Originally Posted By: rationull
I've always been under the impression that to get fastest warm up time you should not only turn off the blower fan, but also turn the temperature down, but from some of the posts here I'm wondering whether that's correct. Do the temperature controls on manual HVAC cars tend to close off the coolant line to the heater core or just operate the blend door?
It depends, probably. My 1990 Integra manual system did both via two bowden cables (variable air flap, variable coolant valve). My 1992 Porsche semi-auto system had electric blending flaps and an on/off coolant valve (vacuum operated via solenoid IIRC). My 1998 BMW auto system has lots and lots of electric blending flaps and a pair of PWM variable flow coolant valves - the air flaps and the valve opening combine to set the vent temperature. It all depends on the system.
Regardless, if there's no or little air flow over the heater core, it isn't sinking any additional heat. The coolant loop will be a little longer and include more coolant with the heater valve(s) open.
"Winter" means different things to different people. If it's 0*F you aren't going to get away with no airflow or using the recirc setting if there are breathing humans in the cabin - you need at least a little bit of dry airflow to keep from fogging. In "quite cold" conditions in the 1.8L Integra I used to put the fan on but with the controls set to cold to keep the windows clear. On the BMW I've found that I set the fan to low but leave the temperature on auto - this will still gently heat the windshield but doesn't place a huge heat sink on the system. The Porsche never saw rain, let alone snow
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The reason I thought this is I'm pretty sure I've read instructions to turn the heat all the way up before draining the coolant when replacing it, which would only make sense if the heat needed to be up in order to drain all of the coolant.
Correct, the heater valve needs to be open to drain/fill the heater core.
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I'm pretty sure I've noticed my Civic warming up faster when the temp is set all the way cold. I haven't done any extensive testing on our Mazda3, but that car seems to warm up faster than the Civic anyway for whatever reason.
It's a combination of the temperature setting and the airflow. Any heat you're putting into the cabin, whether a little bit of scorching air, or max speed fan of warm air, is heat being taken from the engine.
I suppose some might think I'm a nut-job for taking this into account at all, but I don't feel my actions are extreme considering our vehicle currently does a fair bit of winter short-tripping (kids) that it didn't do in the past (pre-kids). It's been pretty nippy here lately (in the 10s*F?) and there's barely a trace of condensation emulsion on my oil fill cap despite the short trips. Different vehicles have different HVAC systems and may require different actions or compromises.
If "winter" for you is 50F, worrying about the heater load affecting engine warm up may be a little extreme.