Closing vents may help. Try closing the vents in the three unused bedrooms and see what happens.

Have you thought about buying a gate to block your bedroom door? This would allow you to keep your bedroom door open unless your cats are very good climbers.

Install a louvered vent through door.

Unfortunately, our cats are champion climbers. Perhaps the louvered vent would be a good solution. I've been reading online a bit about closing vents and apparently it can strain the system so is not recommended.

If you are having trouble with a dual zone HVAC system, in one particular area of the home this points to design problem. There is little you can do yourself other than what you already know to be true. (leave the doors open).

Design problems can only be solved by redesigning. In your situation, this would be best suited by creating a third zone, that zone being the master suite.

Closing vents at the register in rooms not used tends to create system turbulence with in the duct work of your HVAC system. It is unlikely to do much good and may make matters worse.

You could move the thermostat of this zone to the master suite, but this would likely make the other rooms cooler. If these rooms are ever occupied at some point, the occupants may be logging complaints on a daily basis.

This is why design problems should be fixed, not just throw some whitewash on them. But when it comes to the residential market it's all about making things pretty aesthetically. Forget about that boring box that sits outside slaving away in the heat of the summer. Poor little guy. He's screaming and no one listens to him.

Austin, thank you for your comments. I did a little investigation last night to see what was going on vent-wise, and I'm even more confused than before! The ductwork for the second floor is in the attic, along with what I assume is a blower (it's a big metal thing attached to the ducts). There is only one return vent on the floor, located in the hallway right above the thermostat (and right below the blower). There are no supply vents in the hallway.

Each of the rooms on the second floor has two supply vents in the ceiling, but one of the vents in the master bedroom is a "vent to nowhere" - there is just an exposed elbow joint sticking out in the attic where it was obviously supposed to attach to the duct work. It looks to me as though they placed the vents in the ceilings before they extended the ductwork along the perimeter of the attic, and simply decided not to extend it far enough to encompass that vent (it would mean losing a good bit of useful finished attic space). Given that the master only has one working supply vent whereas the others have two, I suppose it's no wonder that it tends to be warmer than the other rooms. Could adding a vent (with or without extending the ductwork) improve this issue?

Furthermore, each of the bathrooms on the second floor (one of which is in the master bedroom), has a supply vent in the wall near the floor, but I don't know where this air is coming from. I assume it is from the first floor zone, but all of the supply vents on the first floor are in the floor, not the ceiling - would it be normal to have two sets of ducts?

"one of the vents in the master bedroom is a "vent to nowhere" - there is just an exposed elbow joint sticking out in the attic where it was obviously supposed to attach to the duct work. "

I recommend stuffing a beach towel in this one vent instead of closeing those in other rooms. I have been a student of mechanical systems for 40 plus years. During that period I've learned it's beneficial to understand individuals in charge of operating those systems. This cause's me to ask,what you plan on doing with information gathered here at houzz. Are you planning to work on the system,hire someone to work on it or do you have another approach?

Klem, I am obviously no expert in these matters so if we decide to have major work done, we will be hiring a pro! I am just hoping to get a better understanding of how the system works so that I am not completely at the mercy of whoever I hire in terms of deciding what to do. Unfortunately, we have encountered quite a few service people who seem to want to take advantage of us as new homeowners (notably, the contractor who told us we needed to replace our stucco with aluminum siding immediately and the chimney guy who tried to sell us a liner when we just wanted some repointing done), so it's nice to have at least a basic understanding of how things work before getting estimates.

Well the one vent that is not attached is a big hole to your attic, allowing heat from the attic into the room. So thats why he says plug it. Otherwise, get the duct operational (connect to supply plenum) and that would increase incoming air to the room which may solve your problem other than the closed door and no return which would require the louvered door since your return is in the hallway and is meant to cover that floor. Would be better if return branched and terminated in each room but we dont know if that is feasible or not.

You are doing yourself a favor by learning. Enough learning and you can DIY. Not everything, but most things. Attaching that duct elbow to the supply plenum is DIY material, cut a hole in plenum, install take off, use your flavor of duct, insulate with duct sock or equivalent, seal with mastic or foil tape. About the only thing you can do wrong is cut the hole at the end of the plenum instead of the side. Here are some other ductwork basics (what the parts are and general design), does not mention sealing and insulation or airflow calculations.

Can you post some photos of the items/areas of concern, including the indoor HVAC equipment (furnaces, blowers, etc.) and especially the "vent to nowhere!" I think photos are always a good thing.

I live in Florida so cooling is definitely a big concern, especially at night. Of course, we have the A/C on all the time and it cycles off and on. In addition, we have a Casablanca fan installed in each room. At night we turn it to the lowest setting and it definitely works to cool the room.

Also, if the attic is finished, I assume it's also conditioned? If so, stuffing something in the "vent to nowhere" (I love that) won't really make a difference. As for the lack of return air vent in the bedroom and the temp problems it may/may not be causing, I think the easiest thing *might* be to install a jump duct over the bedroom door.

Sammy, the attic is not conditioned. The previous owner said that since the ducts are running around the perimeter of the attic, it would be easy to just cut vents in the existing ductwork to heat/cool the attic. However, I have a feeling it would send our energy bills through the roof (no pun intended!), not to mention that I'm sure it would negatively affect the cooling on the second floor. The attic is a bonus room so we don't HAVE to be up there daily - we just avoid it on extremely hot or cold days. Or I use it as a built-in bikram yoga studio. I have closed the vent to nowhere so hopefully the cool air is no longer escaping into the attic, but I can stuff it with something as well if that would help further.

As for a jump duct, I just googled it and it looks as though it would at least allow air transfer while the door is shut. Would it do a substantially better job than a louvered vent through the door? That seems like something I could easily do myself, while the jump duct would require a professional.

ETA: Just saw your post regarding photos - I'm not at home currently but I can take some and post when I get home later.

All things considered,suggesting a "jump duct over the bedroom door" is invititation for disaster. The space above the door is occupied by very important structural framing members that should not be disturbed. I suggested through the door for good reasons. #1 because op is an admitted novice and worried about hireing green horns calling themselves pros. Both parties risk cutting into structrial members,elcectrical devics,plumbing,alarm/fire protection or something ,,,,,,,,,anywhere else except in the door. #2 because it's the single most inexpensive to put back good as new in the event of mistake or if one change's their mind later on. #3 Unlike cutting holes in walls,ceiling or floor,nothing you do can render the place dangerous or uninhabitable. Let's try and keep it simple. BTW, "student for 40 plus years"doesn't mean it has took that long to learn the trade,more like still expanding on knowledge,how better to apply it.

jump ducts go in the ceiling, jumping from bedroom to hallway so air can reach main return in hallway.

transfer grills go above doors, in doors or in walls as long as they are located

within the same stud bay.

if your attic above bonus room is hard to get to then adding a jump duct would be problematic.

so you might focus on transfer grills. I find offsetting the two grills (one for

each side of the wall) minimizes noise & light transfer. I've installed transfer grills over doors or in the walls...again staying within same stud bay. one is higher, the other lower.

definitely attach duct to supply box. but no one can answer where the

air for the bathrooms comes from without seeing the set up. sometimes

if it is via internet/pics you still won't get necessary information. a visual

walk through by a pro would be in your favor.

best of luck.

Consider checking and adding insulation and making sure that the bedroom window sealing is up to snuff.