Solve an argument for me.
 

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

Which car? I would say......it depends.

100%.

In an 80s-00s MB: a valve opens to let hot coolant through the heater core. Air always flows through the heater core.

In an 85+ VW: hot coolant always flows through the heater core. A blend door directs air to and from the heater core to control temp.

In a 92 Dodge Ram: hot coolant normally flows through the heater core. A blend door directs air to and from the heater core to control temp. A vacuum valve shuts off the coolant flow when Max AC is selected.

The blend door system has one advantage: You can change the temp very quickly from full cold, to full hot, back to full cold. In the MB system you have to wait for the heater core to warm up (this happens quickly), then cool back off again (this takes longer).

-J

Correct....

MOST (not all) modern cars use a blend door system for the reasons stated but also because it allows a smaller radiator. The heater core rejects heat just like the radiator and you have to have it anyway so the manufacturers figure you might as well make use of it. The hot air is directed away from the passenger compartment when not needed for heat and thru and into the passenger compartment when you do need it.

Older cars had a water control valve (in American cars called a RANCO valve after the most common manufacturer) that was essentially a cable controlled 1/4 turn ball valve. This was another part that could be eliminated with the blend door system.

So basically, your friend is operating on dated information but he was not completely wrong depending on the vehicle in question. For most current vehicles you are right on the money. I don't know what the bet was but it looks like you're going Dutch!

Dan

Well, I appreciate the responses. Like you said, it looks like we are both partially correct. So, in my 83 300D, will hot coolant flow through the heater hoses but not the heater core itself if the heater itself is not turned on? I'm wondering, as I know several people that use WVO use a flat plate heat exchanger and plumb it into the heater hoses. Are they not getting hot coolant through the heat exchanger if the heater is off??

thanks,

Glenn

MB vehicles- (specifically 123)- When A/C/ cold is selected (temp wheel) the mono valve is energized and keeps coolant from flowing through the heater core. Air is flowing past both the evaporator and heater core when the blower is running. The vacuum operated flaps direct the air to the desired location (except when a pod is faulty).

Yes they would be getting flow regardless of the in cabin heat being on or off. Most common spot to tap in would be a T in the hose that runs over the oil filter housing near #5 cylinder.

On my W123 300D with no air conditioning there's just a little tap on the engine bulkhead that stops the flow through the heater core.