Technical Deep Dive
OFTEC Kerosene Oil Combustion Engineering
Oil boilers atomize Class C2 kerosene at 8–12 bar pressure through a precision nozzle, mixing with combustion air for high-efficiency heat transfer.
Off-Grid Heating • Engineering Sizer
Oil Boiler Size Calculator
Precise sizing for kerosene 28-second oil-fired combi, system, and external regular boilers.
OFTEC Technical Books & BS 5410 •
BS EN 12831 Verified
•
Dual Unit (kW & BTU)
OFTEC & BS 5410 Engineering Engine
Kerosene Class C2 (28-Second)
Oil Boiler Sizing, Nozzle & Kerosene Burner Calculator
Target Heating Output (kW): 21 kW (Standard 3-4 Bed)
15 kW (Small Cottage) 21–26 kW (Standard Rural Home) 36+ kW (Large Farmstead)
Burner Oil Pump Pressure (bar / PSI): 8.5 bar (123 PSI)
Recommended Burner Nozzle
0.60 USgal/hr 80°
OFTEC-matched Danfoss / Delavan oil nozzle size calibrated for clean, soot-free combustion at 93%+ ErP efficiency.
Kerosene Fuel Consumption: 2.18 Litres / hour
Gross Heat Input (at 93% ErP): 22.6 kW (77,100 BTU)
1,000L Oil Tank Full-Fire Runtime: ~458 hours burn
Oil Feed Pipe Diameter: 10mm Copper (with Tigerloop)
🛢️ De-Aeration (Tigerloop) Tip: For external oil tanks below the burner level, a de-aerator (Tigerloop) eliminates air locks without needing a return pipe to the oil tank.
The Danger of Oversizing: Why Bigger Is Not Better
The Hidden Cost of Installing an Oversized Boiler
For decades, installers applied a sloppy rule of thumb: Add 30% extra capacity just in case. In modern condensing gas boilers, oversizing causes severe short-cycling, wastes 10–15% of annual fuel, and causes premature component failure.
Interactive Thermodynamics Simulator
Installed Boiler Output vs Real Heat Loss (6 kW Requirement): Short-Cycling & Condensing Loss Simulator
Installed Boiler Output vs Real Heat Loss (6 kW Requirement): 18 kW (300% Sized)
6 kW (100% Perfect Match) 18 kW (Typical 3x Oversized) 36 kW (6x Severely Oversized)
Ignition Cycles / Hour
14 cycles
Ideal is 1 to 2 continuous long modulation burns.
Return Water Temp
62°C
❌ Condensing HALTED (>54°C)
True Seasonal Efficiency
83.5%
ErP rated at 94% on lab test bench.
Wasted Fuel Cost
£215 / yr
Wasted in pre-purge exhaust & lost condensation.
1
Short-Cycling & Fuel Waste
When a boiler is too powerful, it dumps heat into the radiators faster than the rooms can absorb it. The water returns hot within 2 minutes, forcing the burner to shut off, only to refire minutes later. This rapid cycling loses 8–15% in standby ignition losses.
2
Disabling Condensing Mode
Modern A-rated boilers only achieve 92–94% efficiency when return water stays below 54°C (130°F). An oversized unit forces temperatures above 65°C immediately, preventing latent heat condensation and dropping true efficiency.
3
Premature Part Failure
Ignition spark generators, gas solenoid valves, circulating pumps, and heat exchanger seams suffer severe thermal stress from 100+ ignitions a day, causing expensive leaks and fan failures within 4 to 6 years.
Thermodynamic Methodology
OFTEC Book 2 / BS 5410-1 Kerosene Fuel Burn Rate & Heat Input Formula Formula & Variables
Mathematical Formulation
Fuel_Rate [L/hr] = Boiler_Output [kW] / ( 10.3 [kWh/L] × Efficiency [0.93] )
Kerosene (Class C2 28-second oil) has a net calorific value of 10.3 kWh per Litre. At 93% condensing efficiency, every 1 kW of heat output requires 0.104 Litres of oil per hour.
