HVAC Systems Encyclopedia

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Radiant Heating Design for HVAC Engineers

Radiant Heating Design for HVAC Engineers

Radiant heating delivers comfort via infrared radiation and minimal air movement. Proper design ensures uniform temperatures, adequate capacity, and system responsiveness while preventing floor surface overheating.

Radiant Panel Output

Heat flux from radiant panel:

$$q = C \times (T_s - T_r)^n$$

Where:

  • $q$ = heat flux (Btu/(h·ft²))
  • $C$ = empirical constant
  • $T_s$ = surface temperature (°F)
  • $T_r$ = room temperature (°F)
  • $n$ = 1.0 to 1.3 (depending on system type)

Floor panel output: 15-30 Btu/(h·ft²) typical

Limitations:

  • Floor surface: < 85°F (comfort limit for occupied areas)
  • Ceiling surface: < 120°F (prevents discomfort from overhead radiation)

Tube Spacing Design

Heat transfer per unit length:

$$q’ = \frac{k_{effective} \times W \times (T_{avg} - T_{floor,bottom})}{thickness}$$

Where:

  • $k_{effective}$ = effective thermal conductivity including tube influence
  • $W$ = panel width per tube
  • Spacing: typically 6-12 inches

Serpentine vs. Counterflow:

  • Serpentine: Simple, temperature gradient along length
  • Counterflow: More uniform, requires more circuits

System Design Procedure

  1. Calculate room heat loss
  2. Determine available floor area (exclude furniture)
  3. Select supply water temperature (typically 95-120°F)
  4. Calculate required surface temperature
  5. Size tube spacing and circuit length
  6. Design manifold and controls

Water Temperature and Flow

Supply temperature: 95-140°F (lower for floor, higher for ceiling)

Temperature drop: 10-20°F per circuit

Flow rate per circuit:

$$GPM = \frac{q}{500 \times \Delta T}$$

Circuit length: Maximum 300-400 ft (pressure drop limit)

Practical Applications

  1. Slab-on-grade: Tubes in concrete slab (high thermal mass, slow response)
  2. Suspended floor: Tubes in subfloor or joist space (faster response)
  3. Ceiling panels: Higher temperatures, faster response
  4. Snowmelt: Driveway/walkway heating (higher capacity, 150-250 Btu/(h·ft²))

Related Technical Guides:

References:

  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 6: Radiant Heating and Cooling
  • Radiant Professionals Alliance Design Guidelines