I recently finish converting an attic space into an game room, that thread is here The space is insulated and I have a local HVAC company tie it into the rest of my homes system with a return and a supply. The supply is a 9” run from my upstairs system that splits into 2 7” lines going to 2 vents.

The room is about 4-5 degrees hotter than the rest of the upstairs. My wife works from home and keeps the upstairs HOT at around 77-78 making that room in the low 80s often. Its not the worst when the kids are in school as the office isn’t in use but it takes hours to cool off in the evening when we want to use the room.

I am looking at Mini Splits but I am not sure I have space. One company wants to do a Duct Mini split and reuse the vents I just had installed removing it from my main system.
I was thinking about putting in a smart dampener only on my wife’s office vents to have it close when it gets to a specific temp, probably 77, but allow the HVAC to continue cooling the rest of the upstairs. Wife’s office is about 400 sqft and my game room is about 220 sqft.

I also have a small closet return that is between the 2 rooms that I am thinking about moving into the game room. It’s a small 4” return so I am not sure that will make a huge difference.

I am thinking of alternatives to having my entire system reworked or spending $5k on a mini split and install. Ill need to have a sub panel installed if I go mini split because I am out of breakers.  

I just had a mini-split installed (Daiken) in a remote bedroom. It only takes up a small space (about 1 x 2.5 feet) on the wall inside the room. I really like it for several reasons:

- The room I put it in is far from the main house central AC. It allowed me to remove an air handler that was required previously.  
- It also cools and heats more efficiently since there are no long ducts etc.
- The condenser unit (outside) is small and VERY quiet.
- It only requires one 4" hole through the wall.
- The evaporator (inside) only takes an hour to install on the wall, including locating studs for mounting etc.
- It provides both heating and cooling, so essentially gives the room complete independence from the rest of the house.

It did need a new 240V line, that was actually the hardest part of the job. The outside work is easy from a construction point of view, but you need HVAC skills to deal with the refrigerant lines etc.  I haven't gone through winter yet, and haven't used it much in summer due to the late installation, but so far I highly recommend them just due to simplicity of retro-fitting into an existing house.

ETA: Also, you can have multiple evaporators all tied in to one condenser if you have multiple rooms to address.

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You can get 9k and 12k in 115v. Airtemp.com

https://mrcooldiy.com/

Never used it but it is DIY friendly.

I'll take some measurements tonight but this might be perfect if I end up needing the Mini Split. Another $5k isn't really what I want to do but I might have to. Summer is almost over so I can probably get away with nothing for another 8 months.  

I just wonder if I can get away with adding that 4" line plus adding a dampener. I am concerned about janking up my HVAC system though. I added a pusher fan to the duct going to the hot room which is helping get more volume in the room when the system is running but my wife doesn't run the system enough for it to make an impact.

A ducted mini split using existing duct would be great and my preferred option. It’s the cleanest option.

If you go mini split. Stay away from the DIY units. They look like shit installed and aren’t properly vacuumed. Buy a low side gauge, a flare kit and a HF vac pump.   The money you save buying s traditional line set pays for the tools.

I like pioneer units. I’ve put in 4 of them and  have no regrets at all.

Thanks for the input. If I go ducted can I   Y   split into the existing ducts so the main floors AC will still push air into the space when it runs or do I need to fully disconnect the duct from the main system and only use the mini split? Would there be pressure issues, unforeseen consequences of doing this, janky red neck hvac system?

I originally wanted to go this route and gett a duel zone system to also splice into my Master Bedroom. Its also hotter than the rest of the house. I also don't want to break my primary systems though.

Some newer dealers, you premeasure your line runs and they send you pre-charges lines.  No need for HVAC skills or dealing with refrigerant.