Tips for dual-hose AC conversions that nobody talks about

I acutally tried this last summer with an older Black+Decker 12,000 BTU unit that was making my bedroom feel like a vacuum. I went the cheap route initially using some 4-inch vinyl dryer venting I had in the garage. Within twenty minutes, the casing felt incredibly hot to the touch and the compressor started making a high-pitched whine I’d never heard before. It was clear the fan was struggling against the resistance just like you mentioned. After seeing how much it was straining, I swapped over to a 6-inch insulated flex duct. It defintely helped with the static pressure issues, but the intake air temperture is the real killer. My window faces west, and pulling in that 100-degree afternoon air meant the vent temperatures coming out of the front were about 8 degrees higher than when it was just pulling 74-degree room air.

The room stopped having that negative pressure draft under the door, wich was nice, but the unit itself runs constantly now beacuse it can't shed the heat fast enough. If anyone else is planning this, definitely don't skimp on the hose size or you'll likely smell burning plastic like I did.

Have you ruled out the latent heat load from drawing in that outside air? It’s one thing to handle 100-degree dry air, but if you’re in a humid area, you’re forcing the coil to dehumidify all that raw outdoor moisture instead of just recycling the drier indoor air. That might be why it feels like it's struggling to cycle off. Also, are you sure the internal cabinet is actually sealed? A lot of these cheap portables have gaps between the fan housing and the outer shell. You might just be leaking some of that hot intake air directly back into the room before it even hits the condenser.

Spot on about the condenser heat. One thing I noticed when I rigged mine up was that I had to add a custom mesh filter to the new intake. Since these units are designed to pull air from inside the house, they ussually only have a filter on the evaporator side. Once you start sucking in raw outdoor air, you're basically sandblasting your condenser coils with dust and pollen. If you don't screen that intake, those fins will get gunked up in a single season, and then your heat exchange drops even further regardless of how big your hose is.

I rigged up an intake plenum using a plastic storage tub instead of just taping the hose to the back. When I first tried taping the 6-inch hose directly to the grill, the middle of the condenser did all the work while the edges got scorching hot.

I cut the bottom out of a small tub, taped that over the whole intake vent, and then mounted the hose to the tub. That extra air gap let the suction pull evenly across the entire coil surface. My compressor stopped cycling off on thermal overload once the whole coil was actualy getting air, not just the center spot.