Sounds like you have a leak in the system as recharging helped. As the unit is 16 years I agree time to replace. You need to replace both the inside and outside units. Unfortunately flexible duct work is what you typically see in residential installations so I wouldn't neccessarily let that be a determining factor unless it has leaks.

Personnaly, I think you'd be better off with a conventional split heat pump system assuming that you get adequate flow from each of the supply vents now( verifying that the duct work doesn't have major leaks). Can only guess at what size unit you have now but would think maybe 1.5 or 2.0 tons for 1000 s.f in basement condo in your area.

I've got a mini split in my large sunroom (400 s.f.) and it does well in that setting. The only downside is that all the flow comes out of that one wall mounted inside unit. With a conventional system there might be 3 supply vents for 400 s.f. resulting in more even air distribution
and less chance for a "drafty" feeling.

Rental of a crane to remove the old unit and hoist up the new unit won't be a deal breaker when compared to price of multi unit mini split model. Also, it's my opinion that mini splits need to be installed and serviced by HVAC technicians familar with the specific brand of mini split...which limits your service options later. Mini splits however can be purchased with inverters and have high SEER ratings...they also are less noisy outside but if it's on the roof not an issue.

Bottom line I'd solicit recommendations from reputable HVAC companies in your area and let them quote on what they think you need.

First off, 4 stories sounds like a big vertical head for residential mini-split equipment just from my casual notice of those requirements. I would check that out first.

I am testing a whole-house mini-split installation right now (Mitsubishi, 7 in, 3 out). I will let you know in a year or so if I like it ;-) On paper they are more efficient due to inherent efficiency and the inherent zoned nature. We have been having pretty hot weather and with my wife out of town this week, I have been pretty much been cooling the bedroom and the kitchen with some intermittent cooling in the utility room where the exercise machine lives. Note that I am not much of a sit and watch TV type.

You are really a leg-up over lots of others on this decision as you have experienced mini-splits. One thing to think about is that, IIRC, most of them don't have back-up heat so you will not have any heat while they are defrosting. (Maybe none of them have electric resistance back-up, but I think I ran across at least one brand that does.) You also have to pay close attention to cold weather performance for the mid-Atlantic region. I know that with Mitsubishi's current residential equipment, the 1:1 systems have better low temp performance than the multi-splits. They are more efficient too. Even if they do have back-up heat, you can kill the efficiency pretty fast if you need back-up very often.

Keep in mind that there are lots of options to the wall-hung indoor units. They cost more, but there are units that can be dropped into ceilings and others that sit on the floor. There are also short, ducted units that would solve that big room problem by splitting the supply and return. (You could also install more than one indoor unit/room.) Condensate needs to be dealt with using pumps.

I am not an HVAC or building pro, so take my observations with a grain of salt. After replacing a ducted system with mini-splits, and talking to my install guys a lot, here are some observations and conclusions.

Installing a mini-split through outside walls, as is it usually promoted, is pretty quick and straightforward (read fast and cheap) compared to installing a new duct system in an existing house with no central AC. My install was done through interior walls running the lines/cables under the raised house and out to the side which was much more complicated. If I were doing this over and I had a good duct system within a house envelope, I would stick with it. The trouble is, what is a good duct system? More often than not, they are outside of the living space. In addition, they are considered good when they leak 5% of the air and they only get worse as they age.

My system was in the attic with parts of the duct system in the attic and partly in a furred-down space over the hallway. The worst part was one duct that started at the AH in the attic, dropped down under the house in a chase and ran for about 30 feet before it reached the first floor register in an addition with a flat roof (thus no attic). It then reduced in size and ran another 25 feet under the house. That part might have been mini-splitted even if the rest of the house retained a ducted system.

By your description, your duct system was not installed very well, but if the design is sound, it could be revamped less expensively than installing minis. In general, I think that ducted systems are trickier to design and install than mini-splits so there is more tolerance for less than careful and knowledgeable installers with mini-splits. Due to their ability to handle variable loads, choosing the right size is not as critical either. (I had 4 tons cooling plus a gas furnace before, now I have about 106% of the cooling and 60% of the heating. The heating was way oversized.) My system will handle cooling fine with the house the way it is, but I should not have trouble after I make improvements to limit infiltration and add insulation.

My installer is knowledgeable, smart, very workmanlike, and experienced (he has taught the trade for 30 years). He would have installed new ducts and a two-speed system for a slightly lower price than the chosen mini-split equipment. I doubt that he would do it for the same price if doing it on a similar home going through interior walls. He did not have much mini-split experience and brought in a collaborator that does, on this project because he was looking forward to a learning experience. I don't think that he lost his shirt, but I don't think he made as much as he usually does.

Thanks for your input roadking & ionized. I was going through all the old condo docs today and found that the condenser only was replaced back in 2001, with a 2-Ton Goodman. So, it's only 10 years old which might not be as bad, but then again, the unit before that only lasted 6-7 years then.

I do think a straight-replacement would be less expensive. I just wonder about the efficiency of that type of system given the constraints of a basement condo. Then again, perhaps I'm only skeptical given that ours has not really worked since living here. The neighbor upstairs, who recently replaced her entire system, said the huge electric bill spikes are just common with our units and have always been that way. But I came from another 1,000sq ft condo in this area, and I am pretty sure this is not the norm.

Does anyone know what type of system does a better job with overall dehumidifying? I remember that was a major issue last summer, and likely always be given this type of unit.

While I do have some experience with mini-splits, the units I lived with were all in China, with Chinese character remotes and Celsius readouts. I could tell each unit worked well, as long as I pressed a variety of buttons I didn't understand-haha!

I prefer a conventional heatpump system versus mini-split if you already have vents.

the ductless systems are great. i would always use them if it is possible. you can get multiple indoor units off one outdoor unit so you have the benefit of true zoning. for your size condo this would prolly work awesome

neohioheatpump, why?

Mini split sellers market their good dehumidification heavily. The variable-speed-blowers and variable speed compressors would help with low-load situations better and single-speed equipment. I am sure that some variable-speed ducted systems can match it.

If you have a lot of air infiltration, you will have a humidity problem no matter what you install. Ducted systems can make a passive infiltration problem into a power-vent infiltration problem under some conditions. If the leaking supply ducts are outside of the house envelope you suck hot, humid air in somewhere else to replace the leaking, humidified air. If the leaking return is sucking air in from outside, conditioned air will leave through holes elsewhere. If supplies and returns are not set up right, you can pressurize some rooms and depressurize others relative to outdoors and get both at the same time.

One advantage of ducted systems that I have considered is their ability to use the capacity of the system in one room of the house. That might sound weird, but consider the situation of a party in your house with a large crowd gathered in the living room. Your HVAC system can be running at full capacity picking up heat from the bodies in the LR. Unless your mini split was sized for a large crowd, it might get warm in there. If you have frequent parties, you might size it that way, or install two. Most people won't do that.

If you go mini splits, don't mount a wall unit above where you will have a bed. I saw advertising that showed that and planned that but changed my mind figuring that at low speed at night the cold air would sort of cascade down over the heads and be uncomfortable. I am glad that I changed my mind about it. I don't know if that problem would occur. What I did find, since we did the installation during heating season, is that the default programming of the heat pump mode makes the units good ceiling heaters. As they approach the set point the blower speed goes way down so hot air gets stratified near the ceiling even though the louvers are pointed down. Setting the blower speed manually to a slightly higher speed fixed that. Putting the indoor units above the bed would have been uncomfortable if only for that reason.