Technical Deep Dive
Grant Vortex Oil Sizing & Renewable Biofuel Future
Grant is the undisputed market leader in oil-fired heating across the UK and Ireland, delivering exceptional condensing efficiency and HVO biofuel compatibility.
Grant Vortex Oil • Engineering Sizer
Grant Boiler Size Calculator
Size high-efficiency Grant Vortex condensing oil boilers for off-grid homes in the UK and Ireland.
Grant Engineering Technical Manuals & OFTEC •
BS EN 12831 Verified
•
Dual Unit (kW & BTU)
Manufacturer Product Range
Official Technical Specs
Grant Engineering Sizing Matrix & Model Lineup
ℹ️
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 Vortex Stainless Steel Condensing Sizing Formula & Variables
Mathematical Formulation
Vortex_Efficiency = 93.3% ErP Seasonal Condensing (HVO Biofuel Compatible)
Grant Vortex condensing oil boilers feature a patented secondary stainless steel heat exchanger that achieves award-winning ErP efficiency on both kerosene and HVO biofuel.
Vortex_Pro
Premium indoor / outdoor oil boiler (15–26 kW, 26–36 kW, 36–46 kW) Unit: 10 Year Guarantee
Vortex_Eco
Compact entry indoor oil boiler (15–21 kW, 21–26 kW) Unit: 5 Year Guarantee
Lookup Standards
Grant Vortex Oil Boiler Sizing Matrix
Complete kilowatt range for Grant Vortex indoor, outdoor, and combi models.
| Grant Vortex Model | kW Output Range | Burner Nozzle Spec | Location / Casing | Ideal Property |
|---|---|---|---|---|
| Grant Vortex Pro Utility 15/26 | 15 kW – 26 kW | 0.60 USgph 80°EH | Indoor Utility Room | 2–4 Bed Rural Home |
| Grant Vortex Pro External 26/36 | 26 kW – 36 kW | 0.85 USgph 80°EH | Outdoor Weatherproof Module | 4–5 Bed Detached Rural Home |
| Grant Vortex Pro Combi 26 | 26 kW | 0.65 USgph + 36L Store | Indoor Floor-Standing | 3–4 Bed Off-Grid (15 L/min DHW) |
| Grant Vortex Pro 46/70 | 46 kW – 70 kW | 1.50 USgph 60°S | Large Commercial Utility | Country Estate / Farmhouse |
Which? Best Buy award winner for condensing oil boilers multiple consecutive years.
Key Principles
Critical Sizing Factors & Engineering Influences
The 2 variables that move the grant boiler result most, and how Grant Engineering Technical Manuals & OFTEC expects each one to be established.
🛡️
Patented Stainless Steel Secondary Heat Exchanger
Captures latent heat from sulfurous oil flue gases without corrosion.
🌱
100% HVO Biofuel Ready
Can be converted from kerosene to fossil-free Hydrotreated Vegetable Oil in minutes.
Prevent Costly Errors
Common Sizing Mistakes & How to Avoid Them
The mistake below costs more than any other on a grant boiler job — specification mismatches that surface only after the unit is installed and commissioned.
Common Pitfall
Operating without Secondary Condensate Neutralizer
Consequence: Acidic oil condensate (pH 3.5) corrodes metal drainage pipes.
Correct Approach: Route through a limestone neutralizer cartridge.
Quality Assurance Audit
Grant Boiler: Installer Consultation Audit
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against Grant Engineering Technical Manuals & OFTEC.
Readiness Score: 0 / 2
⚖️
CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against Grant Engineering Technical Manuals & OFTEC (Grant Engineering & OFTEC) for Grant Boiler Size Calculator.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
2 questions we are asked most about grant boiler sizing, answered against Grant Engineering & OFTEC guidance.
The Grant Vortex Pro 15/26 kW is the most popular oil boiler model for 3 to 4 bedroom rural homes.
Yes, all modern Grant Vortex condensing oil boilers are 100% HVO (Hydrotreated Vegetable Oil) biofuel compatible.
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
Open Calculator →
System Boiler Sizer
Calculate space heating kW + unvented cylinder recovery capacity.
Conventional
Open Calculator →
Regular Boiler Sizer
Open-vented gravity systems with loft header tanks.
Commercial
Open Calculator →
Commercial Cascade Sizer
Cascade systems and plate heat exchangers (50 kW – 500+ kW).
Steam & BHP
Open Calculator →
Steam Boiler Sizer
Calculate Boiler Horsepower (BHP) and steam flow in lbs/hr.
Heavy Process
Open Calculator →
Industrial Boiler Sizer
High-capacity industrial thermal process and manufacturing loads.
Natural Gas
Open Calculator →
Gas Boiler Sizer
Mains natural gas condensing output with modulation analysis.
Gas Combi
Open Calculator →
Gas Combi Sizer
Peak gas volume flow and instantaneous hot water demand.
