Ice Rink Sub-Floor Heating Systems

Overview

Sub-floor heating systems installed beneath ice rink concrete slabs prevent frost penetration into underlying soil, eliminating frost heave damage and maintaining structural integrity. The heating system maintains the soil-concrete interface above freezing while the ice surface operates at 20-25°F, creating a controlled thermal gradient through the assembly.

Without sub-floor heating, continuous heat extraction through the refrigerated ice slab drives freezing temperatures deep into foundation soils. Frost-susceptible soils undergo volumetric expansion during freezing, generating heave pressures exceeding 50,000 psf that buckle concrete slabs and damage refrigeration piping networks.

Thermal Requirements

Frost Penetration Depth

Maximum frost depth without heating follows the modified Berggren equation:

$$Z_f = \sqrt{\frac{48 \cdot k \cdot F}{\pi \cdot L \cdot \rho}}$$

Where:

• $Z_f$ = frost penetration depth (ft)
• $k$ = soil thermal conductivity (Btu/hr·ft·°F)
• $F$ = freezing index (°F-days)
• $L$ = latent heat of fusion (144 Btu/lb for water)
• $\rho$ = soil dry density (lb/ft³)

The sub-floor system must supply sufficient heat flux to maintain the concrete undersurface at 38-45°F.

Heat Loss Through Slab

Required heating capacity balances downward heat loss from the ice slab system:

$$q_{sf} = \frac{T_{sf} - T_{soil}}{\frac{t_{conc}}{k_{conc}} + \frac{t_{ins}}{k_{ins}} + R_{soil}}$$

Where:

• $q_{sf}$ = sub-floor heat flux (Btu/hr·ft²)
• $T_{sf}$ = sub-floor target temperature (40°F)
• $T_{soil}$ = deep ground temperature (50-55°F)
• $t_{conc}$ = concrete slab thickness (ft)
• $t_{ins}$ = insulation thickness (ft)
• $k$ = thermal conductivity (Btu/hr·ft·°F)
• $R_{soil}$ = soil thermal resistance (hr·ft²·°F/Btu)

Typical heat flux requirements range from 3-8 Btu/hr·ft² depending on refrigeration load intensity and ground conditions.

System Configuration

graph TB
    subgraph "Ice Rink Floor Assembly"
        A[Ice Surface<br/>20-25°F]
        B[Refrigerant Piping<br/>Embedded in Concrete]
        C[Concrete Ice Slab<br/>4-6 inches]
        D[Rigid Insulation<br/>R-10 to R-15]
        E[Sub-Floor Heating Coils<br/>Embedded in Sand/Concrete]
        F[Base Concrete Slab<br/>6-8 inches]
        G[Compacted Subgrade<br/>Maintained Above 32°F]
    end

    A --> B
    B --> C
    C --> D
    D --> E
    E --> F
    F --> G

    H[Heat Source<br/>Glycol/Electric/Waste Heat] --> E

    style A fill:#e3f2fd
    style C fill:#90caf9
    style D fill:#ffb74d
    style E fill:#ef5350
    style G fill:#8d6e63

The sub-floor heating coils embed in a 2-4 inch sand-cement mixture or directly in the structural base slab, positioned 2-3 inches below the bottom insulation surface. Coil spacing typically ranges from 9-18 inches on center, with tighter spacing at perimeter zones experiencing edge heat losses.

Temperature sensors embedded near heating coils enable closed-loop control maintaining the interface setpoint. Multiple control zones accommodate varying heat loss rates across the rink footprint.

Heating System Comparison

Glycol Heating Loop

Closed-loop glycol systems (typically 25-35% propylene glycol) circulate through PEX or HDPE tubing at 45-60°F supply temperature. A heat exchanger interfaces with the facility’s heating plant or dedicated boiler. Flow rates of 0.3-0.5 gpm per 100 ft² maintain adequate heat distribution.

Electric Heat Tracing

Self-regulating or constant-wattage heat cables install at 6-12 inch spacing, controlled by embedded thermostats. Power density ranges from 3-8 W/ft² depending on climate severity and insulation R-value. Electric systems provide simplest installation but incur highest operating costs in most utility rate structures.

Refrigeration Heat Recovery

Capturing condenser rejection heat provides the most energy-efficient solution. The refrigeration system generates 12,000-15,000 Btu/hr·ton of waste heat at suitable temperature levels (80-120°F). A heat exchanger transfers this energy to the sub-floor glycol loop, achieving heating COP values of 2.5-3.5 when accounting for parasitic pump energy.

During peak refrigeration load, waste heat often exceeds sub-floor heating demand. Surplus energy integrates into facility heating, domestic hot water preheating, or snow-melt systems.

Design Considerations

Insulation placement critically affects heat flux distribution. The rigid insulation layer (typically extruded polystyrene, R-10 to R-15) must provide continuous coverage without gaps that create thermal bridges. Insulation boards lap at least 6 inches and stagger joints.

Vapor barriers below the insulation prevent ground moisture migration into the assembly. Moisture accumulation degrades insulation performance and can lead to concrete deterioration.

Edge effects at rink perimeters require increased heating capacity or tighter coil spacing. Heat losses to adjacent spaces or exterior walls create localized cold spots without compensation.

Soil conditions determine frost heave susceptibility. Clay soils with high moisture content present the greatest risk, while granular soils with good drainage pose minimal heave concerns. Geotechnical investigation establishes site-specific heating requirements.

ASHRAE Recommendations

ASHRAE Applications Handbook Chapter 44 (Refrigerated Facilities) provides guidance for ice rink sub-floor heating:

• Maintain sub-slab temperature between 38-45°F
• Design for worst-case refrigeration load at peak ambient conditions
• Include redundancy for critical systems serving Olympic or professional facilities
• Monitor temperature at multiple locations across the rink footprint
• Commission systems before initial ice-making to verify uniform heat distribution

Sub-floor heating system failure leads to progressive frost heave damage requiring extensive remediation. Proper design, installation quality, and continuous monitoring ensure decades of reliable operation with minimal maintenance intervention.