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.
Oil Combi Systems • Engineering Sizer
Oil Combi Boiler Size Calculator
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
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.
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
OFTEC Technical Guide Oil Combi Thermal Store Energy Storage Formula & Variables
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 variables that move the oil combi boiler result most, and how OFTEC Book 2 & BS 5410 expects each one to be established.
🔋
Internal Thermal Buffer Heat Store
Stores pre-heated water to bridge the delay between tap opening and oil burner ignition.
📐
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
The mistake below costs more than any other on a oil combi boiler job — 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: Installer Consultation Audit
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against OFTEC Book 2 & BS 5410.
Readiness Score: 0 / 2
⚖️
CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against OFTEC Book 2 & BS 5410 (OFTEC & BSI) for Oil Combi Boiler Size Calculator.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
2 questions we are asked most about oil combi boiler sizing, answered against OFTEC & BSI guidance.
Modern oil combis deliver 15 to 20 Litres per minute of hot water assisted by an internal thermal store.
Yes, because of the oil burner assembly and internal thermal store, oil combis are floor-standing and resemble a slimline washing machine.
Heating Engineering Suite
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