Technical Deep Dive
Gas Combi Flow Sizing & Plate Heat Exchangers
Gas combination boilers provide instantaneous hot water on demand, eliminating the need for cold storage tanks and cylinders.
Gas Combi Engine • Engineering Sizer
Gas Combi Boiler Size Calculator
Size natural gas combination boilers for optimum hot water flow and condensing efficiency.
BS 6891 & CIBSE Guide G •
BS EN 12831 Verified
•
Dual Unit (kW & BTU)
Bedrooms
2. Property Type & Bedrooms
3
Bathrooms
3. Number of Bathrooms
2
Radiators
Radiators
12
5. Home Insulation Quality
Average / Standard
3. Radiators & Underfloor kW / °C
Recommended Boiler Size
28
kW
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
Simultaneous Outlet Physics Can a Combi Boiler Run Two Showers at Once?
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:
Incoming Mains Cold Water Flow Limit 16.0 L/min
10 L/min 16 L/min 26 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
BS EN 12831 Gas Combi Flow & Heat Exchange Formula & Variables
Mathematical Formulation
kW_dhw = ( L/min × ΔT × 4.186 ) / 60
Gas combi boilers fire at maximum burner capacity to heat incoming cold mains water in a single rapid pass through a plate heat exchanger.
kW_dhw
Instantaneous domestic hot water firing rate Unit: kW
L/min
Shower delivery flow rate Unit: Litres per minute
ΔT
Temperature rise across plate exchanger Unit: °C
Lookup Standards
Gas Combi Output vs Shower Flow Rates
Instant hot water delivery benchmarks across gas combi sizes.
| Combi Model Size | Flow Rate @ 35°C Rise | Flow Rate @ 40°C Rise (Winter) | Shower Capability | Gas Pipe Sizing |
|---|---|---|---|---|
| 24 kW Combi | 9.8 L/min | 8.6 L/min | 1 Standard Shower | 15mm (Short) / 22mm |
| 30 kW Combi | 12.3 L/min | 10.8 L/min | 1 High Flow Shower | 22mm Dedicated Line |
| 35 kW Combi | 14.3 L/min | 12.5 L/min | 1 Rainfall Shower | 22mm / 28mm Line |
| 40 kW Combi | 16.4 L/min | 14.3 L/min | 1 Rainfall + Basin | 28mm Primary Line |
Tested with 1.5 bar dynamic mains water supply pressure.
Key Principles
Critical Sizing Factors & Engineering Influences
The 2 variables that move the gas combi boiler result most, and how BS 6891 & CIBSE Guide G expects each one to be established.
🔄
Plate Heat Exchanger Surface Area
Larger plate packs (e.g. 24 plates) transfer heat faster with less hydraulic resistance.
❄️
Winter Incoming Mains Temperature
Mains water drops to 5°C in winter, requiring 40°C temperature elevation.
Prevent Costly Errors
Common Sizing Mistakes & How to Avoid Them
The mistake below costs more than any other on a gas combi boiler job — domestic hot water assumptions that collapse the moment two outlets run together.
Common Pitfall
Installing 35 kW Combi on 15mm Gas Pipe
Consequence: Flame starvation triggers F28/EA error codes.
Correct Approach: Run 22mm copper gas pipe from meter.
Quality Assurance Audit
Gas Combi Boiler: Installer Consultation Audit
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against BS 6891 & CIBSE Guide G.
Readiness Score: 0 / 2
⚖️
CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against BS 6891 & CIBSE Guide G (Gas Safe Register & CIBSE) for Gas Combi Boiler Size Calculator.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
2 questions we are asked most about gas combi boiler sizing, answered against Gas Safe Register & CIBSE guidance.
A 30kW gas combi boiler requires a minimum 22mm copper gas supply to prevent pressure drop under full fire.
A 35kW combi produces approximately 14.3 Litres per minute at a standard 35°C temperature rise.
Heating Engineering Suite
40+ Dedicated Heating Utilities Specialized Boiler Calculators & Engineering Tools
Explore our complete suite of dedicated heating engines, pipe sizing tools, house archetype guides, and brand comparison calculators:
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