Radiant Home Heating Radiant Home Heating SystemsSystems

Dan KorthDan Korth

Corey ChristensenCorey Christensen

Project ObjectivesProject Objectives

Compare CFD and analytical models of Compare CFD and analytical models of the heat transfer rate through two the heat transfer rate through two plumbing pipe materials. plumbing pipe materials.

Determine why PEX pipes are used in Determine why PEX pipes are used in place of copper in radiant heating place of copper in radiant heating systems.systems.

System LayoutSystem 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.

Pipe CharacteristicsPipe Characteristics

GeneralGeneral Inner diameter: 0.5 inInner diameter: 0.5 in Wall thickness: 1/16 inWall thickness: 1/16 in Length: 76.3 ft (10’x10’ room)Length: 76.3 ft (10’x10’ room)

Copper:Copper: Thermal conductivity: 387.6 (W/mK)Thermal conductivity: 387.6 (W/mK) $3.68/ft$3.68/ft

PEXPEX Thermal conductivity: 0.461 (W/mK)Thermal conductivity: 0.461 (W/mK) $0.33/ft$0.33/ft

Mathematical ModelMathematical Model

Closest correlation found using Sieder Closest correlation found using Sieder equation to calculate Nusselt number:equation to calculate Nusselt number:

Copper Pipe:Copper Pipe:q = 320 kWq = 320 kWΔΔT = 10 KT = 10 K

PEX PipePEX Pipeq = 16.4 kWq = 16.4 kWΔΔT = 0.6 KT = 0.6 K

CFD Results Using Gambit / FluentCFD Results Using Gambit / Fluent

Copper Pipe:Copper Pipe: q = 292 kW (9% error)q = 292 kW (9% error) ΔΔT = 8 KT = 8 K

PEX Pipe:PEX Pipe: q = 8.7 kW (87% error)q = 8.7 kW (87% error) ΔΔT = .007 KT = .007 K

Cost AnalysisCost Analysis

Copper:Copper: Much more (Much more (≈10x) ≈10x) expensive to purchase and install.expensive to purchase and install. High thermal conductivity means the building heats up High thermal conductivity means the building heats up

quicker but requires much more energy to maintain quicker but requires much more energy to maintain ambient temperature.ambient temperature.

PEXPEX Cheaper hardware costs.Cheaper hardware costs. Low thermal conductivity means a longer time to heat Low thermal conductivity means a longer time to heat

the building but much lower energy requirements.the building but much lower energy requirements.

ConclusionsConclusions

PEX pipes are used in radiant heating systems PEX pipes are used in radiant heating systems because of:because of: Lower hardware / operating costs.Lower hardware / operating costs. Easier installation process (flexible pipes).Easier installation process (flexible pipes).

CFD and mathematical models correlate within CFD and mathematical models correlate within reason. reason. Good numerical models require a lot of preparation Good numerical models require a lot of preparation

and work to get things right.and work to get things right. Experience with tools is priceless.Experience with tools is priceless.