DHW Flow Physics • Engineering Sizer

Combi Boiler Size Calculator

Q: What size combi boiler do I need for a 3 bedroom house?

A 28 kW to 32 kW combi boiler is ideal for a standard 3-bedroom house with 1 to 2 bathrooms and 10 to 14 radiators, providing 11.5 to 13.1 L/min of instant hot water.

Q: Can a combi boiler run two showers at the same time?

Standard 24–30 kW combis struggle with simultaneous showers. To run two showers concurrently, you require a 38 kW to 42 kW storage combi boiler or an unvented cylinder system.

Q: Why is combi boiler kW higher than space heating need?

Central heating only needs 6 to 10 kW, but heating freezing tap water instantly to 45°C requires 28 to 35 kW for 10–14 L/min flow.

Q: Does mains water pressure affect combi sizing?

Yes. If incoming mains flow is only 11 L/min, installing a 38 kW combi (capable of 16 L/min) will not produce 16 L/min without a mains boost pump or accumulator.

Combi boilers must be sized primarily for peak domestic hot water (DHW) demand. Calculate your exact kilowatt requirements for simultaneous showers.

CIBSE Domestic Heating Design Guide & BS EN 12831 • BS EN 12831 Verified • Dual Unit (kW & BTU)

1. Select Boiler Type

Combi Instant Hot Water System Unvented Cylinder Regular Loft Tank & Cylinder

2. Property Type & Bedrooms

Bedrooms

2. Property Type & Bedrooms

Bathrooms

3. Number of Bathrooms

Radiators

5. Home Insulation Quality

Average / Standard

3. Radiators & Underfloor kW / °C

Radiators 3. Radiators & Underfloor

Recommended Boiler Size

95,500 BTU/hr

Recommended Boiler Output Range: 24 kW – 30 kW

Domestic Hot Water (DHW) Peak 11.5 L/min @ 35°C ΔT

Space Heating Heat Loss Load 14.2 kW +15% margin

Recommended Modulation Ratio 1:7 Ratio

Min: 3.5 kW Max: 28 kW

Hydraulic Flow Simulator

Can a Combi Boiler Run Two Showers at Once?

Simultaneous Outlet Physics

Homeowners often wonder why their shower turns freezing or drops to a trickle when someone runs the kitchen tap. Unlike system boilers with hot water cylinders, a combi boiler must heat water on-the-fly. Test how simultaneous taps impact water temperature and flow rate below:

1. Select Installed Combi Boiler Size

Incoming Mains Cold Water Flow Limit 16.0 L/min

10 L/min 16 L/min 26 L/min

Toggle Active Simultaneous Outlets in Use

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Master Ensuite Shower 9.0 L/min

🚿

Family Bathroom Shower 9.0 L/min

🚰

Kitchen Hot Tap 5.5 L/min

🧺

Washing Machine Fill 4.5 L/min

Hydraulic Delivery State Optimal Flow

Delivered Tap Water Temperature

45.0°C

🔥 45°C (@ 35°C Rise)

Total Demanded Flow Rate: 9.0 L/min

Max Boiler Thermal Delivery (@45°C): 13.1 L/min

Verdict:

The boiler comfortably supplies the shower at full mains pressure and temperature.

💡 If you frequently run 2 showers simultaneously, choose an 18–24 kW System Boiler + 210L Cylinder instead of an oversized combi.

Thermodynamic Methodology

Instantaneous Domestic Hot Water (DHW) Formula & Variables

BS EN 12831 / CIBSE Guide G

Mathematical Formulation

kW = ( Flow_Rate [L/min] × ΔT [°C] × 4.186 ) / 60

Unlike space heating which is continuous, domestic hot water must be generated instantaneously as cold mains water passes across the plate heat exchanger.

Flow_Rate

Water delivery flow rate required by shower or tap Unit: Litres per minute (L/min)

ΔT

Temperature rise needed (e.g. 10°C cold mains to 45°C hot outlet = 35°C) Unit: °C

4.186

Specific heat capacity of water Unit: kJ / (kg·°C)

Time conversion factor from minutes to seconds Unit: seconds

Lookup Standards

Combi Boiler Sizing & Shower Delivery Benchmarks

Standard kilowatt ratings matched to flow rates and household bathroom configurations.

Combi Rating (kW)	DHW Flow Rate (@ 35°C Rise)	Central Heating Power	Optimal House Archetype	Shower Capability
24 kW	9.8 L/min (2.6 GPM)	18–20 kW	1 Bed Flat / Studio	1 Standard Mixer Shower
28 kW	11.5 L/min (3.0 GPM)	24 kW	2–3 Bed Terraced / Semi	1 High-Pressure Shower
32 kW	13.1 L/min (3.5 GPM)	28 kW	3–4 Bed Semi-Detached	1 High-Flow Shower + 1 Tap
36 kW	14.7 L/min (3.9 GPM)	30 kW	4 Bed Detached Home	1 Rainfall Shower + Basin
40 kW	16.4 L/min (4.3 GPM)	32 kW	4–5 Bed Executive Home	2 Showers (Moderate Flow)
42+ kW Storage Combi	18.5+ L/min (4.9 GPM)	35 kW	Large 5 Bed / 3 Bath Home	2 High-Pressure Showers

Tested at 10°C incoming winter mains water temperature raised to 45°C delivery.