Off-Grid Oil
Open Calculator →
Oil Boiler Sizer
Kerosene 28-second oil-fired combi, system, and external units.
Oil Combi
Open Calculator →
Oil Combi Sizer
Thermal store oil combination boilers for rural homes.
100% Electric
Open Calculator →
Electric Boiler Sizer
Kilowatt capacity and single-phase 60A/100A main fuse limits.
Electric Combi
Open Calculator →
Electric Combi Sizer
Direct electric combination flow rates and amperage requirements.
LPG Propane
Open Calculator →
Propane Boiler Sizer
Bulk tank and 47kg cylinder LPG condensing boiler sizing.
Wood Pellets
Open Calculator →
Biomass Boiler Sizer
Wood pellet boilers and 50L/kW thermal buffer accumulator tanks.
Log Gasification
Open Calculator →
Wood Boiler Sizer
Batch-fired log gasification boilers and heat accumulators.
Outdoor Furnaces
Open Calculator →
Outdoor Wood Boiler
Outdoor hydronic wood furnaces for homes and workshops.
BS 7074 Spec
Open Calculator →
Expansion Tank Calculator
Calculate expansion vessel volume in Litres and pre-charge bar.
BS 6891 Spec
Open Calculator →
Gas Pipe Size Calculator
Calculate copper pipe diameter (15-28mm) for ≤ 1.0 mbar drop.
Hydronic Flow
Open Calculator →
Heating Pipe Size Sizer
Flow velocity & diameter (15mm to 35mm) by kW circuit load.
BS 5440 Spec
Open Calculator →
Flue Size Calculator
Concentric 60/100mm & 80/125mm equivalent length and bend penalties.
BS 7593 Spec
Open Calculator →
System Volume Calculator
Water volume in Litres and chemical inhibitor bottle dosing.
ASME Spec
Open Calculator →
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
Open Calculator →
Boiler vs Heat Pump Sizer
Compare 70°C gas vs 45°C heat pump radiator surface uprating.
Dual Output
Open Calculator →
Stove Back Boiler Sizer
Split heat calculation: output to room vs output to radiators.
1-2 Bed Flat
Open Calculator →
2-Bedroom House Sizer
24 kW combi sizing for compact flats and terraced properties.
3-Bed Semi
Open Calculator →
3-Bedroom House Sizer
28–30 kW combi or 15–18 kW system boiler recommendations.
4-Bed Detached
Open Calculator →
4-Bedroom House Sizer
High-demand family homes with 2+ bathrooms and en-suites.
5-Bed Executive
Open Calculator →
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
Open Calculator →
Boiler with Underfloor Heating
Low-temperature manifold blending and minimum burner modulation.
In-Floor Loops
Open Calculator →
Radiant Heat Boiler Sizer
Hydronic radiant floor loops and low-temperature condensing.
UK Part L
Open Calculator →
Boiler Size Calculator UK
CIBSE domestic heating design guide and Gas Safe standards.
SEAI Ireland
Open Calculator →
Boiler Size Calculator Ireland
Irish home heating standards and SEAI energy upgrade guidelines.
Irish Kerosene
Open Calculator →
Oil Boiler Ireland Sizer
Dedicated oil boiler sizing for rural Irish detached homes.
OFTEC UK
Open Calculator →
Oil Boiler Calculator UK
OFTEC standards for kerosene central heating in rural UK.
CSA B149 Canada
Open Calculator →
Boiler Sizing Canada
Severe cold climate sizing (-30°C) in MBH and kW.
Greenstar Series
Open Calculator →
Worcester Bosch Sizer
Greenstar 2000, 4000, and 8000 series sizing comparison.
ecoTEC Series
Open Calculator →
Vaillant Boiler Sizer
ecoTEC pro, plus, and exclusive series kilowatt matching.
600 / 800 Series
Open Calculator →
Baxi Boiler Sizer
Cupboard-fit Baxi 600, 800, and Platinum combi/system sizing.
Vitodens Series
Open Calculator →
Viessmann Boiler Sizer
1:10 Inox-Radial Vitodens 050-W and 100-W efficiency matrix.
Logic & Vogue
Open Calculator →
Ideal Boiler Sizer
Ideal Logic+ and Vogue Max boiler capacity matching.
Energy Series
Open Calculator →
Glow-worm Boiler Sizer
Glow-worm Energy and Compact combi/system output guide.
Vortex Oil
Open Calculator →
Grant Boiler Sizer
Grant Vortex internal and outdoor condensing oil boilers.
Outdoor Modules
Open Calculator →
Grant Oil Boiler Sizer
Weatherproof external and utility room kerosene oil units.
British Gas Tiers
Open Calculator →
British Gas Boiler Sizer
Independent comparison of British Gas standard & premium tiers.
US Baseboard
Open Calculator →
Slant/Fin Boiler Sizer
North American cast iron Sentry, Bobcat, and Intrepid MBH models.