BS 6891 Gas Safe • Engineering Sizer

Boiler Gas Pipe Size Calculator: Pressure Drop & Sizing

Q: Can I run a 30kW boiler on a 15mm gas pipe?

Only if the pipe run is extremely short (under 2–3 meters). For runs over 3m, 22mm copper is mandatory to prevent burner starvation.

Q: What are the symptoms of an undersized gas pipe?

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.

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

Boiler Gas Pipe Size & Pressure Drop Calculator

Max Allowable Drop: 1.0 mbar (Mains Gas)

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:

90° Elbows (0.5m eq):

90° Bends (0.3m eq):

Tee Fittings (1.0m eq):

Gas Pipework Material:

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.

⚠️ Statutory Gas Safety Notice (BS 6891 & UK Law) Under the Gas Safety (Installation and Use) Regulations 1998, physical gas installation, pipe alteration, and pressure testing must be performed exclusively by a certified Gas Safe Registered engineer. These outputs are for preliminary planning and educational estimation only.

Burner Engineering

Boiler Modulation & Turndown Ratio (1:3 vs 1:10)

Anti-Cycling Firing Physics

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)

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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.

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Burner Output 10.0 kW (Minimum)

Short-Cycling (18/hr)

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.

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Burner Output 4.5 kW (Perfect Match)

Continuous Smooth Burn

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

Pole's Low-Pressure Gas Flow Formula Formula & Variables

BS 6891 / IGEM/UP/2

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).

Gas volumetric flow rate required by burner Unit: m³/hr

Internal pipe diameter (e.g. 22mm copper = 20.2mm ID) Unit: mm

Permissible gas pressure drop across run (maximum 1.0 mbar) Unit: mbar

Specific gravity of natural gas relative to air (typically 0.60) Unit: ratio

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 primary variables that determine actual heat demand, and how BS 6891: Specification for Gas Pipework calculates each.

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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.

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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.

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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.

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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

Component specification errors that trigger system faults — 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 Size Calculator: Pressure Drop & Sizing: Installer Consultation Audit

Verification checkpoints to audit with your technician before pipework and components are commissioned (BS 6891: Specification for Gas Pipework).

Readiness Score: 0 / 5

Working Pressure Tested at Boiler Gas Valve Measured with boiler firing at 100% maximum output (must be ≥ 20.0 mbar).

Standing vs Working Drop ≤ 1.0 mbar Verified total pressure drop between meter outlet and boiler does not exceed 1.0 mbar.

Gas Pipe Diameter Sized for Full DHW Firing Installed 22mm or 28mm copper line for combi boilers rated 28 kW to 42 kW.

Gas Tightness Test Conducted (BS 6891) Tested with digital manometer at 20 mbar for 2 minutes with zero permissible drop.

Emergency Gas Isolation Valve Installed Fitted lever isolation ball valve accessible adjacent to the boiler casing.

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.

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CIBSE & BS EN Standards-Compliant Framework BENCHMARKED

Formulations benchmarked directly against published British & European standards: BS 6891: Specification for Gas Pipework (British Standards Institution & Gas Safe Register) for Boiler Gas Pipe Size Calculator: Pressure Drop & Sizing. Maintained by the Boiler Size Calculator Technical Editorial Desk.

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Standards Revision: 2026

FAQ Knowledgebase

Frequently Asked Questions

Direct engineering guidance for common boiler gas pipe size calculator: pressure drop & sizing questions, referenced against British Standards Institution & Gas Safe Register.

Can I run a 30kW boiler on a 15mm gas pipe?

What are the symptoms of an undersized gas pipe?

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