Technical Deep Dive
Grant Vortex Fine-Tuning & Burner Calibration
Grant Vortex boilers offer flexible adjustable output brackets. A qualified OFTEC technician can set the burner nozzle to match the building heat loss precisely.
Grant Oil Matrix • Engineering Sizer
Grant Oil Boiler Size Calculator
Calculate the right Grant oil boiler model and nozzle size for off-gas grid country homes.
Grant Engineering & OFTEC Guidelines •
BS EN 12831 Verified
•
Dual Unit (kW & BTU)
Manufacturer Product Range
Official Technical Specs
Grant Oil Boilers Sizing Matrix & Model Lineup
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Independent Engineering Guide: Boiler Size Calculator is an independent sizing resource and is NOT affiliated with, authorized, endorsed, or sponsored by Grant Engineering.
Grant Vortex condensing oil boilers are the industry benchmark across the UK and Ireland for rural off-grid heating, offering exceptional 93%+ ErP seasonal fuel efficiency.
| Model Range | Boiler Type | Available Outputs | DHW Flow Rate | Modulation Turndown | Standard Warranty | Optimal Property Archetype |
|---|---|---|---|---|---|---|
| Grant Vortex Pro Internal | System & Regular Oil | 15/21 kW, 21/26 kW, 26/36 kW | Via Unvented Cylinder | 2-Stage Kerosene Burner | 5–10 Years | Utility Room Rural Installations |
| Grant Vortex Outdoor Module | External Weatherproof Oil | 15/21 kW, 21/26 kW, 26/36 kW | External System | OFTEC Compliant | 5–10 Years | Freeing up Internal House Space |
| Grant Vortex Pro Combi | Oil Combi (with Store) | 21 kW, 26 kW, 36 kW | 15.0 – 20.0 L/min | Internal Heat Store | 5–10 Years | Instant Hot Water for Off-Grid Homes |
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 Grant Oil Output Calibration Formula & Variables
Mathematical Formulation
kW_grant = Heat_Loss [kW] + Cylinder_Allowance [3.0 kW]
Grant Vortex boilers can be fine-tuned by changing the Danfoss burner nozzle and pump pressure to match exact house heat loss.
Grant_15/21
Adjustable from 15 kW to 21 kW via burner nozzle Unit: kW
Grant_21/26
Adjustable from 21 kW to 26 kW via burner nozzle Unit: kW
Grant_26/36
Adjustable from 26 kW to 36 kW via burner nozzle Unit: kW
Lookup Standards
Grant Oil Boiler Selection Guide
Model specifications by property type and radiator count.
| Grant Oil Model | Adjustable Output | Radiators Handled | Water Storage Type | Location |
|---|---|---|---|---|
| Grant Vortex Eco 15/21 | 15 – 21 kW | 8 – 12 Radiators | External Cylinder | Utility Room / Garage |
| Grant Vortex Pro 21/26 | 21 – 26 kW | 12 – 16 Radiators | External Cylinder | Indoor Utility / Kitchen |
| Grant Vortex Pro Outdoor 26/36 | 26 – 36 kW | 16 – 22 Radiators | External Cylinder | Outdoor Weatherproof Module |
| Grant Vortex Pro Combi 26 | 26 kW | 10 – 14 Radiators | Internal 36L Store | Indoor Floor-Standing |
Riello RDB burner technology fitted as standard.
Key Principles
Critical Sizing Factors & Engineering Influences
The 1 variable that move the grant oil boiler result most, and how Grant Engineering & OFTEC Guidelines expects each one to be established.
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Riello RDB Pressure-Jet Burner
World-renowned reliability with precision flame photocell and oil pump.
Prevent Costly Errors
Common Sizing Mistakes & How to Avoid Them
The mistake below costs more than any other on a grant oil boiler job — specification mismatches that surface only after the unit is installed and commissioned.
Common Pitfall
Leaving Burner at Factory Max Setting
Consequence: Overheats heating circuit if house only requires 18 kW.
Correct Approach: Engineer should recalibrate nozzle to match measured heat loss.
