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)

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

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
No longer selling Cluster Needles Paint
No longer selling New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more

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

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Correct....

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

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

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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)

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).

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All Diesel Fleet
1985 R107 300SLD TURBODIESEL
2005 E320 CDI (daily)

LOTS of parts for sale!


EGR block kit
http://www.peachparts.com/shopforum/diesel-discussion/355250-sale-egr-delete-block-off-plate-kit.html

1985 CA emissions 617 owners- You Need This!
Sanden style A/C Compressor Mounting Kit for your 616/ 617 For Sale + Install Inst.

Sanden Instalation Guide (post 11):
http://www.peachparts.com/shopforum/diesel-discussion/367883-sanden-retrofit-installation-guide.html

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.

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

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.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!