I'm a little over my head here but let me clarify - there are 2 kinds of "hydronic" heat. The primary heat is hydronic radiant in-floor Viega heat panels. The hydronic coil in the air handler was recommended by the HVAC contractor because he doubted that the radiant heat would be sufficient. However, we don't really know if he is correct since he is only familiar with a different brand of radiant system and might just have been trying to get us to buy more from him. But we did it because he alarmed us. If we ran ducts from the air handler into the bonus room for heat and a/c we would want to seal up the supply ducts in the rest of that zone in the winter because the radiant heat thermostat would be affected by that heat in those spaces. Please tell me more about your thoughts about time to raise the temperature with radiant heat. We've been told to keep the heat at a steady temperature and not touch it. Since this will be a second home for at least a year, and we won't be there full time, we've been told to leave it at 60 degrees all the time. When we are there for more than a day or two we can raise it to a more comfortable setting (65-68) but it will take a few days to get it there. That's what we were told. Is that not correct? That was another reason for the heating coil in the air handler - to get the living spaces warmer quickly until the floor could catch up. As far as installing radiant heat in the floor of the bonus room on a separate zone, it would require different piping, manifolds etc. Something I know nothing about. The system was not designed for expansion in separate zones (as in a sub-zone off the second floor) although I suspect it could be done by someone a little more sophisticated than our plumber. What I was told is that it would cost a few thousand more than including it as part of the current second floor zone NOW. If we don't do that we can add radiant heat at a later time but will have to pump it up as a separate zone from the basement at considerably greater cost. Right now the bonus room floor is unfinished and is directly connected to the second floor. It would be easy to connect the tubing to what's already in the hallway outside it. It's the issue of heating a less-used space unnecessarily that makes this a difficult decision for me. Thanks again for your views.

Maybe you have already done this, but rereading the comments, it jumps right out at me now. I am combining mike-home's suggestion of window units with the need to determine the capacity that you need. Sometimes load estimates are very good and sometimes they are way off. Sometimes they are way off because it is impossible to get a good number for how good insulation and are sealing are, or some other parameter. I suggest that, if you can pull it off, run with a couple of window units for a time with heavy demand. Get data for a few weeks, at least. Pay attention to, or somehow meter, how much the units are running. That way you can have the best possible estimate given the capacity of the window units is accurate.

Do you have a basement so condensate can run down? I have no experience with pumps to push it up, but that should work. If possible, I'd install the drains through a closet or something by placing them near the closets. That way you can get to them in case there is a clog or leak. You might consider covering several areas with the small, flat AHUs that connect to short ducts. I do have some advice for how to combine indoor and outdoor parts. Smaller systems are convenient in the case of a power outage because you can run parts of it with a smaller backup genset. In addition to that, group indoor units that will be running at times with low demand, low output, together. That way the outdoor unit driving them can modulate better rather than toggling on and off. i don't know why refrigerant lines in the attic would be a problem, but keep them as short as practical.

If the rest of the house has forced air, why wouldn't you extend that to the bonus room? Didn't specify if attached/detached... If detached, depends how you heat the garage. If in-floor heat, obviously extend that upstairs. If modine/infrared, I'd run the gas line upstairs to a gas fireplace. If its a portable shop heater or something, electric fireplaces today throw out a decent amount of heat. You can do mantle units to slide around the floor. Heatilator/Simplifire make wall mounted units you can either recess in or hang like a TV, and they put out decent heat. Otherwise I have a Dyson hot fan that oscillates and heats up decent, but depends how much juice you'll have to this garage