Boiler_Output
Required central heating and cylinder heating output Unit: kW
10.3
Net calorific value of standard 28-second kerosene Unit: kWh / Litre
Efficiency
OFTEC seasonal condensing ErP efficiency Unit: 0.92 to 0.94
Nozzle_USGPH
Danfoss / Delavan burner nozzle flow rate at 7.0 bar test pressure Unit: USgal/hr
Lookup Standards
Oil Boiler Output Tiers & Nozzle Sizing Benchmark
Standard factory kW brackets, burner nozzle selections, and kerosene consumption rates.
| Oil Output Bracket | Factory Burner Nozzle | Oil Pump Pressure | Fuel Consumption | Optimal Property Type |
|---|---|---|---|---|
| 15 kW – 21 kW | 0.50 – 0.55 USgph 80°EH | 8.0 – 9.0 bar | 1.55 – 2.18 L/hr | 2–3 Bed Rural Cottage / Bungalow |
| 21 kW – 26 kW | 0.60 – 0.65 USgph 80°EH | 8.5 – 10.0 bar | 2.18 – 2.70 L/hr | 3–4 Bed Detached Rural Home |
| 26 kW – 36 kW | 0.75 – 0.85 USgph 80°EH | 9.0 – 11.5 bar | 2.70 – 3.75 L/hr | 4–5 Bed Large Farmhouse |
| 36 kW – 46 kW | 1.00 – 1.10 USgph 60°S | 10.5 – 12.5 bar | 3.75 – 4.80 L/hr | Large Estate / Multi-Zone |
Nozzle spray angles calibrated for high-efficiency multi-pass secondary condensing heat exchangers.
Key Principles
Critical Sizing Factors & Engineering Influences
The 2 variables that move the oil boiler result most, and how OFTEC Technical Books & BS 5410 expects each one to be established.
🛢️
Limited Burner Modulation Turndown
Unlike gas burners that modulate down to 1:10, most pressure-jet oil burners are fixed 1-stage or 2-stage fire.
🔄
Tigerloop De-Aeration Oil Supply Lines
When the oil storage tank is located below burner level, a Tigerloop de-aerator eliminates air bubbles without a tank return line.
Prevent Costly Errors
Common Sizing Mistakes & How to Avoid Them
The mistake below costs more than any other on a oil boiler job — nozzle and combustion mismatches that waste fuel and soot the heat exchanger.
Common Pitfall
Oversizing Oil Boiler on Fixed-Output Burner
Consequence: Severe short-cycling causes soot buildup on baffles.
Correct Approach: Match burner nozzle strictly to building heat loss.
Quality Assurance Audit
Oil Boiler: Installer Consultation Audit
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against OFTEC Technical Books & BS 5410.
Readiness Score: 0 / 2
⚖️
CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against OFTEC Technical Books & BS 5410 (Oil Firing Technical Association (OFTEC) & BSI) for Oil Boiler Size Calculator.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
2 questions we are asked most about oil boiler sizing, answered against Oil Firing Technical Association (OFTEC) & BSI guidance.
Standard oil boiler models range from 15–21 kW, 21–26 kW, and 26–36 kW. Most 3 to 4 bed rural homes use a 21–26 kW unit.
OFTEC recommends annual servicing to replace the burner nozzle, clean the photocell, check oil pump pressure, and scrape heat exchanger baffles.
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:
Instant DHW
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Combi Boiler Sizer
Calculate DHW L/min & GPM flow capacity for instantaneous showers.
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Conventional
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Regular Boiler Sizer
Open-vented gravity systems with loft header tanks.
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Cascade systems and plate heat exchangers (50 kW – 500+ kW).
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Calculate Boiler Horsepower (BHP) and steam flow in lbs/hr.
Heavy Process
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Oil Boiler Sizer
Kerosene 28-second oil-fired combi, system, and external units.
