A friend and I were discussing the heating/cooling system in cars. He says that hot coolant only flows through the heater hoses and heater core when the heating system fan is turned on an the temperature selector is set to hot. I told him that hot coolant is always flowing through the heater core no matter what the fan or temp setting. All the fan and temperature control do is determine how much of the radiant heat coming from the heater core goes into the passenger cabin.

Who is correct?

Thank you

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)