Ductless mini-split HVAC systems are simple, inexpensive, and energy efficient. They can be installed by a do-it-yourselfer with no special equipment other than a vacuum pump, which can be rented. Because they run refrigerant to each of the indoor units, they do not require ducts. This reduces the accumulation of dust and mold in the HVAC system. They use heat pump technology to both heat and cool the house. They consist of an outdoor unit (the heat pump with a fan) and one or more indoor units. Each indoor unit is connected to the outdoor unit with a set of two refrigerant lines (high and low pressure) and an electrical cable. These indoor units also have a drainpipe to discharge condensate when the unit is in cooling mode. The outdoor unit must be wired into a small subpanel (a miniature breaker box) to meet code requirements. The system you purchase will contain the specifications for the subpanel.
I would recommend a system like this, which provides a cassette for every bedroom and one for the main living area. Install the indoor cassette units prior to installing drywall and run the refrigerant and electrical lines to the area where the outdoor unit will be installed. The refrigerant lines are made of soft copper, so they should be bent carefully and gradually to avoid kinking them. Strap the lines to the side of one of the rafters with plastic pipe strap clamps, midway between the top and the bottom of the rafter. Install ¾” PVC drains for each indoor unit, running straight outside through the rafter bay. Make sure that the slope for the drain is consistent, so that it does not sag anywhere. Use a 45 elbow to turn the drain so that it discharges straight down from the rafter bay 12 inches from the outside wall of the house.
Install the outdoor unit on a small concrete pad poured for a foundation. The pad should be at least two inches wider than the footprint of the unit, and at least 3.5 inches thick. Ideally, it should be reinforced with welded wire mesh of #3 rebar to prevent or minimize cracking over time. The outdoor unit can be attached to the concrete pad with galvanized bolts placed in the wet concrete, or anchor bolts drilled into the concrete after it has cured. Follow the manufacturer’s instructions to connect the wiring and refrigerant tubes between the outdoor and indoor units, as well as to attach the outdoor unit to a dedicated subpanel on the wall 48” above the ground near the outdoor unit.
After you have installed the outdoor unit and all the indoor units, ensure all refrigerant lines are tight. Then pull a vacuum on the lines using a vacuum pump to evacuate the lines and ensure there are no leaks. Release the refrigerant from the outdoor unit according to the manufacturer’s installation instructions. If possible, ensure that the system is functioning properly before installing the drywall.
Install bathroom vents near the center of the ceiling in both bathrooms, following the manufacturer’s instructions for wiring and venting. The vent ducts should be routed straight out to the soffit board between the rafters, with an outside vent cut into the soffit board for each bathroom vent. A similar vent should be installed for the drier. 4-inch flexible aluminum ducting is typically used for both. Check the manufacturer’s recommendations on the dryer and the bathroom vents before purchasing.