What would cause a musty smell from my portable A/C unit?
What I have found is the drain has gotten plugged in tha back outside section, dust builds up and plugs the hole , the other thing I have found that causes portables to smell musty is when they are installed level. There is supposed to be a tilt to the back so the condensate will run to the back and drain outside. There are special cleaners for AC systems make sure to use non acid based cleaners and flush with clear water. Acid based cleaners are very hard on the cooling fins and can start corrosion that will shorten the units life. I would want to flush my unit if musty and make sure it is draining completely or the smell will come back after cleaning .