I have a nice Boiler system heating glycol to two fan coils running high velocity air in a 3500 sq ft house with a 1500 sq ft basement.

I want to add air-conditioning. Two problems i am worried about. 1. if I put a second exchanger in each of the two plenums, I will affect the air velocity in those 3 inch flex lines down stream 2. the compressed cooled air will give up it's moisture in the 3 inch lines as it's pressure drops at the exit vents leading to mould potential.

Question:: is it feasible to create tee's in the supply and return glycol lines from the boiler to the fan coils and a boiler bypass such that when there is a call for cooling, the boiler goes into closed loop and the air conditioner cools the glycol circuit effectively turning the heat exchangers into the cooling radiators? Doing this would allow me to avoid putting a second exchanger in the two plenums and make for a slick solution to boot. (doesn't solve the humidity question though - if that is actually a problem)