Mini-split v Package Unit Humidity Control; Sizing and Configuration
I have a 1920’s house in Tallahassee, Florida – 2×4 frame construction with shiplap siding, no vapor barrier or insulation except in the attic and one wall that was installed later (originally a screened in porch), and a fair number of windows. I have sealed the floor and walls of the dirt crawlspace using heavy plastic with vapor-barrier tape at the seams, but there is still appreciable air flow between the foundation wall and the girders. The main floor is ~1,600 sq. ft.; approximately ~600 sq. ft. of the attic has been finished into a bedroom and bath. We tend to just keep windows open for four months out of the year in the spring and fall, heat occasionally over the 2-3 months of cool-ish weather we get, and generally keep the AC on between June and September unless it gets cooler than average The HVAC system (presently a 4-ton gas/AC package unit for the main floor, which I believe was grossly oversized) and a 1.5 ton heat pump for the upstairs) needs to be replaced. I am considering a Mistsubishi heat pump (MXZ series ) with a 2-ton ducted MVZ air handler for the main floor and a 1 or 1.5 ton wall unit upstairs.
Humidity has been an issue – the interior humidity generally almost matches the exterior when the AC is off, and given that we set the temperature to ~76-78, often stays high even when the AC is on. This sometimes leads to mildew growth on leather and cloth. My question is whether my newly proposed configuration would tend to provide better humidity control than the current package unit, and whether the Mitsubishi systems allow humidity control independent of temperature control. If anyone has input on the sizing and configuration, that would be appreciated, as well.
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