Wall Heating

Overview

Hydronic radiant wall heating systems deliver thermal comfort through warm vertical surfaces, providing an alternative to floor-based radiant systems where floor construction limitations exist or furniture placement would block floor heating. Wall heating operates at surface temperatures between 85-110°F, transferring heat through combined radiation and natural convection. These systems prove particularly effective in spaces with high thermal mass floors, retrofit applications, or where floor coverings with high thermal resistance preclude floor heating.

Embedded Wall Tube Configuration

Wall heating employs flexible PEX (cross-linked polyethylene) or PEX-AL-PEX tubing embedded within wall construction at depths of 0.5-1.5 inches from the finished surface. Tube spacing typically ranges from 6 to 12 inches on center, with closer spacing near exterior walls and wider spacing on interior partitions. Loop length limits to 250-300 feet maintain adequate flow rates and temperature differential below 10-15°F.

Tube routing follows serpentine or spiral patterns, with serpentine layouts providing simpler installation and spiral arrangements delivering more uniform surface temperature distribution. Supply and return tubes must enter and exit from the same wall edge to minimize thermal bridging through structural elements. Tube support employs plastic clips, wire mesh, or dedicated track systems secured to studs or masonry backing.

Plaster Wall Systems

Traditional plaster wall heating embeds tubing within gypsum plaster or lime-sand plaster applications over masonry, concrete, or lath backing. The tubing installation occurs after scratch coat application, with tubes secured by wire ties or clips. Brown coat plaster surrounds and encapsulates tubes, followed by finish coat application achieving minimum 0.5-inch coverage over tube centerlines.

Plaster thermal conductivity (3-5 Btu·in/hr·ft²·°F) exceeds drywall compound, improving heat transfer effectiveness and surface temperature uniformity. The high thermal mass of plaster systems creates substantial response time lag, with warm-up periods extending 4-8 hours after system activation. This characteristic suits buildings with stable occupancy patterns but proves less ideal for spaces requiring rapid temperature adjustment. Plaster moisture content during curing requires controlled system startup, gradually raising water temperature from 75°F to design conditions over 3-5 days to prevent cracking.

Drywall Systems

Drywall-based wall heating installs tubing against the rear face of drywall panels or between drywall layers in double-layer construction. Aluminum heat transfer plates attach to drywall backing, securing tubing while distributing heat laterally to improve surface temperature uniformity. Tube centerline spacing of 8-12 inches provides adequate coverage with heat transfer plate assistance.

Drywall thermal resistance (approximately 0.45 hr·ft²·°F/Btu for 0.5-inch thickness) limits heat transfer compared to plaster systems, requiring higher water temperatures to achieve equivalent surface output. The lower thermal mass yields faster system response, with warm-up times of 1-2 hours. Joint compound application must avoid tubing contact to prevent localized stress concentrations. Fastener penetration requires careful coordination, with fastener-free zones specified along tube runs.

Surface Temperature Control

Wall surface temperature regulation maintains thermal comfort while preventing excessive heat output that creates uncomfortable radiant asymmetry. ASHRAE Standard 55 recommends maximum radiant temperature asymmetry of 10°F between opposing walls to avoid localized discomfort. Wall surface temperatures typically limit to 95-105°F for occupied spaces, with higher temperatures permissible in circulation zones or industrial applications.

Control strategies include outdoor reset of supply water temperature based on heating load variation, constant temperature operation with zone valve cycling, or thermostatic radiator valve (TRV) integration for individual room control. Mixing valves blend supply water from primary loop temperatures of 120-140°F down to distribution temperatures of 85-115°F. Differential temperature between supply and return should not exceed 15°F to maintain uniform surface conditions.

Heat Transfer Analysis

Wall heating heat flux depends on surface temperature, room air temperature, and mean radiant temperature according to: q = h_c(T_s - T_a) + h_r(T_s - T_mr), where h_c represents the convective coefficient (0.5-1.0 Btu/hr·ft²·°F for vertical surfaces) and h_r equals the radiative coefficient (approximately 1.0 Btu/hr·ft²·°F). Typical heat flux ranges from 15-35 Btu/hr·ft².

Surface heat output decreases with height due to boundary layer development in natural convection flow. Lower portions of heated walls transfer 10-15% more heat than upper sections for constant surface temperature. Panel orientation affects convection coefficients, with vertical surfaces producing lower coefficients than floors but higher than ceilings. Heat distribution within wall assemblies follows one-dimensional conduction analysis: q = k·A·ΔT/Δx, modified by tube spacing geometry.

Application Considerations

Wall heating suitability depends on wall availability, furniture placement, and window locations. Exterior walls benefit most from heating panels, counteracting conductive heat loss and cold radiation from fenestration. Interior walls provide heating capacity without offsetting specific loads, requiring careful system balancing to prevent overheating. Wall space occupied by cabinetry, shelving, or large furniture should exclude heated panels to avoid blocked heat transfer and potential damage to furnishings.

Bathroom applications leverage wall heating effectively, providing comfort in spaces where floor drains or thin floor slabs complicate floor heating. Kitchen installations must account for cabinet blocking and heat-sensitive equipment. Historic building retrofits often favor wall heating over floor modifications that would compromise original flooring or require excessive floor buildup. Load calculations must account for the reduced heating capacity per square foot compared to floor systems, typically requiring 40-60% more wall area than equivalent floor heating area.

Thermal Response Characteristics

Wall system thermal response depends on assembly thermal mass and tube embedding depth. Lightweight drywall systems achieve 90% output capacity within 2 hours of startup, while heavy plaster or masonry assemblies require 6-12 hours. Response time calculations employ: τ = ρ·c·V/(h·A), where ρ represents density, c is specific heat, V equals volume, h denotes the combined heat transfer coefficient, and A is surface area.

Setback strategies must account for warm-up time requirements. Night setback of 5-8°F proves practical for light construction, recovering within typical morning warm-up periods. Heavy mass systems typically operate continuously with minimal temperature swing. The thermal flywheel effect moderates room temperature fluctuations from solar gain or internal loads, improving comfort stability but reducing responsiveness to thermostat adjustment.

Design Best Practices

Wall heating design begins with room-by-room heat loss calculations at design outdoor conditions, accounting for transmission losses, infiltration, and any ventilation heating requirements. Panel area requirements derive from heat loss divided by design heat flux, typically 20-30 Btu/hr·ft² for 95-100°F surface temperatures. Panel placement prioritizes exterior walls, particularly below windows and along walls with minimal furniture obstruction.

Circuit balancing employs manifold valve adjustment or circuit length matching to achieve uniform supply temperatures across all zones. Flow rates of 0.5-1.0 gpm per circuit maintain turbulent flow within tubes, with minimum Reynolds numbers above 3,000 ensuring adequate heat transfer. System pressure drop calculations include tubing friction losses, manifold losses, and control valve authority, with circulator selection providing adequate head at design flow rates. Proper air elimination through high-point vents or automatic air separators prevents circulation problems and noise.