1. Radiant Home Heating Systems Dan Korth Corey Christensen

2. Project Objectives • Compare CFD and analytical models of the heat transfer rate through two plumbing pipe materials. • Determine why PEX pipes are used in place of copper in radiant heating systems.

3. System Layout House is heated by hot water flowing through pipes imbedded in the floor. Pipes are positioned between the floor joists. The top surface of the pipe is in direct contact with the underside of the sub floor.

4. Pipe Characteristics • General • Inner diameter: 0.5 in • Wall thickness: 1/16 in • Length: 76.3 ft (10’x10’ room) • Copper: • Thermal conductivity: 387.6 (W/mK) • $3.68/ft • PEX • Thermal conductivity: 0.461 (W/mK) • $0.33/ft

5. Mathematical Model • Closest correlation found using Sieder equation to calculate Nusselt number: • Copper Pipe: • q = 320 kW • ΔT = 10 K • PEX Pipe • q = 16.4 kW • ΔT = 0.6 K

6. CFD Results Using Gambit / Fluent • Copper Pipe: • q = 292 kW (9% error) • ΔT = 8 K • PEX Pipe: • q = 8.7 kW (87% error) • ΔT = .007 K

7. Cost Analysis • Copper: • Much more (≈10x) expensive to purchase and install. • High thermal conductivity means the building heats up quicker but requires much more energy to maintain ambient temperature. • PEX • Cheaper hardware costs. • Low thermal conductivity means a longer time to heat the building but much lower energy requirements.

8. Conclusions • PEX pipes are used in radiant heating systems because of: • Lower hardware / operating costs. • Easier installation process (flexible pipes). • CFD and mathematical models correlate within reason. • Good numerical models require a lot of preparation and work to get things right. • Experience with tools is priceless.