Technical Deep Dive
Vaillant German Engineering & Stainless Steel Longevity
Vaillant boilers are renowned for high-grade stainless steel heat exchangers and whisper-quiet operation. The ecoTEC plus series is the premier choice for reliability.
Vaillant ecoTEC Matrix • Engineering Sizer
Vaillant Boiler Size Calculator
Find the ideal Vaillant ecoTEC boiler model and output tailored to your heating and hot water needs.
Vaillant Group Technical Engineering Standards •
BS EN 12831 Verified
•
Dual Unit (kW & BTU)
Manufacturer Product Range
Official Technical Specs
Vaillant 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 Vaillant Group.
Vaillant ecoTEC boilers feature precision German engineering and high-efficiency stainless steel heat exchangers. Use this matrix to match your household heating load to ecoTEC model sizes.
| Model Range | Boiler Type | Available Outputs | DHW Flow Rate | Modulation Turndown | Standard Warranty | Optimal Property Archetype |
|---|---|---|---|---|---|---|
| ecoTEC intro | Combi | 24 kW, 28 kW | 9.6 – 11.5 L/min | 1:4 (6.0 kW min) | 2–5 Years | Budget / Rental 1-2 Bed Properties |
| ecoTEC pro | Combi & System | 24 kW, 28 kW, 30 kW | 9.6 – 12.2 L/min | 1:4 (5.5 kW min) | 5–7 Years | Standard 2–3 Bed Semi-Detached |
| ecoTEC plus | Combi, System, Regular | 25, 32, 35, 38 kW | 10.4 – 15.9 L/min | 1:6 (3.8 kW min) | 7–10 Years | Executive 3–5 Bed Homes & Unvented Cylinders |
| ecoTEC exclusive Green iQ | Combi (with ExtraCondens) | 35 kW, 43 kW | 14.5 – 17.8 L/min | 1:10 (3.5 kW min) | 10 Years | High-Demand Showers & Low Running Costs |
Burner Engineering
Anti-Cycling Firing Physics Boiler Modulation & Turndown Ratio (1:3 vs 1:10)
The maximum kW rating of your boiler only matters on the 3 or 4 coldest days of winter. For the remaining 95% of the heating season, your home requires just 2 kW to 6 kW of continuous heat. A high modulation turndown ratio (1:7 to 1:10) allows the burner to throttle down to a tiny flame without shutting off, preventing short-cycling and saving money.
Simulate Outdoor Seasonal Temperature: +8°C (Mild Autumn / Spring Day)
-5°C (Freezing Winter Peak - 10 kW Load) +8°C (Typical Season - 4.5 kW Load) +16°C (Mild Spring - 1.8 kW Load)
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Current Real-Time Heating Demand
House Needs 4.5 kW of Thermal Power
Target Room Temp: 21°C Constant
Narrow 1:3 Modulation
Budget / Older Boiler (1:3 Turndown)
Rated: 30 kW Maximum → 10 kW Minimum Limit. Because the house only needs 4.5 kW, the 10 kW burner overheats the water in 2 minutes and cuts out repeatedly.
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Burner Output 10.0 kW (Minimum)
Ignition Cycles / Hour: 16 – 20 cycles
Return Water Temp: 62°C (Condensing Off)
Seasonal Gas Efficiency: 82.5%
❌ Rated: 30 kW Maximum → 10 kW Minimum Limit. Because the house only needs 4.5 kW, the 10 kW burner overheats the water in 2 minutes and cuts out repeatedly.
Elite 1:10 Modulation
Premium Boiler (1:10 Turndown)
Rated: 30 kW Maximum → 3.0 kW Minimum Limit. Burner modulates smoothly to match the exact demand, sustaining condensing mode without stopping.
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Burner Output 4.5 kW (Perfect Match)
Ignition Cycles / Hour: 1 continuous cycle
Return Water Temp: 46°C (Peak Condensing)
Seasonal Gas Efficiency: 94.2%
✅ Rated: 30 kW Maximum → 3.0 kW Minimum Limit. Burner modulates smoothly to match the exact demand, sustaining condensing mode without stopping.
