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I noticed this a few years ago, that the hot coolant expansion tank is right next to the cabin air intake.
My AC works but is barley cold and if I turn the AC off and have it on LO, it blows warm air.
I thought I had a shield missing.
I have removed the engine bay seal, for a bit of heat dispersion, but left the bulkhead seal in place.
Anyone else have this?

Yep, mine only blows very warm air when the ac is off. You can monitor the temp coming out the vents with Vagcom and mine showed 27 degrees, getting hotter when car was driven hard. Makes for a very uncomfortable cabin. Not sure why it does this, but with ac on it blows as low as 3 degrees in to the cabin.

Think I'm going to make a better sheild between the expansion tank and intake, next week.
I'll let you know.

I would be interested to hear the results

So I've made pretty much the same shield as last time, with car washing sponges. I know, not the best.
It seems to help, but, the other day, I drove out the garage with the AC on, (I don't usually do that) and it stayed nice and cool while driving around, which makes sense, as (if I'm right) it goes straight to recirc mode with the AC on, so wouldn't be dragging warm air from around the expansion tank.
So, like some of the other guys on here, I think I'll just use the AC more.

Yeah, probably the best idea. The AC likes to be used anyway!

Yeah, my AC is always on.

Echo the above.
My a/c is always on.

Set temp and choose auto, forget about it unless you need t tweak a degree or 2

Mine is always on in the intelligent recirculation mode.

Whilst I’m sure isolating the inlet box could help, I fear some of the actual problem is the design of the blower housing and routing of air.

I believe as the air comes into the unit it always goes through rhe AC evaprator. If AC is on the it gets cooled down.

Depending on the mixer flaps position air either goes up, down or both ways. If up it goes straight out to the vents. If down it goes through the heater core then to the vents.

I assume that in full COLD position all air should go up, however some of the air actually goes down to the heater element and swirls back round as the flap is after it.

This must heat up the air. Ideally you want a flap before the heater core. Our cars do not have valves in/out the heater core so it’s always got hot coolant though it.

D is the AC evaporator
E is heater core
F is additional heater that most of us won’t have.

Good points. Well analysed.
Is that from the Bentley service manual? (or as my mate calls mine, War and Peace).

Yep. Normally quite expensive but you can pick them up cheap second hands sometimes.

I battled my AC for a while. It was cold, but not quite 100% cold enough on very hot days.

It turned out to be the AC expansion valve on my car was a bit blocked (and was maybe in the wrong way round). It actually sits underneath the coolant tank I’d wager that not many people have changed theirs as it’s hidden.