Underfloor Heating Calculation: Flow Temp & Spacing
Understanding Underfloor Heating Calculation
This guide covers the key concepts, formulas, and practical methods for underfloor heating calculation. All methods referenced are based on ASHRAE Fundamentals and GB 50736 standards.
Key Formula
The primary formula used in heating load calculations is based on the heat balance method described in ASHRAE Fundamentals Chapter 18 (nonresidential) and Chapter 19 (residential). The total heating load is the sum of conduction losses through the building envelope and infiltration/ventilation losses.
Qtotal = Qwalls + Qwindows + Qroof + Qfloor + Qinfiltration + Qventilation
Each component: Q = Area × U-value × ΔT where ΔT = indoor design temp − outdoor design temp (ASHRAE 99% heating value for your location).
Step-by-Step Example
Scenario: 150 sq ft bedroom in New York (outdoor design temp 14°F, indoor 70°F, ΔT=56°F).
Given: 2 exterior walls (total 180 sq ft, U=0.077), 1 window (15 sq ft, U=0.5), ceiling (150 sq ft, U=0.03), infiltration (0.7 ACH).
Walls: 180 × 0.077 × 56 = 777 BTU/h
Window: 15 × 0.5 × 56 = 420 BTU/h
Ceiling: 150 × 0.03 × 56 = 252 BTU/h
Infiltration: 0.018 × 0.7 × (12×12×8) × 56 = 813 BTU/h
Total: 777 + 420 + 252 + 813 = 2,262 BTU/h
Standards Reference
- ASHRAE Fundamentals — Chapter 18 (Nonresidential), Chapter 19 (Residential) cooling and heating load calculations
- ASHRAE Handbook — Chapter 25 (fenestration), Chapter 26 (thermal insulation)
- GB 50736-2012 — Design Code for Heating Ventilation and Air Conditioning of Civil Buildings (China)
- ACCA Manual J — Residential load calculation (8th Edition)
Frequently Asked Questions
What flow temperature do I need for underfloor heating?
Typical flow temperatures: 35–45°C (95–113°F) for standard UFH, 25–35°C for heat pump systems. Lower flow temps = higher efficiency but larger heat exchange area needed. The floor output depends on floor covering, pipe spacing, and flow temperature.
What pipe spacing should I use?
Standard spacing: 150mm (6 inch) for main living areas, 200mm (8 inch) for bedrooms, 100mm (4 inch) for bathrooms and high-heat-loss areas. Closer spacing = higher output per square meter.
How much heat does underfloor heating produce?
Standard UFH output: 80–120 W/m² at 45°C flow temperature with 150mm spacing and tile flooring. With carpet: reduce by 20–30%. With laminate: reduce by 10–15%. Maximum floor surface temperature: 29°C (EN 1264).