Technical Deep Dive
Modern Digital Tooling for Heating Professionals
Say goodbye to complex paper charts. Our free mobile web app provides instant, accredited calculations for heating engineers, plumbers, and homeowners on any device.
Mobile PWA App • Engineering Sizer
Boiler Size Calculator App
Free, mobile-friendly heating calculator app for on-site boiler sizing, radiator heat loss, and gas pipe calculations.
CIBSE Domestic Heating & BS EN 12831 Mobile Reference •
BS EN 12831 Verified
•
Dual Unit (kW & BTU)
Bedrooms
2. Property Type & Bedrooms
3
Bathrooms
3. Number of Bathrooms
2
Radiators
Radiators
12
5. Home Insulation Quality
Average / Standard
3. Radiators & Underfloor kW / °C
Recommended Boiler Size
28
kW
95,500 BTU/hr
Recommended Boiler Output Range: 24 kW – 30 kW
Domestic Hot Water (DHW) Peak 11.5 L/min @ 35°C ΔT
Space Heating Heat Loss Load 14.2 kW +15% margin
Recommended Modulation Ratio 1:7 Ratio
Min: 3.5 kW Max: 28 kW
Diagnostic System Selector
4-Step Engineering Diagnostic Which Boiler Type Do You Actually Need?
Before calculating exact kilowatt ratings, you must choose the right system architecture. An incorrect boiler type leads to weak shower pressure, cold taps, or wasted living space. Answer the four diagnostic questions below to find your optimal hydronic configuration:
Diagnostic Recommendation 98% Match
Instantaneous Hot Water
Combi Boiler (Combination)
Calculate Exact kW for this System: 28 kW – 32 kW
Engineering Assessment:
With 1 bathroom and strong incoming water pressure, a Combi boiler provides instantaneous hot water on demand with zero cylinder heat loss and frees up your airing cupboard space.
✓ No cylinder heat loss or waiting for hot water to reheat.
✓ Wall-hung unit only
⚠️ Cannot run two high-pressure showers simultaneously.
Thermodynamic Methodology
BS EN 12831 Mobile Tool Instant Mobile Sizing Algorithm Formula & Variables
Mathematical Formulation
kW_total = Σ( Room_Volume × Fabric_U_Value ) + DHW_Peak
Our free progressive web app runs entirely in your mobile browser with zero installation required, saving offline calculation history on site.
Offline_Mode
Runs without cell reception inside basements and plant rooms Unit: PWA Cache
Instant_PDF_Export
Generate customer quote sizing reports on your phone Unit: Export
Lookup Standards
Boiler Sizing Mobile App Features & Toolsets
Included calculations and technical modules available on mobile.
| Tool Module | Target Use Case | Key Calculation | Engineering Standard |
|---|---|---|---|
| Instant Boiler Sizer | Fast Customer Consultation | Combi / System kW output | CIBSE Domestic Guide |
| Room Heat Loss Engine | Detailed Survey Quotation | Room-by-room Watts & BTU | BS EN 12831 |
| Gas Pipe Pressure Sizer | Gas Safe Installation Check | 15mm vs 22mm mbar drop | BS 6891 |
| Radiator BTU Calculator | Emitter Upgrade Sizing | Type 11, 21, 22 ΔT50 outputs | BS EN 442 |
| Expansion Vessel Sizer | Sealed System Diagnostics | Diaphragm Litres & precharge | BS 7074 |
Installable to your home screen with 1 tap.
Key Principles
Critical Sizing Factors & Engineering Influences
The 2 variables that move the boiler size calculator app result most, and how CIBSE Domestic Heating & BS EN 12831 Mobile Reference expects each one to be established.
📱
Offline Plant Room Operation
Engineered as a Progressive Web App (PWA) that operates flawlessly in basements without mobile signal.
📄
One-Tap Customer PDF Sizing Reports
Export certified heat loss calculations directly to homeowners to validate your equipment choice.
Prevent Costly Errors
Common Sizing Mistakes & How to Avoid Them
The mistake below costs more than any other on a boiler size calculator app job — sizing shortcuts that lock in an oversized output before the survey is even finished.
Common Pitfall
Guessing Boiler Sizing without Documented Proof
Consequence: Homeowners may dispute oversizing or undersizing issues later.
Correct Approach: Save and email the on-site calculation summary from the app.
Quality Assurance Audit
Boiler Size Calculator App: Installer Consultation Audit
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against CIBSE Domestic Heating & BS EN 12831 Mobile Reference.
Readiness Score: 0 / 2
⚖️
CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against CIBSE Domestic Heating & BS EN 12831 Mobile Reference (Heating Engineers Technical Hub) for Boiler Size Calculator App.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
2 questions we are asked most about boiler size calculator app sizing, answered against Heating Engineers Technical Hub guidance.
Yes! The entire suite of boiler sizing, radiator BTU, gas pipe, and expansion vessel tools is 100% free to use on mobile and desktop.
Yes, as a Progressive Web App (PWA), once loaded it caches locally and can be used offline in basements or rural areas without mobile signal.
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.