommended: 210L indirect cylinder

Recovery: With 20kW boiler, recovery rate β‰ˆ 382 L/h. Reheat from 70% drawdown: ~23 minutes.

Example 2: 4-Bedroom Property, 5 Occupants, 3 Bathrooms

Assumptions: 5 people, 4–5 showers/day, 3 baths/week, gas boiler (indirect).

Daily demand: ~220–250L with high simultaneity factor of 1.5Γ—.

Recommended: 250–300L indirect cylinder to comfortably support 2 simultaneous showers during the morning peak without pressure or temperature drop.

Recovery: With 24kW boiler, recovery rate β‰ˆ 458 L/h. Reheat from 70% drawdown (175L): ~23 minutes.

Example 3: Large Family Home with Air Source Heat Pump

Assumptions: 6 occupants, 3 bathrooms, 11kW ASHP with dedicated cylinder coil.

Cylinder sizing: 300–350L recommended due to lower flow temperature (50Β°C) and slower recovery. Oversize by 30–40% compared to boiler-fed equivalent.

Recovery: ASHP recovery rate at Ξ”T=35Β°C: (11 Γ— 860) / 35 β‰ˆ 270 L/h. Full reheat may take 50–70 minutes β€” plan for off-peak or continuous heating strategies.

Energy Efficiency & Standing Heat Loss

Modern unvented cylinders are highly insulated, but standing heat loss remains a key factor in overall system efficiency. ERP ratings, insulation quality, and smart controls all contribute to reducing energy waste.

ERP Rating Guide

Energy-Related Products ratings range from A+ (best) to G (worst). Most modern stainless steel unvented cylinders achieve B–A ratings. Look for cylinders with ERP band A or B for optimal efficiency.

Insulation Standards

High-quality cylinders feature 50–75mm of CFC-free polyurethane foam insulation. Standing heat loss for a 210L cylinder should be under 1.7 kWh/24h β€” equivalent to less than 50p/day at current energy prices.

Smart Controls

Pairing your cylinder with smart heating controls, off-peak electricity tariffs (Economy 7), or solar PV diversion can significantly reduce running costs. Timed heating avoids unnecessary reheating during vacant periods.

Installation Requirements & G3 Regulations

Unvented hot water cylinders are classified as controlled fittings under Building Regulations Part G3 (England & Wales) and must be installed by a G3-certified competent person. Failure to comply is a criminal offence and can invalidate home insurance.

Essential Safety Components

  • Pressure Reducing Valve (PRV) – reduces mains pressure to 3 bar
  • Expansion Vessel – accommodates thermal expansion (typically 8–25L)
  • Tundish – visible air gap for discharge warning
  • Temperature & Pressure Relief Valve (TPRV) – 90–95Β°C / 7 bar
  • Discharge Pipework – correctly sized and routed to safe location

Compliance Checklist

  • βœ… G3-registered installer (e.g., Gas Safe, OFTEC, or competent person scheme)
  • βœ… Annual servicing by qualified engineer
  • βœ… Discharge pipe terminates safely (no freezing risk)
  • βœ… Expansion vessel correctly sized and pre-charged
  • βœ… Tundish visible and accessible
  • βœ… Mains pressure: minimum 1.5 bar dynamic, 2–3 bar recommended
  • βœ… Building Regulations notification completed

Flow Rate & Pressure Requirements

Unvented cylinders deliver mains-pressure hot water, but performance depends on adequate incoming flow and pressure. Below are the key metrics to verify before installation.

ParameterMinimumRecommendedNotes
Static Pressure2.0 bar3.0–5.0 barMeasured at incoming main
Dynamic Pressure1.5 bar2.5+ barWith one outlet flowing at 10L/min
Flow Rate (Incoming)15 L/min25+ L/minAt kitchen cold tap
Shower Flow Rate8 L/min10–15 L/minPer shower outlet
Bath Fill Rate15 L/min20+ L/minFor acceptable bath fill time

If incoming flow is below 15 L/min, consider an accumulator or booster pump. Note that unvented cylinders cannot improve inadequate mains supply β€” they can only deliver what the mains provides.

Frequently Asked Questions – Unvented Cylinder Sizing

Expert answers to the most common questions from homeowners, installers, and building services engineers.

