This is a long-term conceptual bit of research I am doing for when that day finally comes that we build an addition onto the house. When we build the addition, we will be removing an existing sunroom that faces east, adding a small walk-in pantry to the kitchen, adding a rear foyer/laundry, and adding a master suite.

The total net increase of square footage is somewhwere around 450-500 SF. We could connect this to our existing HVAC with no sizing problems, but I am trying to determine whether or not it is worth it to put the master suite on its own system. The reason I ask is because we currently run a window AC unit in our bedroom because we like it around 68 in the bedroom at night, and it is much more efficient to run AC in the bedroom only, instead of cooling the entire house.

Would it make sense to install a second central HVAC system for a ~450 SF suite? It would be a pretty small unit, so I can't imagine a ton of cost over and above labor.

If not, any other suggestions?

you could always add a split unit to the room. much more efficient then a window unit and much less cost vs. a whole other HVAC unit.

If the existing system is capable of handling the additional square footage (500 sq ft per ton) I would recommend adding a zone system. Zone systems are perfect for additions and rooms that don't always need the full capacity of the HVAC system.

Here is some info:
http://www.bayareaservice.com/Manuals/zone_design.pdf



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We sort of need the opposite - we want to run minimal or no HVAC in the rest of the house at night and just run HVAC in the master suite.

Split system might be a good option.

What he's suggesting with zoning should enable you to just run the A/C in just the living area, just the master, or both. Is that not what you're looking for, or maybe I'm misreading?

Well that would make more sense, but the next question would be can my existing system be set up for that? I just replaced my entire system last year.

Ryan- You have a great idea but you will have problems with it. Yes, if you have an independent system in your bedroom, you can turn off the unit to the rest of the house and save money.
Things to think about.
1. 500 sqft is 1 ton roughly (not knowing anything about your house). It is very tough to find a 1 ton “system.” They are out there but they are 1 phase voltage, not built well, and usually very expensive/ ton relative to normal sized units. The normal 3 phase residential units start at 3 tons (way to big). If you do a window unit, it will work but hard to get to 68. You could go with a Mitsubishi “ductless” model but it will be expensive.

2. If you do a zoning set up, you will add a duct to the existing system you have and install dampers on each supply line. You program a more sophisticated controller to turn off areas as not needed. This does not turn off (or down) the Condensing unit or evaporator and thus DOES not save money. If you want to save money with this you need temp sensors (like thermostats) in each room and then tell the system which ones are active at what time of day($$$$$).

So with all my years in the industry, run a duct to the room and be done with it. Hope I helped.