Technical Deep Dive
BS 6891 Gas Safe Pressure Drop Compliance
Under BS 6891, the gas governor maintains 21 mbar at the meter. Permissible pressure drop to the boiler must not exceed 1.0 mbar to prevent combustion instability.
BS 6891 Gas Safe • Engineering Sizer
Boiler Gas Pipe Size Calculator
Verify copper gas pipe diameter (15mm, 22mm, 28mm) to ensure pressure drop from meter to boiler stays below 1.0 mbar.
BS 6891: Specification for Gas Pipework •
BS EN 12831 Verified
•
Dual Unit (kW & BTU)
BS 6891 & IGEM/UP/2 Standard
Max Allowable Drop: 1.0 mbar (Mains Gas)
Boiler Gas Pipe Size & Pressure Drop Calculator
Boiler Gross Heat Input (kW): 32 kW (Combi Boiler)
15 kW (System) 30–35 kW (Standard Combi) 60 kW (Large Boiler)
Straight Pipe Distance from Meter (meters): 12 meters
Fittings & Bends Allowance:
Recommended Pipe Diameter
22 mm Copper
✓ Complies with BS 6891 (< 1.0 mbar)
Natural Gas Volumetric Flow: 3.05 m³/hr
Total Equivalent Pipe Length: 15.6 meters
Calculated Pressure Drop: 0.68 mbar
Inlet Pressure at Boiler (from 21mbar): 20.32 mbar
⚠️ Why 15mm vs 22mm matters: Upgrading from an old 15 kW boiler to a 32 kW combi on an existing 15mm pipe run will starve the burner, triggering fault codes (e.g. EA/F28) when the shower demands maximum gas flow.
Burner Engineering
Anti-Cycling Firing Physics Boiler Modulation & Turndown Ratio (1:3 vs 1:10)
The maximum kW rating of your boiler only matters on the 3 or 4 coldest days of winter. For the remaining 95% of the heating season, your home requires just 2 kW to 6 kW of continuous heat. A high modulation turndown ratio (1:7 to 1:10) allows the burner to throttle down to a tiny flame without shutting off, preventing short-cycling and saving money.
Simulate Outdoor Seasonal Temperature: +8°C (Mild Autumn / Spring Day)
-5°C (Freezing Winter Peak - 10 kW Load) +8°C (Typical Season - 4.5 kW Load) +16°C (Mild Spring - 1.8 kW Load)
🌡️
Current Real-Time Heating Demand
House Needs 4.5 kW of Thermal Power
Target Room Temp: 21°C Constant
Narrow 1:3 Modulation
Budget / Older Boiler (1:3 Turndown)
Rated: 30 kW Maximum → 10 kW Minimum Limit. Because the house only needs 4.5 kW, the 10 kW burner overheats the water in 2 minutes and cuts out repeatedly.
🔥
Burner Output 10.0 kW (Minimum)
Ignition Cycles / Hour: 16 – 20 cycles
Return Water Temp: 62°C (Condensing Off)
Seasonal Gas Efficiency: 82.5%
❌ Rated: 30 kW Maximum → 10 kW Minimum Limit. Because the house only needs 4.5 kW, the 10 kW burner overheats the water in 2 minutes and cuts out repeatedly.
Elite 1:10 Modulation
Premium Boiler (1:10 Turndown)
Rated: 30 kW Maximum → 3.0 kW Minimum Limit. Burner modulates smoothly to match the exact demand, sustaining condensing mode without stopping.
🔥
Burner Output 4.5 kW (Perfect Match)
Ignition Cycles / Hour: 1 continuous cycle
Return Water Temp: 46°C (Peak Condensing)
Seasonal Gas Efficiency: 94.2%
✅ Rated: 30 kW Maximum → 3.0 kW Minimum Limit. Burner modulates smoothly to match the exact demand, sustaining condensing mode without stopping.
Thermodynamic Methodology
BS 6891 / IGEM/UP/2 Pole's Low-Pressure Gas Flow Formula Formula & Variables
Mathematical Formulation
Q = 0.0071 × sqrt( ( d^5 × h ) / ( s × L ) )
BS 6891 strictly mandates that gas pressure drop between the gas meter outlet (typically 21 mbar) and boiler gas inlet must NOT exceed 1.0 mbar (minimum 20 mbar working pressure at full fire).