What size unvented cylinder do I need?
For most UK homes, a 180–250L cylinder suits 3–5 occupants with 1–2 bathrooms. Use our interactive calculator above for a personalised recommendation based on your exact household details.
Is a 210 litre cylinder enough for 4 people?
Yes, a 210L cylinder is typically sufficient for a 4-person household with 1–2 bathrooms, provided recovery is via an indirect gas boiler coil. For direct electric heating, consider 250L+ due to slower recovery.
What size cylinder for 2 bathrooms?
For a property with 2 bathrooms, a 210–250L cylinder is recommended. This supports up to 2 simultaneous showers during peak demand periods when paired with adequate boiler recovery.
How long does an unvented cylinder stay hot?
A well-insulated modern cylinder loses only 1–2Β°C per hour. Water heated to 60Β°C at 8am will typically still be above 50Β°C after 12–16 hours, remaining usable for evening demand without reheating.
What pressure is needed for an unvented cylinder?
A minimum dynamic pressure of 1.5 bar and flow rate of 15 L/min at the incoming main is required. Optimal performance is achieved at 2.5–3.5 bar with 25+ L/min flow.
Can a heat pump use an unvented cylinder?
Yes, but the cylinder must have an appropriately sized coil (3.0–4.5 mΒ² surface area) designed for lower flow temperatures. Standard boiler coils are inadequate for heat pump applications.
Are larger cylinders more efficient?
Not necessarily. Larger cylinders have higher standing heat losses. An oversized cylinder wastes energy reheating unused water. Size correctly β€” a cylinder that meets demand with a 1.3Γ— storage factor is optimal.
How many showers can run from a 250L cylinder?
A 250L cylinder at 60Β°C can typically support 2–3 simultaneous showers (at 10 L/min mixed to 40Β°C) for 15–20 minutes before noticeable temperature drop, assuming adequate boiler recovery.
What is the recovery time of a hot water cylinder?
Recovery time depends on the heat source. With a 20kW boiler and indirect coil, a 210L cylinder recovers from 70% drawdown in approximately 22–28 minutes. Direct electric cylinders may take 3–6 hours.
What is the best cylinder size for a family home?
For a typical UK family home (4–5 occupants, 2 bathrooms), a 210–250L indirect unvented cylinder offers the best balance of capacity, recovery speed, and energy efficiency.
What size unvented cylinder for a 4-bedroom house?
A 4-bedroom house typically houses 4–6 people. A 250L cylinder is the standard recommendation, providing ample capacity for multiple showers and occasional bath usage.
What size hot water cylinder for 5 people?
For 5 occupants, specify a 250–300L cylinder. The larger size accounts for higher simultaneous demand during morning and evening peaks.
How to calculate cylinder size?
Calculate daily hot water demand (occupants Γ— 40L + showers Γ— 60L + baths Γ— 120L), then multiply by a storage factor of 1.2–1.6 depending on peak simultaneity. See our formulas section for detailed methodology.
What is the lifespan of an unvented cylinder?
Stainless steel unvented cylinders typically last 15–25 years with proper maintenance. Annual servicing of safety devices is essential and legally required under G3 regulations.
Direct vs indirect unvented cylinder – which is better?
Indirect cylinders (heated by boiler) offer faster recovery and lower running costs. Direct cylinders (electric immersion) suit properties without gas but have higher operating costs. For most UK homes with gas boilers, indirect is the better choice.
What size cylinder for an air source heat pump?
Oversize by 25–50% compared to boiler-fed equivalent. For a 4-person home, specify 250–300L with a dedicated heat pump coil of 3.0+ mΒ² surface area.
What size cylinder for solar thermal?
Allow 50–80L of cylinder capacity per square metre of solar collector. A typical 3 mΒ² array pairs well with a 180–250L twin-coil cylinder.
How many litres should my hot water cylinder be?
As a rough guide: 90–120L for 1–2 people; 150–180L for 3–4 people; 210–250L for 4–5 people; 300L+ for 5+ people. Always verify with a detailed demand calculation.
What is standing heat loss?
Standing heat loss is the energy lost from a stored hot water cylinder to its surroundings over 24 hours, measured in kWh. Modern ERP-rated cylinders lose 1.0–2.0 kWh/day depending on size and insulation quality.
Do I need a G3 certificate for an unvented cylinder?
Yes. Installation, servicing, and any work on the safety devices of an unvented cylinder over 15 litres must be carried out by a G3-certified competent person. The installer must notify Building Control.
What is a tundish and why is it required?
A tundish is a visible air gap fitting installed in the discharge pipework from safety valves. It provides a visible warning if water is being discharged, indicating a potential fault requiring investigation.
Can I install an unvented cylinder myself?
No. UK law requires unvented cylinders to be installed by a G3-certified competent person. DIY installation is illegal, dangerous, and will invalidate warranties and home insurance.
How much does an unvented cylinder cost?
Cylinder unit costs range from Β£400–£1,200+ depending on size and brand. Professional installation typically adds Β£500–£1,000 including safety components, pipework, and Building Regulations notification.
What is the difference between vented and unvented cylinders?
Vented cylinders rely on a cold water storage tank (usually in the loft) and provide low-pressure hot water. Unvented cylinders connect directly to the mains, delivering high-pressure hot water without a header tank.
How do I know if my mains pressure is adequate?
Use a pressure gauge on an outside tap or washing machine connection. Measure static pressure (no flow) and dynamic pressure (with one tap running at 10 L/min). A G3 installer will verify this during the survey.
What size expansion vessel do I need?