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Thermal store oil combination boilers for rural homes.
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Electric Combi Sizer
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Wood Boiler Sizer
Batch-fired log gasification boilers and heat accumulators.
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Outdoor Wood Boiler
Outdoor hydronic wood furnaces for homes and workshops.
BS 7074 Spec
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Calculate expansion vessel volume in Litres and pre-charge bar.
BS 6891 Spec
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Calculate copper pipe diameter (15-28mm) for ≤ 1.0 mbar drop.
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Flow velocity & diameter (15mm to 35mm) by kW circuit load.
BS 5440 Spec
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Concentric 60/100mm & 80/125mm equivalent length and bend penalties.
BS 7593 Spec
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System Volume Calculator
Water volume in Litres and chemical inhibitor bottle dosing.
ASME Spec
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Boiler Tube Size Chart
Firetube & watertube OD, BWG gauge, and thickness lookup table.
Emitter Sizing
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Radiator BTU Calculator
Room-by-room Watts & BTU for Type 11, 21, and 22 radiators at ΔT50.
BS EN 12831
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Whole-House Heat Loss
Fabric U-values, air infiltration, and winter design temperature.
Financial Loss
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Boiler Cycling Cost Sizer
Calculate annual fuel wasted from oversized boiler short-cycling.
Low Carbon
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Boiler vs Heat Pump Sizer
Compare 70°C gas vs 45°C heat pump radiator surface uprating.
Dual Output
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Stove Back Boiler Sizer
Split heat calculation: output to room vs output to radiators.
1-2 Bed Flat
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2-Bedroom House Sizer
24 kW combi sizing for compact flats and terraced properties.
3-Bed Semi
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3-Bedroom House Sizer
28–30 kW combi or 15–18 kW system boiler recommendations.
4-Bed Detached
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4-Bedroom House Sizer
High-demand family homes with 2+ bathrooms and en-suites.
5-Bed Executive
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5-Bedroom House Sizer
Large 24–32 kW system boilers + 300L unvented cylinders.
Square Footage
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Sizing by Square Footage
Square feet / m² heat loss conversion across climate zones.
Low-Temp UFH
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Boiler with Underfloor Heating
Low-temperature manifold blending and minimum burner modulation.
In-Floor Loops
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Radiant Heat Boiler Sizer
Hydronic radiant floor loops and low-temperature condensing.
UK Part L
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Boiler Size Calculator UK
CIBSE domestic heating design guide and Gas Safe standards.
SEAI Ireland
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Boiler Size Calculator Ireland
Irish home heating standards and SEAI energy upgrade guidelines.
Irish Kerosene
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Oil Boiler Ireland Sizer
Dedicated oil boiler sizing for rural Irish detached homes.
OFTEC UK
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Oil Boiler Calculator UK
OFTEC standards for kerosene central heating in rural UK.
CSA B149 Canada
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Severe cold climate sizing (-30°C) in MBH and kW.
Greenstar Series
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Greenstar 2000, 4000, and 8000 series sizing comparison.
ecoTEC Series
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Vaillant Boiler Sizer
ecoTEC pro, plus, and exclusive series kilowatt matching.
600 / 800 Series
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Cupboard-fit Baxi 600, 800, and Platinum combi/system sizing.
Vitodens Series
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Viessmann Boiler Sizer
1:10 Inox-Radial Vitodens 050-W and 100-W efficiency matrix.
Logic & Vogue
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Ideal Logic+ and Vogue Max boiler capacity matching.
Energy Series
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Glow-worm Energy and Compact combi/system output guide.
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Grant Vortex internal and outdoor condensing oil boilers.
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Grant Oil Boiler Sizer
Weatherproof external and utility room kerosene oil units.
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Independent comparison of British Gas standard & premium tiers.
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Slant/Fin Boiler Sizer
North American cast iron Sentry, Bobcat, and Intrepid MBH models.