Before making any changes,
run a fuel use based heat load calculation using the Biasi as the measuring instrument. In your location use either +2F (London's
99% design temperature) or +1F (Hamilton's design temp) for this calculation. If you need help running these numbers and doing the unit conversions we can do them here. If your boiler is like most in the US it's probably 3x oversized for the actual heating load.
For sanity checking and another datapoint (and to get cooling load numbers) run the load numbers with an online Manual-J tool, using the most aggressive possible assumptions about insulation values, air tightness of both the ducts and house etc..
Loadcalc is quick & easy to use (and has datasets for both London & Hamilton), but even with aggressive assumptions it tends to oversize, but by 15-35%, not 200-300%.
Also, the Biasi B10 series comes in a number of different sizes. The burner output is important when selecting water-to-air "hydro-air) air handlers. If the air handler coil is oversized for the burner the return water to the boiler can be cold enough to destroy the boiler in less than one heating season. If the air handler is undersized it can short-cycle the boiler into low efficiency.
Doing a self-designed hack of a fan coil or air handler isn't really recommended. Right sized air handlers with both a hydronic heating coil and a cooling coil just aren't all that expensive, and they're fairly flexible & easy to design with. But if the boiler & heating coil are 3x oversized for the heating load, the matching cooling coil may be even more grotesquely oversized for the load.
A boiler located in a garage on the opposite side of an insulated wall is an inefficient configuration. If only the exterior walls of the garage are insulated, and not insulated in the partition wall between the garage & house the boiler's standby and distribution losses (both pipes, and ducts, with the new system) are still inside and heating the house. If it's on the other side of an insulated wall the losses are truly lost.
Poorly designed ducted systems create room to room pressure differences that use "the great outdoors" as part of the return path, increasing the load, and wasting a lot of energy. This stuff has to be designed and implemented well to come close to the efficiency of heating with radiators, but there is always at least some duct losses, even when it's all inside the thermal & pressure boundary of the house.
In your area a cold climate ductless heat pump system would be cheaper to heat with than #2 oil, and would provide high efficiency cooling as well. If you run room by room load numbers using Loadcalc it's possible to figure out a ductless or mini-ducted heat pump approach too.
Full basement, crawlspace, or slab on grade?
Single story house or two (or three?)