Expansion vessel sizing depends on cylinder capacity and incoming pressure. Typically: 8–12L for cylinders up to 150L; 12–18L for 150–250L; 18–25L for 250–400L cylinders.
Can an unvented cylinder be used with a combi boiler?
Generally no β€” combi boilers heat water instantaneously and do not have a dedicated heating circuit for a cylinder. A system boiler or regular boiler is required for indirect cylinder heating.
What temperature should an unvented cylinder be set to?
Store water at 60–65Β°C. This prevents legionella bacteria growth (which thrives below 50Β°C) while avoiding excessive energy waste and scalding risk. A thermostatic mixing valve should reduce delivery temperature to 38–48Β°C at outlets.
How often should an unvented cylinder be serviced?
Annually, by a G3-certified engineer. The service includes checking all safety valves, expansion vessel charge, tundish, pressure reducing valve, and thermostat calibration.
What does ERP rating mean on a cylinder?
Energy-Related Products (ERP) rating indicates the energy efficiency class of the cylinder, from A+ (most efficient) to G (least efficient). The rating accounts for standing heat loss and reflects compliance with EU energy labelling regulations.
Can I use an unvented cylinder with Economy 7?
Yes. Direct electric unvented cylinders work well with Economy 7 tariffs. Specify a larger cylinder (250–300L for 4 people) to store enough hot water heated during the off-peak period for all-day use.
What is a twin-coil cylinder?
A twin-coil cylinder has two separate heating coils: one for the primary boiler and a second (usually lower) coil for a renewable heat source such as solar thermal panels or a heat pump.
How do I prevent legionella in an unvented cylinder?
Maintain stored water at 60Β°C minimum. Many cylinders include a weekly anti-legionella cycle that heats the entire cylinder to 65Β°C+ for a set period. Avoid prolonged periods below 50Β°C.
What are the best unvented cylinder brands in the UK?
Leading UK brands include Megaflo (Heatrae Sadia), Gledhill, Joule, RM Cylinders, Telford, and Worcester Bosch. All should be WRAS-approved and meet ERP efficiency standards.
What is the warranty on unvented cylinders?
Most stainless steel unvented cylinders carry a 10–25 year manufacturer warranty on the inner vessel, subject to annual servicing and G3-compliant installation. Components like valves typically have 1–2 year warranties.
Can an unvented cylinder explode?
In theory, if all safety devices fail simultaneously β€” which is extremely rare with properly maintained equipment. The multiple redundant safety systems (TPRV, expansion vessel, thermostat) make catastrophic failure virtually impossible in a serviced cylinder.
What pipe size is needed for unvented cylinder discharge?
Discharge pipework must be minimum 15mm copper (or equivalent) for the first section, increasing to 22mm+ for longer runs. Pipe must terminate safely at ground level or over a drain, with no risk of freezing.
How much space is needed for an unvented cylinder?
Allow the cylinder footprint plus 100mm clearance above and 50mm around all sides for installation access. A typical airing cupboard for a 210L cylinder needs to be approximately 700mm Γ— 700mm Γ— 1800mm high.
What size unvented cylinder for a flat?
For a 1–2 bedroom flat with 1–2 occupants, a 90–120L direct or indirect cylinder is usually sufficient. Consider a slimline model if space is constrained.
What size cylinder for a commercial kitchen?
Commercial applications require significantly larger storage. A small office kitchenette may use a 90–150L unit, while a restaurant may need 400L+ with high-recovery coils or multiple cylinders in parallel.
Can I use a smaller cylinder with a faster recovery boiler?
Yes, to an extent. A high-output boiler (24kW+ dedicated to the cylinder) with a fast-recovery coil can reduce the required storage volume by 10–20%. However, storage is still needed for simultaneous demand peaks.
What is a buffer cylinder?
A buffer cylinder (or thermal store) stores heated water for space heating rather than domestic hot water. It is distinct from an unvented DHW cylinder but may be used alongside one in renewable heating systems.
How does the cylinder coil affect recovery?
The coil's surface area determines heat transfer rate. Standard coils are 0.8–1.2 mΒ²; fast-recovery coils are 1.5–2.5 mΒ²; heat pump coils are 3.0–4.5 mΒ². Larger coil area = faster recovery for a given boiler output.
Are slimline unvented cylinders available?
Yes, many manufacturers offer slimline models with reduced diameter (typically 450–500mm) for tight spaces. Capacities typically range from 90–180L in slimline format.
What is the weight of a full unvented cylinder?
Water weighs 1 kg per litre. A 210L cylinder weighs approximately 250–262 kg when full (including the cylinder's own weight). Ensure the supporting floor or platform is structurally adequate.
Can I move my unvented cylinder to a different location?
Yes, but it requires a G3-certified installer to disconnect, move, and recommission the cylinder, including re-running discharge pipework and checking all safety devices. Building Regulations may require notification.
What causes an unvented cylinder to discharge water from the tundish?
Continuous discharge usually indicates a failed pressure reducing valve or expansion vessel. Intermittent dripping during heating is normal thermal expansion β€” but constant flow requires urgent investigation by a G3 engineer.
How energy efficient are modern unvented cylinders?
Modern ERP A-rated cylinders lose as little as 1.0–1.5 kWh per 24 hours β€” equivalent to about 30–45p per day. This is a fraction of the energy stored (10–14 kWh), representing over 85% efficiency in storage terms.
What is the best cylinder size for high-demand households?
For households with 5+ people, 3+ bathrooms, or frequent simultaneous usage, specify 300–400L with a fast-recovery indirect coil and a boiler of 24kW+ dedicated output. Consider multiple cylinders for very large properties.