Q
Gas volumetric flow rate required by burner Unit: m³/hr
d
Internal pipe diameter (e.g. 22mm copper = 20.2mm ID) Unit: mm
h
Permissible gas pressure drop across run (maximum 1.0 mbar) Unit: mbar
s
Specific gravity of natural gas relative to air (typically 0.60) Unit: ratio
L
Equivalent pipe length including elbow and tee bend resistance Unit: meters
Lookup Standards
BS 6891 Maximum Gas Pipe Lengths for ≤ 1.0 mbar Drop
Maximum permissible straight run in copper tube before exceeding 1.0 mbar pressure drop.
| Boiler Output (kW) | Gas Flow Rate (m³/hr) | 15mm Copper Tube (13.6mm ID) | 22mm Copper Tube (20.2mm ID) | 28mm Copper Tube (26.2mm ID) |
|---|---|---|---|---|
| 18 kW System | 1.90 m³/hr | 6.0 meters | 38.0 meters | 120.0 meters |
| 24 kW Combi | 2.54 m³/hr | 3.5 meters | 22.0 meters | 75.0 meters |
| 28 kW Combi | 2.96 m³/hr | 2.5 meters | 16.0 meters | 55.0 meters |
| 32 kW Combi | 3.38 m³/hr | 1.8 meters | 12.0 meters | 42.0 meters |
| 36 kW Combi | 3.81 m³/hr | 1.2 meters | 9.5 meters | 32.0 meters |
| 42 kW Combi | 4.44 m³/hr | 0.8 meters (Unsuitable) | 6.5 meters | 23.0 meters |
Each 90° elbow adds 0.5m equivalent length. Each tee adds 1.2m equivalent length.
Key Principles
Critical Sizing Factors & Engineering Influences
The 4 variables that move the boiler gas pipe result most, and how BS 6891: Specification for Gas Pipework expects each one to be established.
🔧
Equivalent Fitting Loss Allowances
Fittings cause directional turbulence. A 90° copper bend adds 0.5m equivalent pipe length, and a 90° tee adds 1.2m.
🍳
Simultaneous Firing of Other Appliances
Gas hobs (7 kW / 0.74 m³/hr) and gas fires (4 kW / 0.42 m³/hr) share the common meter manifold, consuming available pressure drop.
📏
Internal Tube Diameter vs Nominal Outer Diameter
Table X copper tube has thinner walls than Table Y. Calculations must use internal bore diameter, not outer sleeve.
⚡
Corrugated Stainless Steel (CSST / TracPipe)
TracPipe corrugated walls have higher frictional resistance, requiring 1 pipe size larger than smooth copper.
Prevent Costly Errors
Common Sizing Mistakes & How to Avoid Them
The 4 mistakes below cost more than any other on a boiler gas pipe job — pipe run and diameter oversights that drop working pressure below the appliance minimum.
Common Pitfall
Reusing Old 15mm Gas Pipe for a 32 kW Combi
Consequence: Gas inlet pressure drops to 14 mbar on full fire, causing loud roaring, explosive ignition, and F28 / EA flame failure error codes.
Correct Approach: Upgrade gas pipe from meter to 22mm or 28mm copper.
Common Pitfall
Measuring Pressure with Boiler Idle
Consequence: Static gas pressure reads 21 mbar at rest, masking severe pressure drop that occurs only when all burners fire.
Correct Approach: Always test working pressure with boiler running at maximum 100% service mode.
Common Pitfall
Ignoring Elbow and Bend Penalties
Consequence: A 10m pipe run with 6 elbows equals 13m equivalent length, causing the design to exceed 1.0 mbar.
Correct Approach: Calculate total equivalent length including all fittings.
Common Pitfall
Forgetting Gas Meter Governor Capacity
Consequence: A U6 domestic meter supplies max 6.0 m³/hr. Large boilers + range cookers can exceed meter capacity.
Correct Approach: Verify total appliance load remains below 6.0 m³/hr for standard domestic meters.
Quality Assurance Audit
Boiler Gas Pipe: Installer Consultation Audit
Put these 5 questions to your heating engineer before you approve a quoted output — each one checks the specification against BS 6891: Specification for Gas Pipework.
Readiness Score: 0 / 5
⚖️
CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against BS 6891: Specification for Gas Pipework (British Standards Institution & Gas Safe Register) for Boiler Gas Pipe Size Calculator.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
2 questions we are asked most about boiler gas pipe sizing, answered against British Standards Institution & Gas Safe Register guidance.
Only if the pipe run is extremely short (under 2–3 meters). For runs over 3m, 22mm copper is mandatory to prevent burner starvation.
The boiler will suffer flame lift-off, explosive ignition, humming/whistling noises, and lockouts (e.g. Worcester EA or Vaillant F28) when running hot water at full draw.
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