Radiant Home Heating Systems

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 (10x10 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
• ?T 10 K
• PEX Pipe
• q 16.4 kW
• ?T 0.6 K

6
CFD Results Using Gambit / Fluent

• Copper Pipe
• q 292 kW (9 error)
• ?T 8 K
• PEX Pipe
• q 8.7 kW (87 error)
• ?T .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.