Greetings All,

I've noticed something with my car (and all AC vehicles I've ever driven) that has me scratching my head. Yesterday the mercury hit 100F, so this had me thinking about this oddity with AC behavior.

On a hot day, starting with a heat-soaked cabin, why does it take so long for the AC duct temperature to drop all the way down? I keep a pocket thermometer clipped in the center vent to monitor AC performance, but the effect is quite obvious so no thermometer is needed. I even noticed this on a brand new vehicle I drove yesterday at work.

I'm not asking why the cabin stays so warm. I get that part. Lots of heat to be dispersed (or whatever the correct term is), so it's going to take 15 or 20 minutes for the cabin itself to cool off. I'm specifically wondering why the duct temperature itself takes 15 or 20 minutes of driving to get to 38F or so. I also realize the ducts themselves would start out warm, but it doesn't seem like 15 minutes would be needed to cool that small amount of plastic. But wouldn't the evaporator be as cold as it's going to get within a few minutes, especially at freeway speeds?

Here's the driving scenario from yesterday:

100F day, vehicle parked in sun. The thermometer sitting in the duct showed 120F when I first sat down.

Started engine, climate control in AUTO, 72F selected. Max AC (recirc) automatically engaged, as was fan to high. I opened the windows for the first couple of minutes to help expel that 120F air. Duct temp around 80F as I left the parking lot, so it's starting to cool. Cabin still toasty. Not an ideal scenario for cooling, as the compressor RPM was low and the ram airflow through the condenser was also low.

Within about 3 minutes, I was on the freeway, doing a steady 60MPH. Still on recirc. The cabin air being drawn back into the AC system was probably about 80F, so it was cooler than the outside 100F air. Best scenario for cooling now, with adequate compressor speed, lots of ram airflow through the condenser, and the air source is cooler than the exterior air. The duct temp slowly dropped to about 60F. This is the part that has me puzzled, as I'd think the evaporator is already as cold as it would ever be, and the ducts have had some time to cool.

After about 7 minutes on the freeway (10 since start), the system automatically switched from recirc to fresh air. Instead of drawing partially cooled air from the cabin, 100F exterior air was now being used. Still good compressor speed and ram airflow, but with hotter inlet air, so this is a less ideal scenario for cooling. The duct temp slowly dropped to about 50F.

After another few minutes at 60MPH, maybe 15 total since start, the duct temp was down to 38F, and stayed that way for the rest of my drive.

Any thoughts? I'm not saying this is a fault, as every AC equipped vehicle I've ever driven was like this. (It doesn't matter if it was an R12 system, an R134a conversion, or new with R134a.) I just don't understand the behavior, that's all. I would think after a couple of minutes, the evaporator should be as cold as it could possibly get, and the duct temp should show that, even if the cabin is still hot. Does it really take that long for the evaporator to cool down, or am I missing some other factor?

Inquiring minds want to know...

 

Karl, I let this one sit a week hoping someone else would weigh-in, because I simply can't remember the temps from reams of testing we did for Ford back in the early/mid-90's during the R-134a retrofit program. Still can't recall the particulars of the test-data but I'll go ahead and postulate that you've already answered your own question in the excerpt I've quoted above!

As you pointed out, the interior of an automobile gets considerably hotter than the ambient temp when sitting closed up and locked. A couple of years ago, I had a meat thermometer in the dash vent of my black Excursion parked facing the western sun and it was not uncommon to witness 155-160 deg F about 5 or 6pm on a 100F day.

So the ductwork is normally a thin-wall plastic or in some cases even less-substantial - a flexible near-paper and wire affair much like a dryer vent hose. The entire cabin area is hot-soaked to 120F in your example. This means everything behind the dash, structure, electronic modules, etc. etc is also starting out at 120F. Heat lives by the simple rule: "I migrate from where there is more of me to where there is less of me."
You have a lot of mass of dash materials surrounding each duct and very little to no airflow outside of the duct. So even though the evaporator coil reaches it's target temperature relatively quickly (assuming sufficient subcooling - a condenser capacity issue) the duct is still transferring a lot of heat from the surrounding structure to internal airstream, resulting in higher-than-you expect discharge air temps. The duct material is just not much of a thermal insulator. Lots of radiant heat emanating from the dash components.
If you had room, you could insulate your ducts and improve the response, but likely no space for that - real-estate between the dashboard and firewall in modern cars is highly-prized and seldom left unused.

If you have WDS or similar equipment that can read the evap sensor, then you could test the postulate.

 

Zane, I believe you hit the bullseye yet again!

I thoroughly enjoy Karls verbage as of late. I believe you had superseded by at least 6 years. I love the members here.

Carry on boys....

 

You do realize it's considered bad form to encourage my verbosity, right?

 

Ooh, I wish I could read the sensor. Great theory, appreciate the response. Solving moot points, that's my specialty.

 

Looks like an elm327 can do it but the magic data is kept secret by car makers (including Jaguar).

I know some of the data fro my car but it has the CCM (aka ACCM) on CAN whereas yours is on SCP.

 

I warned you:

https://www.jaguarforums.com/forum/s...ontrol-185002/

 

Going to have to read that one later!