Boiler Size Calculator
Grant Vortex Oil • Engineering Sizer

Grant Boiler Size Calculator: Vortex Oil & HVO Guide

Size high-efficiency Grant Vortex condensing oil boilers and calibrate burner nozzles 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

Grant Engineering Sizing Matrix & Model Lineup

Official Technical Specs
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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

Short-Cycling & Condensing Loss Simulator

Installed Boiler Output vs Real Heat Loss (6 kW Requirement):
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

Grant Vortex Output & Nozzle Calibration Formula & Variables

OFTEC Book 2 & Grant Technical Specs
Mathematical Formulation
kW_grant = Heat_Loss [kW] + Cylinder_Allowance [3.0 kW] (Calibrated via Danfoss Nozzle)

Grant Vortex boilers can be fine-tuned by selecting the correct Danfoss burner nozzle (0.60 to 1.50 USgph) and calibrating Riello oil pump pressure to match exact house heat loss.

Vortex_Pro_15/26
Adjustable 15 kW to 26 kW via burner nozzle Unit: 10 Year Guarantee
Vortex_Pro_26/36
Adjustable 26 kW to 36 kW via burner nozzle Unit: 10 Year Guarantee
Vortex_Pro_46/70
Commercial rural estate output Unit: High Output
Lookup Standards

Grant Vortex Oil Boiler Sizing Matrix

Complete kilowatt range for Grant Vortex indoor, outdoor, and combi models.

Grant Vortex ModelkW Output RangeBurner Nozzle SpecLocation / CasingIdeal Property
Grant Vortex Eco 15/21 15 kW – 21 kW 0.55 USgph 80°EH Indoor Utility / Kitchen 2–3 Bed Rural Home (8–12 Rads)
Grant Vortex Pro Utility 15/26 15 kW – 26 kW 0.60 USgph 80°EH Indoor Utility Room 3–4 Bed Rural Home (12–16 Rads)
Grant Vortex Pro External 26/36 26 kW – 36 kW 0.85 USgph 80°EH Outdoor Weatherproof Module 4–5 Bed Detached Rural Home (16–22 Rads)
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

Riello RDB burner technology fitted as standard. Award-winning condensing efficiency.

Key Principles

Critical Sizing Factors & Engineering Influences

Core hardware and load parameters governing model selection, benchmarked against Grant Engineering Technical Manuals & OFTEC.

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Patented Stainless Steel Secondary Heat Exchanger

Captures latent heat from sulfurous oil flue gases without corrosion.

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100% HVO Biofuel Ready

Can be converted from kerosene to fossil-free Hydrotreated Vegetable Oil in minutes.

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Riello RDB Pressure-Jet Burner

Precision combustion photocell and calibrated oil pump for quiet, dependable firing.

Prevent Costly Errors

Common Sizing Mistakes & How to Avoid Them

Common installer miscalculations when specifying Grant Boiler Size Calculator: Vortex Oil & HVO Guide equipment — 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.
Common Pitfall

Leaving Burner at Factory Max Setting

Consequence: Overheats heating circuit if house only requires 18 kW.

Correct Approach: OFTEC engineer should recalibrate nozzle to match measured heat loss.
Quality Assurance Audit

Grant Boiler Size Calculator: Vortex Oil & HVO Guide: Installer Consultation Audit

Review these 3 essential questions with your heating installer to ensure your Grant Boiler Size Calculator: Vortex Oil & HVO Guide quote meets Grant Engineering Technical Manuals & OFTEC.

Readiness Score: 0 / 3
Technical Deep Dive

Grant Vortex Condensing Sizing, Nozzle Calibration & Biofuel Future

Grant is the leading manufacturer of oil-fired heating across the UK and Ireland, delivering exceptional 93%+ seasonal condensing efficiency and full HVO biofuel compatibility.

Burner Nozzle Calibration & Fine-Tuning

Unlike modulating gas boilers, oil boilers utilize multi-stage or calibrated pressure jet burners (Riello RDB). A Grant Vortex 15/26 or 26/36 can be adjusted by your OFTEC technician by installing specific Danfoss oil burner nozzles (e.g. 0.60, 0.65, or 0.75 USgph) and setting oil pump pressure (typically 8.0 to 10.0 bar) to match the building heat loss precisely.

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CIBSE & BS EN Standards-Compliant Framework BENCHMARKED

Formulations benchmarked directly against published British & European standards: Grant Engineering Technical Manuals & OFTEC (Grant Engineering & OFTEC Ireland / UK) for Grant Boiler Size Calculator: Vortex Oil & HVO Guide. Maintained by the Boiler Size Calculator Technical Editorial Desk.

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Standards Revision: 2026
FAQ Knowledgebase

Frequently Asked Questions

Direct engineering guidance for common grant boiler size calculator: vortex oil & hvo guide questions, referenced against Grant Engineering & OFTEC Ireland / UK.

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