Have a 24x24 room with 8' flat ceilings. R13 wall insulation, 4 skylights, 1 Anderson French Door and 1 Anderson sliding door, 4 24x48 windows, 1 48x48 window. Windows and Sliding door on exterior walls facing South West/North East. South-East wall is brick faced and an interior wall to rest of house. North-West wall is interior wall to attached insulated garage (set back 10 feet from front of 24x24 room). Room was added in 1991 (good contractors used, decent insulation for the time (R13 walls, R19 floor, R38 ceiling). Vapor barrier installed under Vinyl Siding. Room has one interior door to laundry room that leads to kitchen. House located in Central MD.

Tile floor, drywall on all interior walls except brick wall. Southwest exterior is brick. No trees/shade. Black asphalt roof with ridge vent and soffit venting (and gable vent that was never sealed ....) Truss attic with 22"x22" access inside of a closet at edge of room.

Current system: Trane 1.5 ton heat pump mounted in attic. 5 supply registers in ceiling with 1 return in wall with filter. System had condensor replaced about 10 years ago (Trane XE1000 replaced original XE800). Evaporator coils were not replaced. Not sure about duct-work condition (was installed 20 years ago by one of the listed/recommended contractors here).

Currently HVAC company diagnosed the following:
Backup electric coils non-functional (need replacing) - stopped working last year
AH Fan relay stuck (needs replacing)

Parts are available to service this unit....

Challenge with replacing system (if we decide to do so):
Unit installed before drywall installed - will need to cut large hole in drywall to replace existing unit in attic. (Not a big deal)
Original contractor installed 1 supply to garage (Big no-no - must be removed ASAP no matter what .... CO poisioning issue ...)

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Options:
1) Repair unit (system cools marginally now, heated marginally last winter) - current system was fine for 18+ years .... HVAC contractor did not test condensor yet .... just diagnosed constant fan and no backup heat problem.
2) Replace unit with modern similar split system (would need to do some major drywall removal, not a huge issue) - manual-j would be requested (do they sell smaller than 1.5 ton??) Current HVAC contractor installed Carrier Performance system in main house, would inquire about Carrier Greenspeed for this room.
3) Abandon current system and install mini-split in room of proper size. Might desire having ceiling-mounted system verus wall mounted system due to low ceilings and little room above windows/doors for wall-mounted system. Current HVAC contractor does not have mini-split experience and expressed concerns about even heating/cooling from one unit mounted on wall (was not aware of ceiling mount units).

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My questions to the pros:
1) Seems a mini-split might be very effecient for a single open room of this size?
2) Should we put money into current old system?
3) Other ideas?