Replace some or all?

17 year old Heat Controller GLUA75-E4A paired with a 30 year old Lennox AC HS18-261. Both very reliable but aging. AC is low use (3-4 weeks x 30 years = not many real years) and more as humidity reducer, rather than a cooler, here in CT. I am 75 myself and thinking I best bite the bullet now and replace furnace at least, rather than be dealing with this in my 80’s. I have proposals for an American Standard Gold S9V2 Furnace and another outfit proposing a similar Trane-branded unit. Questions: Is it really best to change out the AC also? If so, the Lennox is showing 26,000 btu and the AS’s seem to be 23,600. (The current unit was purposely a bit undersized for optimizing moisture removal and it is amazing in that regard.) Maybe best to go up to 2.5 ton on the AS unit? (Don't want to hurt the moisture removal.) If so, which model as high SEER will never pay out for me. You are wondering if my duct work can handle probably, so I measured my returns as follows at the registers. Flex duct throughout with any closeable room containing dedicated return: 4 x 12’s – Four, 4 x 10 – One (metal), 19 x 10 for large open area, gabled, one 6” round (5% fresh air intake from outdoors). So 2.0 or 2.5 if it were your place? (No worries, a heat loss/gain calculation will be done before final unit choices.)

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