illini-heel

Is the request due to heat "leeching" into the cabin? I'm just curious as to why so manu people have apparently done this.

MainePorsche

I'm not seeing it either.
One may be for the most part in a hot, sweltering environs but one might also travel to a place much cooler. Wouldn't one want some measure of heating to be available if needed ? I would without having to 'wrench' it back. I lived in the High Desert in SoCal where heat was almost never needed, but when driving on the PCH to our room after having dinner at McClintocks with the top open, I'm glad I did have a little heat coming through. Was very convenient.

GlenL

That's why: the heat comes on because of some malfunction that's not easily solved.

I actually do this on my 944. On that car the hot water valve isn't below the air filter so it's a quick swap. The valve is identical to the 928s. The problem is that the interior temp sensor has an intermittent short, or open, and it'll decide there's heat needed on the hottest Summer day. In my priority list this doesn't rate an actual fix.

On the 928 it's common to have vacuum leaks in the HVAC system and the hot water valve is "sucked" closed so it's always open and the mixing isn't right, either.

dr bob

On the 928, the heater valve is pulled closed by vacuum, so the 'failed' position of the valve is open. Vacuum is applied when the HVAC controller is powered up, there is vacuum in the reservoir, no leaks in between, and the target temp is lower than the temp measured in the cabin. That's with everything intact and working correctly, and it;s pretty effective. Until you shut down the engine, and the solenoid for the HCV is relaxed, no power applied by the HVAC controller. When that happens, the solenoid valve allows air to bleed into the heater control valve diaphragm, and the spring in that valve causes it to go to its 'failed' position, open. But since the car isn't running and the water pump isn't turning, it's only convective circulation of heat that moves warmer coolant up into the heater core. So park your car after a drive, then come back in a bit, and find that the first air coming from the vents is rich with heat that's migrated up through the system from the engine.

For most folks, a good heater valve seals off tightly when it's pulled closed by vacuum. If the valve leaks by on the liquid side, or is perhaps installed backwards, a small amount of hot water circulates through the heater core even when vacuum is applied. If your valve does seal tightly, it's not impractical to tie the valve closed with a plastic tie during the hot months, and slow the migration of heat when the car is running or parked. Even with the valve closed, there's convective migration of heat to the heater core. For those who want to keep even that small amount of heat out of the heater core and cabin, you'll want to put a valve in the return line from the heater core back to the pipe under the reservoir. You really don't want to bypass the heater valve, you really want to just block the flow through that loop completely.

Options include the manual valve mentioned above by others. You can add another vacuum-operated valve in the return line, vacuum-close like the original, and T the HCV vacuum to both valves. Lots of options.

For my black S4 in Los Angeles, the HCV is tied closed starting in late spring, and it's left that way until after the fall "santa ana wind" season. A cool morning usually reminds me to remove the tie.


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Cars with tired motor and/or transmission mounts may bleed hot air from under the car up into the cabin whenever the windows or sunroof are open. The insulation above the torque tube drops and leaves space for hot air to be drawn into the cabin. Is the console hot with windows open? Verify motor mounts and trans mounts for proper height. Folks without good AC might be forced to drive with windows open, and that extra heat coming into the cabin through the console makes it even worse.

MainePorsche

Don't mean to hijack, but I wanted to address dr bob's point.
Interestingly my motor mounts are 1.5 years old without many miles on them and the intake sits high above the factory cross beam. The engine does the 'shake' as it should. I think in my case it is that insulation shell that sits under my shifter and over the central shaft (I think they call it the sound absorber) that has thinned over time which is allowing the ingress of warm air. My transmission mounts are now a week old and I would be interested to see a change, but I doubt it. Actually that little ingress of warmth is kind of nice with the top open while driving. Is interesting that I don't get it with the windows open, and actually opening a window stops the ingress if the top is open.
Now back to the thread...

Tails

+1 to dr bob's post.

My steel "y" pipe to my reservoir and heater failed over 12 years ago (corroded with holes) and my vacuum diaphragm in the heater valve failed, so I has 3 problems, hot water into heater core, heater valve in open failed position and a USRS (Un-uSable and Rat S..T) "y" pipe with a consequence of hot air into into HVAC, which the HVAC system had to cool, especially as the heater core is before the evaporator core.

I fabricated a new "y" pipe from 3/4'' copper pipe, installed a SUS ball valve into the system at the end of the copper pipe to heater core, renewed the heater valve (Porsche supplied) and renewed the small section of heater pipe.

In the summer time I shut the ball valve, however, due to the design of the HVAC system with the heater core at the high point of the system, when the engine is turned off the hot water from the engine thermocycles to the highest point in the system, via the "failed safe" opened heater valve, heating up the heater core. When the engine is started the outside hot air is blown into the cabin through the heater core and then through evaporator core. If the A/C is off you will get a good blast of hot air and if the A/C is on you will still get a smaller quantity of hot air until the A/C cools the evaporator to such an extent until the air is cooled.

By installing this type of shut off system it will increase the efficiency of the A/C system in hot summer condition well over 40degrees C

I have used this system for 12 year with no ill effects as I use a good cooling water treatment and my heater valve is still working after 12 years.