Commercial Buying Guide: Choosing the Right Unvented Cylinder

When purchasing an unvented cylinder, consider these key factors beyond just capacity:

βœ… WRAS Approval

Ensure the cylinder is WRAS-approved, confirming compliance with UK water regulations and material safety standards.

βœ… ERP Energy Rating

Choose ERP A or B rated cylinders to minimise standing heat loss and comply with building regulations for new installations.

βœ… Warranty Period

Look for 10–25 year vessel warranties. Longer warranties often indicate higher-grade stainless steel and better manufacturing quality.

βœ… Coil Configuration

Single coil for boiler-only; twin-coil if you plan to add solar thermal or a heat pump in the future. Future-proof your investment.

βœ… Physical Dimensions

Measure your available space carefully. Consider slimline models if width is limited, or horizontal cylinders for loft installations.

βœ… After-Sales Support

Choose brands with strong UK-based technical support and readily available spare parts. This matters for long-term maintenance.

Summary: Getting Your Cylinder Size Right

Choosing the correct unvented cylinder size is the single most important decision for hot water system performance. An undersized cylinder leads to cold showers and family frustration; an oversized cylinder wastes energy and money. Use the engineering principles, charts, and calculator on this page to specify the right capacity for your household.

Key takeaways:

  • Calculate daily demand, apply a storage factor of 1.2–1.6Γ—
  • Match cylinder size to peak simultaneous demand, not average usage
  • Indirect cylinders offer faster recovery; direct cylinders suit off-gas properties
  • Oversize by 25–50% for heat pump applications
  • Always use a G3-certified installer β€” it's the law
  • Annual servicing protects your investment and ensures safety

Try the Calculator Again

Β© 2026 Plumbing Engineering Hub. This guide is for informational and educational purposes. Always consult a G3-certified plumbing professional for installation advice. Building Regulations Part G3 compliance is mandatory for all unvented cylinder installations in the UK.

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