Key Principles

Critical Sizing Factors & Engineering Influences

The 4 variables that move the combi boiler result most, and how CIBSE Domestic Heating Design Guide & BS EN 12831 expects each one to be established.

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Incoming Mains Water Dynamic Pressure

A 38 kW combi cannot deliver 16 L/min if incoming street mains is choked to 11 L/min at the boundary stopcock.

❄️

Winter Incoming Water Temperature

Mains water drops from 18°C in summer down to 5°C in mid-winter, increasing the required temperature rise from 27°C to 40°C.

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Simultaneous Outlet Concurrency

Turning on a kitchen mixer tap while someone is showering splits the hot water flow across both outlets.

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Burner Modulation Turndown

High combi output (32 kW) must modulate down to 3 kW in space heating mode to prevent spring short-cycling.

Prevent Costly Errors

Common Sizing Mistakes & How to Avoid Them

The 4 mistakes below cost more than any other on a combi boiler job — domestic hot water assumptions that collapse the moment two outlets run together.

Common Pitfall

Sizing Combi purely on Radiator Count

Consequence: Installing a 24 kW combi for an 8-radiator house that has a 14 L/min rainfall shower creates weak, lukewarm showers.

Correct Approach: Always size combis based on domestic hot water flow requirements first.

Common Pitfall

Ignoring Gas Supply Pipe Diameter

Consequence: Running a 35 kW combi on a 15mm gas pipe causes flame starvation and boiler error lockouts.

Correct Approach: Run a dedicated 22mm or 28mm copper gas pipe from meter to boiler.

Common Pitfall

Over-specifying kW with Low Mains Flow

Consequence: Paying extra for a 40 kW boiler when mains water pressure only supports 10 L/min delivers zero hot water improvement.

Correct Approach: Measure dynamic mains flow with a flow weir cup before selecting boiler size.

Common Pitfall

Overlooking Minimum Turndown Ratio

Consequence: A boiler with poor 1:3 modulation short-cycles 80 times daily during mild autumn weather.

Correct Approach: Choose models with 1:10 modulation turndown (e.g. Worcester 4000 or Viessmann 100-W).

Quality Assurance Audit

Combi Boiler: Installer Consultation Audit

Put these 5 questions to your heating engineer before you approve a quoted output — each one checks the specification against CIBSE Domestic Heating Design Guide & BS EN 12831.

Readiness Score: 0 / 5

Measured Dynamic Mains Flow & Pressure Installer verified incoming cold mains delivers at least 15 L/min at 1.5 bar working pressure.

Gas Pipework Capacity Verification Gas Safe engineer confirmed 22mm/28mm gas line ensures < 1.0 mbar pressure drop at full fire.

Minimum Modulation Turndown ≤ 3.5 kW Boiler throttles down low enough to heat 6–8 radiators in spring without short-cycling.

Weather Compensation Controls Specified Outdoor sensor connected to enable condensing mode (below 54°C return) all winter.

System Flush & Magnetic Filter Installed Central heating circuit power-flushed and protected with Adey MagnaClean / Fernox TF1 filter.

Technical Deep Dive

Thermodynamic Principles of Combi Boiler Sizing

A combination boiler performs two distinct thermodynamic functions inside a single compact wall-hung casing: continuous low-power space heating for radiators and high-power instantaneous domestic hot water (DHW) via an internal brazed plate heat exchanger.

The Concurrency Dilemma in Modern Homes

Because a combi boiler transfers energy instantaneously, it possesses no stored volume. When a thermostatic shower mixer opens, the motorized diverter valve immediately prioritizes DHW, directing all burner output into the secondary heat exchanger. If a second tap is opened downstream, the hydraulic flow splits, causing immediate pressure drop and temperature fluctuations.

⚖️

CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED

Formulations benchmarked against CIBSE Domestic Heating Design Guide & BS EN 12831 (Chartered Institution of Building Services Engineers) for Combi Boiler Size Calculator.

Updated: February 2025

FAQ Knowledgebase

Frequently Asked Questions

4 questions we are asked most about combi boiler sizing, answered against Chartered Institution of Building Services Engineers guidance.

What size combi boiler do I need for a 3 bedroom house?

Can a combi boiler run two showers at the same time?

Why is combi boiler kW higher than space heating need?

Does mains water pressure affect combi sizing?

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Instant DHW

Combi Boiler Sizer

Calculate DHW L/min & GPM flow capacity for instantaneous showers.

Combi Boiler Size Calculator

Bedrooms

Bathrooms

Radiators

5. Home Insulation Quality

Room Dimensions & Building Physics

Hydraulic Flow Simulator

Toggle Active Simultaneous Outlets in Use

Recommended Boiler Sizing & Specification

Embed Widget

Can a Combi Boiler Run Two Showers at Once?

Instantaneous Domestic Hot Water (DHW) Formula & Variables

Combi Boiler Sizing & Shower Delivery Benchmarks

Critical Sizing Factors & Engineering Influences

Incoming Mains Water Dynamic Pressure

Winter Incoming Water Temperature

Simultaneous Outlet Concurrency

Burner Modulation Turndown

Common Sizing Mistakes & How to Avoid Them

Sizing Combi purely on Radiator Count

Ignoring Gas Supply Pipe Diameter

Over-specifying kW with Low Mains Flow

Overlooking Minimum Turndown Ratio

Combi Boiler: Installer Consultation Audit

Thermodynamic Principles of Combi Boiler Sizing

The Concurrency Dilemma in Modern Homes

Frequently Asked Questions

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