Thermodynamic Methodology
Vaillant ecoTEC Technical Specs Vaillant IoniDetect Flame Modulation Formula & Variables
Mathematical Formulation
Burner_Turndown = 1:10 IoniDetect Combustion Management
Vaillant ecoTEC plus models feature IoniDetect technology which monitors flame ionization, automatically adapting to gas quality variations and modulating down to 10% load.
ecoTEC_pro
Entry/mid-range combi (24 kW, 28 kW, 30 kW) Unit: 1:4 Turndown
ecoTEC_plus
Flagship stainless steel combi & system (25 kW – 38 kW) Unit: 1:10 Turndown
ecoTEC_exclusive
Premium Green iQ with integrated Green IQ FGHR (35 kW, 43 kW) Unit: 1:10 Turndown
Lookup Standards
Vaillant ecoTEC Model Sizing Matrix
Complete comparison of Vaillant combi and system boilers by output and flow rate.
| Model Range | Model Numbers | Heating Output (kW) | DHW Flow (@ 35°C Rise) | Warranty Period |
|---|---|---|---|---|
| ecoTEC pro Combi | pro 24, pro 28, pro 30 | 19.0 – 24.5 kW | 9.6 – 12.3 L/min | Up to 7 Years Warranty |
| ecoTEC plus Combi | plus 826, 832, 836, 838 | 24.5 – 30.0 kW | 10.5 – 15.9 L/min | Up to 10 Years Warranty |
| ecoTEC plus System | plus 612, 615, 620, 624, 630, 635 | 12.0 – 35.0 kW | Paired with uniSTOR Cylinder | Up to 10 Years Warranty |
| ecoTEC exclusive with Green iQ | exclusive 835, 843 | 30.0 – 38.0 kW | 14.5 – 17.8 L/min | Up to 10 Years Warranty |
Features premium German-engineered stainless steel heat exchangers.
Key Principles
Critical Sizing Factors & Engineering Influences
The 2 variables that move the vaillant boiler result most, and how Vaillant Group Technical Engineering Standards expects each one to be established.
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High-Grade Stainless Steel Heat Exchanger
Vaillant’s laser-welded stainless steel coil provides exceptional corrosion resistance in hard water areas.
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eBUS Digital Communication Protocol
Enables two-way data exchange with vSMART and sensoCOMFORT modulating controls.
Prevent Costly Errors
Common Sizing Mistakes & How to Avoid Them
The mistake below costs more than any other on a vaillant boiler job — specification mismatches that surface only after the unit is installed and commissioned.
Common Pitfall
Connecting Simple On/Off Thermostat to eBUS Boiler
Consequence: Disables Vaillant’s intelligent flame modulation, turning it into a simple on/off appliance.
Correct Approach: Install a Vaillant sensoHOME or vSMART eBUS modulating thermostat.
Quality Assurance Audit
Vaillant Boiler: Installer Consultation Audit
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against Vaillant Group Technical Engineering Standards.
Readiness Score: 0 / 2
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CIBSE / Gas Safe Peer-Reviewed Engineering Engine VERIFIED
Formulations benchmarked against Vaillant Group Technical Engineering Standards (Vaillant Group & Gas Safe Register) for Vaillant Boiler Size Calculator.
Updated: February 2025
FAQ Knowledgebase
Frequently Asked Questions
2 questions we are asked most about vaillant boiler sizing, answered against Vaillant Group & Gas Safe Register guidance.
The ecoTEC plus has a wider modulation range (1:10 vs 1:4), higher hot water flow rates, a step-down stainless steel casing, and a longer 10-year warranty.
A Vaillant ecoTEC plus 836 combi (or ecoTEC plus 624 system with a 210L uniSTOR cylinder) is ideal for a 4-bedroom home.
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
Open Calculator →
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
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
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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
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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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Baxi Boiler Sizer
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 Boiler Sizer
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.