I recently bought a portable air conditioner and was wondering if it is absolutely necessary to vent it to the outside. I plan on using it in a room that doesn't have a window as was wondering if I could vent it to another room in the house without doing any damage (eg mold growing in the house). I understand the air coming out of the AC will be warm and would warm up the room it was venting into.
It's possible but it will be creating a situation where your a/c is fighting against itself, because you'll simply be redistributing the heat inside your house, instead of putting it on the outside.
Why? Because the warm air will creep back into the room you were cooling, so the a/c will be acting like a big do-nothing machine - to a degree anyway.
Mold isn't going to be too concerning, however, because you won't be taking moisture out of the one room and putting it into the other - the moisture in the air in the warm room may increase a little (because warm air can hold more water vapor than cold air, so if moisture is there, it'll get picked up) but not to the degree it would if you were, say, venting a dryer into that room, where you'd be introducing lots of NEW moisture.
All that said - I do not recommend this idea. Better to create a vent/opening (properly, of course, but that's a different question) in the outside wall to allow the warm air to go where it is supposed to.
Moisture is a problem. Air conditioners actively remove moisture from the cooled space and deliver it into the warmed space. One of the main features of air conditioning is dehumidification. The warmer air in the discharge room may already be heavily saturated with moisture, and the even hotter air coming from the discharge chute will add to that moisture. Prolonged use could lead to mold problems.
Further, interior walls of houses are rarely insulated, so the walls between the cooled room and now super-heated room will transmit the heat back to the cooled room. Also, air leaks will filter some warm air back. This will not totally overcome the cooling effect (we often air-condition one room and not another) but it will diminish the value and efficientcy, since the now heated room is much warmer than it normally would be.
Overall it is a bad idea. It is possible to create a small panel captive in a window that can hold the discharge chute of the air conditioner to vent outside.
Yes, it will work in terms of cooling the room, but it won't be as efficient as an external vent.
If you have the door to the cooled room closed and as air tight as possible, the adjacent room being warm is like the outside of the house being warm. Yes, there will be some unwanted heat transfer but there will still be a temperature differential. The best thing is to try it and see if it is cool enough for you and if the cost in power is worth it.
As for your question about moisture, the answer is "it depends". A typical full house AC has no moisture connection between the cool side and the warm side. All water taken out of the cooled air goes into a drip tray and then a drain.
The more modern window AC are designed so that the water from the drip tray can hit the condenser coils to help cool them off, improving their SEER. This is a moisture path to the exhaust.
Some portable AC have a drain hose and that means no added moisture in the exhaust.
DeLonghi portable air conditioners exhaust warm moisture-filled air out their exhaust hoses, so there is less condensate to empty manually. But this means the exhaust has the extra moisture removed from the cooled air.
However, if the cooled room does not have a constant source of additional moisture, this may not be a problem for the room receiving the exhaust. It would be best if that room had an open window or two.
I have the same situation, only a sliding glass door. Next to it is a floor vent where the heat comes out during the winter.
To fix the problem I removed the vent cap, and connected the hot air flow vent to the floor to blow into my duct work. The air actually cools off before it gets to any other room.
I set the AC on a boot pan, and ran a round drainage hose out the sliding glass door. The door remains open about an inch, and I put a strip of foam down the door. Then I cut a piece of broomstick to wedge the door closed so it will not open for security purposes.