As mentioned previously my home has an oversized furnace that is quick cycling, loud, and uncomfortable. I had considered a few options to make it better including downsizing a new furnace, dual stage, condensing, venting to the side of the house (it is the flue in the wall between 2 bedroom that's loud), going mini split HVAC, etc. Today I was reading a few post on the internet and came across a home water heater (not a dedicated boiler) based hydronic system, using a double wall heat exchanger to keep the hydronic side water away from the potable water in the water heater. I also ran into the fan coil unit in commercial systems and was wondering: is it possible to use the same concept and put a radiator inside the duct, right after it come out of the furnace, heated by the water heater through the heat exchanger, to warm the air going through the duct? The disadvantage of this system would obviously be lower efficiency (assume water heater is only 60% efficient), but it would be quiet, and tune-able to adjust the vent temperature via water flow rate, to make the system always on. It would be very comfortable if I can tune the coolant temperature in the duct (radiator) to be slightly above my desired thermostat temperature, and let the circulation fan run continuously. It would put some extra load on the water heater but it should be fine for my moderate climate. Let's say I can gather all the parts (i.e. potable grade pump, valves, hydronic grade heat exchanger, copper flexible pipe, small radiator, long life coolant), and I can do the electrical signal wiring myself (splice the thermostat output into the pumps), what would be the reason not to do it? Also why aren't there residential size fan coil unit? My goal is to get a quiet (as in no forced air combustion sound) adjustable temperature system with minimal conversion risk and minimal hired labor. I can live with a slightly reduced water heater life, and the drop of heating efficiency from 80% to 60%. My other options of side venting + new furnace or mini split HVAC would likely cost me 5-10k, so it is really not worth the energy efficiency difference. Most importantly, I can remove this rig if I ever want to go back, and I can avoid ever replacing my furnace again.