Radiant & UFH Hydronics • Engineering Sizer

Underfloor & Radiant Heat Boiler Size Calculator

Q: Can a boiler run radiators upstairs and underfloor heating downstairs?

Yes, using a dual-temperature system where radiators receive full boiler temperature while a thermostatic blending valve tempers water down to 40°C for the ground floor UFH manifold.

Q: Does underfloor heating make boilers more efficient?

Yes! Low return water temperatures (30°C–35°C) from UFH guarantee that condensing boilers operate in maximum condensing mode 100% of the time.

Q: What water temperature does radiant floor heating use?

In-slab radiant systems use 32°C to 43°C (90°F–110°F), while joist staple-up systems require 50°C to 65°C (122°F–149°F) due to subfloor thermal resistance.

Q: Can I run radiant floor heating off a combi boiler?

Yes, using a radiant mixing manifold or primary/secondary low-loss header to temper the water down to safe floor temperatures.

Size central heating boilers paired with low-temperature underfloor heating (UFH) manifolds, in-slab concrete loops, and joist staple-up radiant panels.

BS EN 1264 & RPA / ASHRAE Radiant Hydronics Standards • BS EN 12831 Verified • Dual Unit (kW & BTU)

BS EN 12831 & ACCA Manual J Engineering

Whole-House Heat Loss Calculation Engine

Thermodynamic Fabric (Q = U × A × ΔT)

Total Floor Area (m²):

Average Ceiling Height (m):

Wall Construction & U-Value Era:

Loft Roof Insulation:

Window Glazing:

Outdoor Winter Design Temperature:

Total Space Heating Load

7.8 kW

26,600 BTU / hr (65 W/m²)

Heat Loss Breakdown:

External Walls: 2.8 kW (36%)

Windows & Doors: 1.9 kW (24%)

Roof & Ceiling: 1.1 kW (14%)

Ground Floor: 0.8 kW (10%)

Air Infiltration & Ventilation: 1.2 kW (16%)

⚡ Boiler Match: A home with 7.8 kW heat loss only requires an 18 kW to 24 kW Combi (sized for hot water) or a compact 9 kW to 12 kW System Boiler.

Hydronic Thermodynamics

Radiator Delta-T (ΔT50 vs ΔT60 vs Low-Temp ΔT30)

BS EN 442 Standard

One of the most common causes of cold rooms is sizing radiators with the wrong temperature difference (Delta-T / ΔT). Older non-condensing boilers operated at ΔT60 (80°C flow). Modern high-efficiency condensing boilers must operate at ΔT50 (75°C/65°C) or lower to keep return water below 54°C. Test how flow temperature scales your radiator surface area below:

Central Heating Flow Temperature 75°C (Standard ΔT50)

45°C (Heat Pump / Low Temp) 65°C (High Condensing) 82°C (Legacy ΔT60)

Target Room Temp

Baseline ΔT50 Wattage

Operating Delta-T: 50.0 K (ΔT50)

CIBSE Derating Factor: 1.000 ×

Condensing Boiler Status: ✅ High Condensing (<54°C Return)

Required Radiator Length Type 22 Double Convector (600mm H)

1,000 mm Width 1,500 Watts Heat Output

Real Thermal Output 1,500 W (5,118 BTU)

Surface Area Change Baseline (100%)

Thermodynamic Methodology

Underfloor & Radiant Heat Thermal Output Formula Formula & Variables

BS EN 1264 / ASHRAE Handbook

Mathematical Formulation

Output_UFH [W/m²] = 8.92 × ( T_floor_surface - T_room )^1.1

Underfloor and radiant hydronic heating operates at low flow temperatures (35°C–45°C / 95°F–115°F). Finished floor surface temperature is comfort-limited to 29°C (84°F) in living areas and 33°C in bathrooms.

T_floor_surface

Mean finished floor surface temperature (max 29°C / 84°F) Unit: °C / °F

T_room

Target living room air temperature (21°C / 70°F) Unit: °C / °F

Output_UFH

Maximum allowable upward radiant thermal emission Unit: 70 – 100 W/m² (22–32 BTU/sq ft)

Lookup Standards

Underfloor & Radiant Heating Output by Floor Assembly & Pipe Spacing

Heat emissions in W/m² (BTU/sq ft) and maximum loop lengths for 16mm PEX pipe.

