The U-factor of a 2x6/R19 wall is also about 0.07 BTU per square foot per degree F, so roughly speaking you have (3' x 80' ) + (2' x 60') = 360' of roughly U0.07 wall. At 70F indoors, 10F outdoors that's a 60F difference, so the wall losses are roughly:
U0.07 x 360' x 60F= 1512 BTU/hr.
There is also some losses out the slab (not much) but you also have the internal gains of the boiler & hot water heater standby losses to the room, the heating system plumbing losses to the basement, etc. which probably more than offset the slab losses when it's actually that cold out (higher duty-cycle on the heating system plumbing). The fact that the basement stays 61F all winter just on the standby losses means the mini-split won't have to supply very much heat to bring it up to 70F.
The Mitsubishi FH06NA is capable of a peak output of 8700 BTU/hr into a 70F room when it's +5F outside. Discount that by about 1/3 to account for defrost cycles, or even down to 1/2 that under saturated rime-icing fog conditions outside (a once in a century event at +10F in MA). So in that absolute worst case event you have more than 4000 BTU/hr of net capacity, and most +10F days it would be able to deliver over 6000BTU/hr net, after defrost. Even if you have another 1500 BTU/hr of load from air infiltration & window losses & below grade walls, and an actual design heat load of ~3000 BTU/hr, even the FH06 would still be on the order 2x oversized for the design load, and probably still has you covered down to -15F.
Heat gain through those walls on a rare 95F day into a 75F room is roughly ~500 BTU/hr (it's 1/3 the delta-T, 1/3 the heat transfer), but more than that is going to be taken up in by the cool slab in a 75F basement. The slab losses probably also more than cover heat gain from the lights, maybe the standby losses of the water heater too. You won't have the heating system running on that day, so no distribution loss heat to be rid of. Even if you're having a polka party with a dozen 200 lb Czechs dancing they're not putting out more than 500 BTU/hr each (figuring half are sitting & slurping beer at any one time). That's 6,000 BTU/hr of sweaty happy slavs heating up the room, and an FH06 still enough capacity to maintain 75F indoors. The thing can deliver 9000 BTU/hr of cooling at a 15F temperature difference, and probably more than 8000 BTU/hr at a 20F difference (I don't have the engineering manuals in front of me.) As long as the outside door isn't standing open all night it's enough air conditioner for a substantial basement party on a sweltering hot night.
So, HELL yes a half ton Mitsubishi is the way to go, or at least a far better choice than a 1 ton Fujitsu!
U0.07 x 360' x 60F= 1512 BTU/hr.
There is also some losses out the slab (not much) but you also have the internal gains of the boiler & hot water heater standby losses to the room, the heating system plumbing losses to the basement, etc. which probably more than offset the slab losses when it's actually that cold out (higher duty-cycle on the heating system plumbing). The fact that the basement stays 61F all winter just on the standby losses means the mini-split won't have to supply very much heat to bring it up to 70F.
The Mitsubishi FH06NA is capable of a peak output of 8700 BTU/hr into a 70F room when it's +5F outside. Discount that by about 1/3 to account for defrost cycles, or even down to 1/2 that under saturated rime-icing fog conditions outside (a once in a century event at +10F in MA). So in that absolute worst case event you have more than 4000 BTU/hr of net capacity, and most +10F days it would be able to deliver over 6000BTU/hr net, after defrost. Even if you have another 1500 BTU/hr of load from air infiltration & window losses & below grade walls, and an actual design heat load of ~3000 BTU/hr, even the FH06 would still be on the order 2x oversized for the design load, and probably still has you covered down to -15F.
Heat gain through those walls on a rare 95F day into a 75F room is roughly ~500 BTU/hr (it's 1/3 the delta-T, 1/3 the heat transfer), but more than that is going to be taken up in by the cool slab in a 75F basement. The slab losses probably also more than cover heat gain from the lights, maybe the standby losses of the water heater too. You won't have the heating system running on that day, so no distribution loss heat to be rid of. Even if you're having a polka party with a dozen 200 lb Czechs dancing they're not putting out more than 500 BTU/hr each (figuring half are sitting & slurping beer at any one time). That's 6,000 BTU/hr of sweaty happy slavs heating up the room, and an FH06 still enough capacity to maintain 75F indoors. The thing can deliver 9000 BTU/hr of cooling at a 15F temperature difference, and probably more than 8000 BTU/hr at a 20F difference (I don't have the engineering manuals in front of me.) As long as the outside door isn't standing open all night it's enough air conditioner for a substantial basement party on a sweltering hot night.
So, HELL yes a half ton Mitsubishi is the way to go, or at least a far better choice than a 1 ton Fujitsu!
