Center Vent Always Emits Warm Air
>I just had my air con fixed. Compressor rebuilt and gas changed over etc.
>I don't seem to be able to adjust it.
>It blows really cold is the temp set low and heats if the temp set high.
>But in the middle settings I have cold air coming out the centre top
>vent and hot air coming out the centre bottom vent!
>Is this normal?
Hot air out of the bottom portion of the middle vent, in particular the left side of the bottom more so than the right side, is quite typical.
This is usually caused by a combination of two problems. The first is that the hot water valve which controls the flow of coolant into the heater core is not functiioning properly and is leaking water into the heater core when it shouldn't. The valve itself is a very poorly designed unit, entirely made of plastic, and resides in the very hot engine compartment, where it is subject to frequent failure. It is vacuum controlled, and therefore a typical failure mode is a diaphragm leak which prevents the valve from closing all the way, or not at all. Even if the diaphragm is OK, the valve may distort due to heat and will consequently leak coolant into the heater core. Fortunately, the valve is cheap ($15) and easy enough for the owner to replace (but still no excuse for Porsche not fixing this common problem with a better part).
The second part of the system to malfunction are the "flaps" which, if functioning properly, close off the flow of incoming air to the heater core, leaving the incoming air to flow only through the evaporator. In theory, if the flaps close completely, then even if hot coolant is flowing to the heater core because of a defective heater valve there will be no hot air because there is no air flowing through the heat exchanger. However, even properly adjusted flaps (also vacuum controlled), may not shut air tight, thus admitting some air into the heater core.
The combination of the moving flaps and the variable heater coolant flow allows the climate control system to mix this hot air with the AC air to achieve an optimum temperature for the air flowing through the center vents (this way its neither just hot air nor just cold air).
To isolate the problem (WITHOUT THE AC ON - the climate control system functions with or without the AC on), if, when the temp selector is set all the way cold, you get hot air, then the heater valve is probably malfunctioning since the heater valve should be closed. This test should only be done while the car is moving, otherwise you may get fooled by the fact that hot ambient air from under the hood is being drawn into the system. If the car is moving, air drawn in should be ambient, not preheated by the engine. If the air is always hot, replace the valve.
If you get relatively cool air, then you can assume that the valve is working. Try moving the temp setting to max and verify that the air gets real hot, then back to cold and the air returns to its initlal temp. If so, try the middle position and see if you get a temperature in between. If all is OK, then the valve is OK.
If you still get a little warm air from the bottom part of the central vent, the flaps can be adjusted to close more tightly. There is a procedure for this in the workshop manual. It is not especially difficult, but it does take some disassembly of under dash components to access the adjustments on the left and right sides of the heater box. I adjusted mine when I had the dash apart for other reasons, and it did improve the close off of warm air, but valve replacement really was the key to eliminating unwanted hot air.
Steve - 87S4/auto/GPW
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