Floor Assembly / Method	Design Water Temp	Tube Spacing	Max Thermal Output	Max Loop Length
Ceramic / Porcelain Tile Screed	35°C – 40°C (95°F – 104°F)	150mm (6" O.C.)	95 W/m² (30 BTU/sq ft)	100 – 110 meters
Concrete In-Slab (4" Slab)	38°C – 45°C (100°F – 113°F)	200mm (8"–12" O.C.)	80 W/m² (25 BTU/sq ft)	100 – 120 meters
LVT / Luxury Vinyl Tile	35°C – 40°C (95°F – 104°F)	150mm (6" O.C.)	80 W/m² (25 BTU/sq ft)	90 – 100 meters
Engineered Hardwood (15mm)	38°C – 45°C (100°F – 113°F)	150mm (6" O.C.)	65 W/m² (20 BTU/sq ft)	90 – 100 meters
Subfloor Staple-Up with Heat Plates	50°C – 60°C (122°F – 140°F)	200mm (8" O.C.)	60 W/m² (19 BTU/sq ft)	80 – 90 meters
Carpet + Underlay (< 2.5 Tog)	45°C – 50°C (113°F – 122°F)	150mm (6" O.C.)	45 W/m² (14 BTU/sq ft)	80 – 90 meters

Flow rate calibrated to 1.5 – 2.5 L/min per loop on manifold flowmeters. Oxygen-barrier PEX mandatory.

Key Principles

Critical Sizing Factors & Engineering Influences

The 4 primary variables that determine actual heat demand, and how BS EN 1264 & RPA / ASHRAE Radiant Hydronics Standards calculates each.

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Thermostatic Blending Valve Manifold

Mixes 70°C boiler flow down to 40°C for underfloor PEX loops while radiators receive full heat.

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Screed Thermal Storage & Slow Warmup Response

75mm sand/cement screed takes 2–4 hours to warm up, acting as a massive thermal battery.

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Extruded Aluminum Heat Transfer Plates

Spreads heat evenly across joist bays in staple-up retrofits, increasing output by 35%.

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Oxygen-Barrier EVOH PEX Pipe

Prevents atmospheric oxygen permeation that oxidizes cast iron boiler impellers into rust.

Prevent Costly Errors

Common Sizing Mistakes & How to Avoid Them

Key engineering traps that lead to oversizing or poor performance on underfloor & radiant heat boiler designs — fabric and ventilation assumptions that quietly inflate the calculated design load.

Common Pitfall

Exceeding 110m on Single 16mm PEX Loop

Consequence: Excessive hydraulic friction starves water flow, leaving the far end of the room cold.

Correct Approach: Split large rooms into multiple balanced loops under 100m.

Common Pitfall

Using Non-Barrier PEX Pipe on Radiant Loops

Consequence: Oxygen permeates pipe walls, oxidizing cast iron boiler impellers and heat exchangers into rust.

Correct Approach: Install oxygen-barrier EVOH PEX or separate with a stainless steel heat exchanger.

Quality Assurance Audit

Underfloor & Radiant Heat Boiler: Installer Consultation Audit

Put these 4 essential questions to your heating engineer before approving an installation quote — benchmarked against BS EN 1264 & RPA / ASHRAE Radiant Hydronics Standards.

Readiness Score: 0 / 4

Blending Valve & Secondary Pump Commissioned Manifold thermostatic mixing valve set to 40°C–45°C.

Manifold Flowmeter Sight Glasses Balanced Adjusted flowmeters to match calculated design L/min per loop.

Sub-Slab Rigid Foam Insulation (R-10+) Verified Checked under-slab polystyrene insulation to prevent downward heat loss into earth.

Oxygen-Barrier EVOH PEX Pipe Certified Verified DIN 4726 oxygen diffusion resistance.

Technical Deep Dive

BS EN 1264 & Radiant Hydronic System Integration

Underfloor heating and radiant panels distribute heat across extensive surface areas at low flow temperatures (35°C–45°C), keeping return water cool enough to maintain continuous condensing efficiency in modern boilers.

Dual-Temperature Hydraulic Separation

When pairing underfloor heating with high-temperature upstairs radiators, a hydraulic separator (low-loss header) or a thermostatic mixing manifold must be installed. This isolates the high-flow, low-temperature radiant circuit from the low-flow, high-temperature radiator circuit, ensuring both circuits receive adequate flow without hydraulic interference.

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CIBSE & BS EN Standards-Compliant Framework BENCHMARKED

Formulations benchmarked directly against published British & European standards: BS EN 1264 & RPA / ASHRAE Radiant Hydronics Standards (British Standards Institution, CIBSE & ASHRAE) for Underfloor & Radiant Heat Boiler Size Calculator. Maintained by the Boiler Size Calculator Technical Editorial Desk.

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Standards Revision: 2026

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