I have had a portable heater/AC installed in the bedroom for years that currently vents out of a window. I recently had AC installed in the house, but found that my wife has gotten so used to the portable machine running as white noise that she has a hard time sleeping without it.
Would there be any potential issues if we ran the machine on heat mode and just let it vent into the room? I'm concerned about moisture issues over time in the window that it vents from.
If you're using it just for the noise to help your wife sleep, an improvement might be to record the noise the machine makes (such as with the sound recorder in a computer or phone), copy the recording to a CD with a computer, and then play it on a stereo in the bedroom when she's trying to get to sleep. That assumes she needs the noise only to fall asleep, but stays asleep without it.
Another possible solution is a white noise phone app, and a phone in a charger on a bedside table. This may also be useful for travel.
The reason to use one of those solutions is that they draw a lot less power than an air conditioner, and don't put unnecessary wear on a mechanical device with lots of moving parts.
Obviously, if you want the heating or air conditioning too, the machine makes sense, but if you just need the noise, then a device that specializes in sound is probably a better solution.
The issue is only one: the AC not venting outside behave like a heater (energy used by the motor is released in inside air). My suggestion is to buy a simple fan (or use the fan-only mode if present on your machine), so you'll have the 'familiar' noise without heating the room nor draining much power (running only the fan is far cheaper than running the full AC).
