Automobile Climate Control

Hi folks. First-time poster, please be gentle.

Every automobile climate control system I have ever seen offers the ability to direct airflow towards the face, the feet, or the windshield for defogging. In addition, they offer the option to split the airflow between the face and feet, or between the feet and the windshield. But I have never seen a system that offers the obvious sixth option, splitting the airflow between the face and windshield. Does anyone know if there’s a reason for this?

Ron Moses
Merrimack, NH

Very interesting question, never thought about it. The only thing that comes to mind is that when it is really frigid and your windshield needs quick defrosting, the maximum heat and blower speed are directed to it. To split it to have part on the face might well be too much, but as you know (as I do, having lived in VT), if it all goes there, your tootsies might freeze up, so good option to have them get some heat. Just a WAG.

Welcome to the Dope!

I would assume that most of the air blown over the windshield is going to hit one in the face too.

Yep. It’s not as direct, and is a little cooler, but it’s going to warm up the car as a whole just slightly less quick.

Your car is best heated from the floor vents, and best cooled from the face vents. It’s more common to use the defrost vents during cold weather that requires heat than hot weather that requires air conditioning, and so this is the natural split for defrost mode. My automatic climate control automatically diverts between floor and face (and if heating, a mixture) based on the temperature it’s trying to achieve, but it still limits me to the same split choices when it comes to defrost.

Cost is certainly an option, too. There are internal flaps that direct the air to the appropriate vent, which are either servomotor driven (rare) or pneumatically driven (common, even on high-end vehicles). In some cases, the flaps are interlocked, or even more often, the flap isn’t just a door (permitting/blocking flow) but a diverter (choosing where the flow goes). So, the more flaps there are, the more pnuematic lines you have to squeeze in, you’d have to use more doors rather than diverters, and overall size of the distribution box grows, all of which contribute to cost and weight.

All those changes aren’t generally worth it in order to blow cold, air-conditioned air into your face in the middle of winter (assuming your engine isn’t warm yet).

Traditionally, this has been controlled by a single valve. The valve has one inlet, and several outlets. There is a rotating piece that covers the unused outlets, and uncovers the desired outlet. Outlets that are next to each other can be partially uncovered when the valve is set at a mid position. If two outlets are not adjacent, there is no way to turn them both on at the same time.

Some newer, and higher end cars use seperate on/off valves for each outlet, typically operated by a vacuum cylinder. In this case they are electrically controlled, and any combination is possible. The limitations are designers still thinking inside the old box, possible user confusion, and the desire to keep the instrument panel “clean” looking.

All I can think of is my ex-girlfriend’s ceaseless griping that the twisting mix of wind in her face and off the windshield was messing up her hair. Perhaps the engineers have similar problems, although I think the two-channel explanation is the more likely one.

It took my a while to understand what you were talking about, then it occurred to me that you’re talking about ancient vehicles that have a mixer! A single knob hooked to a cable as you describe, and you basically have infinite control of the output mix from windshield to floor. Are there still cars – even low end – that have that?

The standard these days is purely selectable. Either electronic valves (automatic climate control, push button selectors) or manually actuated valves (rotary knobs) divert the engine vacuum to the correct pneumatic actuators, and flip the correct flap for the function. It’s conceivable to open them all or in any combination, but it’s an open/closed deal – no gradual output. Higher-end cars will use electric servos instead of vacuum, but generally only if there’s not a lower end model (you can use the same big parts that way).

I am most familiar with Volvos (obviously)(Oops! Sorry RonMoses you’re new here, you don’t know that I worked for Volvo as an instructor for 15 years)
On Volvo cars as far back as 1973 (140/240/260) you were able to select windshield/face/floor in any combo. The 740/940/760/960 did not have this ability due to using vacuum motors to drive the air distribution doors. Starting with the 1993 850 Volvo has used either electric motors with feedback potentiometers, stepper motors or a LIN bus motor. On these cars any combo of air distribution you can think of can be achieved. These cars also have individual controls for driver and passenger temps.
If the car uses vacuum control a door is either open or closed. This limits your ability to open some combos. However, when you use electric motors, you can open each door as much or as little as required.
To give you an idea of the flexibility of the system on our current generation cars if the system sees the windshield wipers come on, it will open the defrost door just a bit to keep the windshield clear. :cool:

I hope this helps to answer your question.

By the way, welcome to the SDMB RonMoses! I hope you will stick around our little corner of the net.

Get off my lawn, you darned hipster! My “new” car (truck actually) just turned 11, and the old one is WELL over 20, and I don’t have any plans to replace either of them.

What he said you whipper snapper…

Except mine are even older… :stuck_out_tongue:

FWIW, older Porsches have adjustable sliders that let you split air between the face and windshield or among the feet, face, and windshield (in varying amounts) at the same time.