The APU is unable to properly cool the cabin
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Hello,
First of all, thank you very much for this amazing add-on! I'm really enjoying it.
I noticed that with the APU turned on, it's impossible to keep the cabin cool in hot weather. I don't know whether the APU is too weak or whether the effect of the open passenger door is exaggerated. However, the APU should be able to maintain a cabin temperature of around 22°C, even when the outside temperature exceeds 30°C.

Greetings
Tim -
See those knobs with Flight Deck and Cabin they have to be turned down, they are very sensitive in the real aircraft and with them about 33 percent of the way up it will get hot. Also APU is not capable of Bleed air whilst airborne it can only provide electrical power
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https://www.pprune.org/tech-log/135762-fokker-f-100-a.html?
Among crews who flew the aircraft, the Fokker 100 developed a reputation for marginal ground cooling performance in hot weather, especially with a full passenger load. This wasn't necessarily because the APU itself was unreliable. It was more that the entire ECS, including the packs supplied by the APU, had limited cooling capacity in high ambient temperatures.
On hot days (30–40°C / 86–104°F), the APU could struggle to provide enough bleed air for both air conditioning packs to cool a fully loaded cabin before engine start.
It's a feature, not a bug

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This is actually something that affects other aircraft irl as well. The A320 also struggles to maintain cabin temperatures in hot conditions. Not as drastically as the F100 though, but sometimes 26°C is the lowest you can get when outside temperatures are very high. The APU and packs in MSFS addons are oftentimes a bit overpowered in this regard.
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This is realistic when on the ground.
Both the F28 and the F100 were designed for temperate Northern European summers and struggled here in the US, especially in the south.
If anything, I think the packs are actually too powerful once you take off. Today the cabin was 34C when I started the APU and it only got down to 31C by takeoff. After takeoff by around 4000 feet it was 19C. Would've been warm a lot longer!