Just finished my dual-hose conversion for the PAC 3800 S — some thoughts

I did the exact same thing with my old DeLonghi Pinguino last summer. That negative pressure is the worst part of portable units; I coud literally feel the hot draft coming from under my bedroom door every time the compressor kicked on. My setup was a bit more haphazard—I used a plastic storage bin cut to size beacuse I was worried about the humidity weakening a cardboard box over time, but the principle is identical.

The biggest difference I noticed wasn't just the room temperture, but how much less the compressor had to cycle. Before the mod, the unit was struggling to keep the living room at 23°C when it was 30°C outside. After I added the intake hose, I could hit 20°C in about twenty minutes. I ended up using some 150mm insulated ducting from a hardware store instead of the standard plastic hose, wich helped keep the heat from radiating back into the room from the exhaust side. One thing to watch out for is the air filter placement. On my unit, the intake I covered had the main mesh filter, so I had to make sure the box was easy to pop off for cleaning every couple of weeks. If you haven't checked that yet, keep an eye on it because the dust buildup can realy choke the airflow when you're pulling from outside.

I tried the cardboard method on my PAC 3500 last year, but I ran into a wierd issue where the compressor started short-cycling because the intake box was too restrictive. These Trotec units pull a massive volume of air, and if that DIY plenum isn't deep enought, the static pressure just kills the efficiency. I eventually switched to using rigid XPS foam board and silver HVAC tape. It is much sturdier than cardboard and acutally provides a bit of thermal insulation so the intake air doesn't get pre-warmed by the unit's casing before it even hits the coils. One thing people often overlook is the heat bleeding off the exhaust hose itself. Even with a dual-hose setup, that thin plastic pipe can hit 50°C and act like a space heater. I wrapped my 150mm exhaust in a reflective bubble-wrap sleeve I found at the hardware store. Combining the dual-hose intake with an insulated exhaust dropped my vent temperatures by another 3 degrees. If you are already taping boxes to your AC, you might as well go the extra mile to stop that radiant heat from fighting your cooling progress.

This is absolutely brilliant and I am 100% trying this tomorrow morning! I just bought my first Trotec and the way it sucks hot air from under the bathroom door is driving me insane. I never whould have thougth of using a cardboard box for an intake plenum. Quick question though—since you mentioned using a floor fan in the corner, have you noticed if that helps with the 'swampy' smell some units get when they've been running all day? I'm super sensitive to smells and wondering if extra circulation stops the condensation from getting funky!

I'm super new to this and just bought my first portable unit last week, so I'm still figuring out how it all works. I noticed mine has a drain plug at the bottom for water. If you’re pulling in that humid outside air directly trough your DIY box, does it make the unit fill up with water way faster? I’m scared I’ll end up with a puddle on my floor if I try this.

Also, how do you even tell wich vent is the intake? My Trotec has like three diffrent plastic grilles on the back and I have no idea which one to cover.

Quick update—I’ve been watching for that short-cycling issue mentioned. So far so good, but the humidity is a real factor; the tank fills way faster now. I noticed the cardboard is even getting a bit soft from the damp air. For those who used a plastic bin, does that help with the moisture? I'm thinking about upgrading the plenum material if this box gives out.