Oil Combi Systems • Engineering Sizer

Oil Combi Boiler Size Calculator: Output & Flow Rate

Q: How fast do oil combi boilers deliver hot water?

Modern oil combis deliver 15 to 20 Litres per minute of hot water assisted by an internal thermal store.

Q: Are oil combi boilers physically larger than gas combis?

Yes, because of the oil burner assembly and internal thermal store, oil combis are floor-standing and resemble a slimline washing machine.

Calculate kerosene oil combination boiler sizing featuring internal thermal heat storage.

OFTEC Book 2 & BS 5410 • BS EN 12831 Verified • Dual Unit (kW & BTU)

OFTEC & BS 5410 Engineering Engine

Oil Boiler Sizing, Nozzle & Kerosene Burner Calculator

Kerosene Class C2 (28-Second)

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)

Boiler Installation Position:

Nozzle Spray Pattern:

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.

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

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Family Bathroom Shower 9.0 L/min

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Kitchen Hot Tap 5.5 L/min

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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

Oil Combi Thermal Store Energy Storage Formula & Variables

OFTEC Technical Guide

Mathematical Formulation

Energy_Stored [kWh] = Volume [L] × 4.186 × ( T_store - T_cold ) / 3600

Because oil pressure-jet burners take 30–60 seconds to fire, oil combis incorporate an internal thermal buffer heat store (24L to 50L) to provide immediate hot water.

Volume

Internal thermal store fluid capacity Unit: Litres

T_store

Store holding temperature (typically 75°C) Unit: °C

Energy_Stored

Instantaneous usable heat energy for initial shower draw Unit: kWh

Lookup Standards

Oil Combi Sizing & Thermal Store Specifications

Output ratings, thermal store capacities, and hot water delivery rates.

Oil Combi Model	Heating Output	Internal Store Volume	DHW Flow Rate (@ 35°C Rise)	Recommended Property
Grant Vortex Pro Combi 21	21 kW	24 Litres Store	15.0 L/min	2–3 Bed Rural Home
Grant Vortex Pro Combi 26	26 kW	36 Litres Store	17.5 L/min	3–4 Bed Rural Home
Grant Vortex Pro Combi 36	36 kW	50 Litres Store	20.0 L/min	4–5 Bed Detached Rural Home
Firebird Envirogreen Combi 26	26 kW	30 Litres Store	16.0 L/min	3–4 Bed Detached Home

Thermal store allows high initial flow rates exceeding standard gas combis.

Key Principles

Critical Sizing Factors & Engineering Influences

The 2 primary variables that determine actual heat demand, and how OFTEC Book 2 & BS 5410 calculates each.

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Internal Thermal Buffer Heat Store

Stores pre-heated water to bridge the delay between tap opening and oil burner ignition.

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Floor-Standing Footprint Requirement

Oil combis are floor-standing appliances weighing 120kg–160kg requiring dedicated floor space.

Prevent Costly Errors

Common Sizing Mistakes & How to Avoid Them

Fuel delivery and burner miscalculations to prevent on oil combi boiler size calculator: output & flow rate installations — nozzle and combustion mismatches that waste fuel and soot the heat exchanger.

Common Pitfall

Expectation of Wall-Hung Dimensions

Consequence: Purchasing an oil combi expecting it to fit in a kitchen wall cupboard.

Correct Approach: Allocate floor space in a utility room, garage, or outdoor module.

Quality Assurance Audit

Oil Combi Boiler Size Calculator: Output & Flow Rate: Installer Consultation Audit

Put these 2 essential questions to your heating engineer before approving an installation quote — benchmarked against OFTEC Book 2 & BS 5410.

Readiness Score: 0 / 2

Thermal Store Temperature Set to 75°C Verified thermostatic store sensor maintains correct buffer heat.

Tigerloop De-Aerator Installed Fitted to ensure instant bubble-free fuel delivery to burner.

Technical Deep Dive

Why Oil Combi Boilers Include Thermal Stores

Unlike gas burners that ignite in 2 seconds, oil pressure-jet burners require pre-purge fan and oil pre-heating. An internal thermal store provides instantaneous hot water.

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CIBSE & BS EN Standards-Compliant Framework BENCHMARKED

Formulations benchmarked directly against published British & European standards: OFTEC Book 2 & BS 5410 (OFTEC & BSI) for Oil Combi Boiler Size Calculator: Output & Flow Rate. Maintained by the Boiler Size Calculator Technical Editorial Desk.

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Standards Revision: 2026

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