Quality Assurance Audit
Grant Oil Boiler: Installer Consultation Audit
Put this question to your heating engineer before you approve a quoted output — each one checks the specification against Grant Engineering & OFTEC Guidelines.
Readiness Score: 0 / 1
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CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against Grant Engineering & OFTEC Guidelines (Grant Engineering & OFTEC Ireland / UK) for Grant Oil Boiler Size Calculator.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
1 question we are asked most about grant oil boiler sizing, answered against Grant Engineering & OFTEC Ireland / UK guidance.
Yes, by fitting a 0.75 USgph nozzle and setting pump pressure to 10.0 bar, the Grant 26/36 runs at exactly 30 kW output.
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
Open Calculator →
Combi Boiler Sizer
Calculate DHW L/min & GPM flow capacity for instantaneous showers.
Stored Water
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System Boiler Sizer
Calculate space heating kW + unvented cylinder recovery capacity.
Conventional
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Regular Boiler Sizer
Open-vented gravity systems with loft header tanks.
Commercial
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Commercial Cascade Sizer
Cascade systems and plate heat exchangers (50 kW – 500+ kW).
Steam & BHP
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Steam Boiler Sizer
Calculate Boiler Horsepower (BHP) and steam flow in lbs/hr.
Heavy Process
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Industrial Boiler Sizer
High-capacity industrial thermal process and manufacturing loads.
Natural Gas
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Gas Boiler Sizer
Mains natural gas condensing output with modulation analysis.
Gas Combi
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Gas Combi Sizer
Peak gas volume flow and instantaneous hot water demand.
Off-Grid Oil
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Oil Boiler Sizer
Kerosene 28-second oil-fired combi, system, and external units.
Oil Combi
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Oil Combi Sizer
Thermal store oil combination boilers for rural homes.
100% Electric
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Electric Boiler Sizer
Kilowatt capacity and single-phase 60A/100A main fuse limits.
Electric Combi
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Electric Combi Sizer
Direct electric combination flow rates and amperage requirements.
LPG Propane
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Propane Boiler Sizer
Bulk tank and 47kg cylinder LPG condensing boiler sizing.
Wood Pellets
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Biomass Boiler Sizer
Wood pellet boilers and 50L/kW thermal buffer accumulator tanks.
Log Gasification
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Wood Boiler Sizer
Batch-fired log gasification boilers and heat accumulators.
Outdoor Furnaces
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Outdoor Wood Boiler
Outdoor hydronic wood furnaces for homes and workshops.
BS 7074 Spec
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Expansion Tank Calculator
Calculate expansion vessel volume in Litres and pre-charge bar.
BS 6891 Spec
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Gas Pipe Size Calculator
Calculate copper pipe diameter (15-28mm) for ≤ 1.0 mbar drop.
Hydronic Flow
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Heating Pipe Size Sizer
Flow velocity & diameter (15mm to 35mm) by kW circuit load.
BS 5440 Spec
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Flue Size Calculator
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
Open Calculator →
Radiator BTU Calculator
Room-by-room Watts & BTU for Type 11, 21, and 22 radiators at ΔT50.
BS EN 12831
Open Calculator →
Whole-House Heat Loss
Fabric U-values, air infiltration, and winter design temperature.
Financial Loss
Open Calculator →
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
Open Calculator →
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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Boiler Sizing Canada
Severe cold climate sizing (-30°C) in MBH and kW.
Greenstar Series
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Worcester Bosch Sizer
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 Boiler Sizer
Ideal Logic+ and Vogue Max boiler capacity matching.
Energy Series
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Glow-worm Energy and Compact combi/system output guide.
Vortex Oil
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Grant Boiler Sizer
Grant Vortex internal and outdoor condensing oil boilers.
Outdoor Modules
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Grant Oil Boiler Sizer
Weatherproof external and utility room kerosene oil units.
British Gas Tiers
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British Gas Boiler Sizer
Independent comparison of British Gas standard & premium tiers.
US Baseboard
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Slant/Fin Boiler Sizer
North American cast iron Sentry, Bobcat, and Intrepid MBH models.