So, you're dealing with a net 2016 sq. ft. including the finished basement. I would not go with 2 systems for this size, but I would go with one properly-sized unit (possibly with zoning). Proper sizing and ductwork design is the key here--sounds like the other unit is doing well, so I assume you'll want to the other to do the same. Find a good contractor to come in and assess the situation. One system for a master bedroom at 672 sq. ft. sounds iffy. A load calculation needs to be done to determine the sizing of the systems (will take into consideration insulation, windows, wall orientation, etc.).

Ryan -- What exactly are the insulation/windows/wall orientation considerations?

The plans are set up so we get bids on doing the addition in stone and on doing it with wood siding (differences of opinion on which is most cost efficient in the long run). Siding seems to make a difference in how the walls are insulated although I don't quite understand how.

The addition faces south, with most windows on that side and some on the east and west faces. The north face is where the addition attaches to the current house so no windows there.

I hope you are right about going with one unit and zones because that appears to cost considerably less. We also realized how important sizing and ductwork design are when we visited some Parade homes recently and found large temperature differences in different parts of brand-new houses. How is a consumer supposed to be able tell ahead of time if the system will work comfortably if professional builders have a hard time getting it right?

I assume when Ryan says go with one system he means one system for the new addition, and if so I agree (except for the sunroom -- see below). I would not disturb your existing system. I can't figure out why your architect is suggesting that you put in separate systems for the master bedroom and for the kitchen and basement. It might be worthwhile to ask, because maybe there's some issue there we can't "see" from this angle.

Personally, I am not an advocate of zoned forced-air systems. I think they tend to be more trouble than they're worth. In 2,000 or so square feet, I really don't think you need zoning if the air handler is placed well and, as Ryan said, if ductwork is designed properly.

If it were me, I'd want separate heating and cooling for the sunroom because its construction and heating/cooling loads are going to be quite different from those of the rest of the house, and there may be significant periods of time when you choose not to heat or cool it. It's only 168 square feet, so you could use electric in-floor radiant, if you wanted, but the time lag from turning those systems on to providing warmth can be long. That's no problem as long as you want to keep it heated 24/7. In a room like this I'd prefer not always to have to heat it, and instead to be able to turn on some heat and feel warm in a few minutes, so I'd want something like a fan-forced electric wall heater, maybe, or a unitary air conditioner with electric heat strips.

It makes sense that if the current system works great for a similar sized structure, then duplicating it for the addition may work best, even without zoning. One of the major reasons we are doing this addition is because we currently live in every inch of the house as it is and are sure we would use every inch of the new space regularly.

The sunroom is meant to be the breakfast/informal dining area. We have a large lot, have large trees in the yard and face an arboretum (only have neighbors on either side and then not very close), so it is quite a bit like living in the country, not the city or the suburbs, with turkeys and deer in the yard regularly. I really expect we will use this space every day of the year and would like it to be comfortable all the time.

Does this mean we need a separate A/C condenser for just that room? If so, that seems like a lot of money to cool one room. And does it make sense to put ducts to feed the room with cool air and heat but supplement with electric in-floor to improve heat comfort? Is the advantage of fan-forced electric wall heat that it works more quickly than in-floor? And why does everybody keep trying to talk to me about hydronic heat for this floor?

Your further explanation indicates my initial assumption about the sunroom space was incorrect. Yes, if you want to use it all the time, I would want it to be heated and cooled by your main system. A separate heat and a/c unit would probably not be extremely expensive (certainly under $1,000) if you did choose to go that way, but based on what you have said I don't think that's warranted.

After being all but abandoned in many parts of the country due to incompatibility with central air conditioning, hydronic heat is all the rage right now. People like the idea of a warm floor. But I think for a space this small, just using the electric in-floor system makes more sense if you want the in-floor heat. It's easier to control, warms up faster if you want to turn it on just when using the room, less expensive to put in, and the electrically-heated floor is going to feel just the same as a hydronically-heated floor.

By A/C unit, californiagirl, I think Ky114 just means a PTAC unit on the wall. Is it going to have doors? Might be okay to just leave it with the rest of the system given that it is shaded and is/will be used often. By insulation/windows/wall orientation/etc. I mean that all theses factors (e.g. insulation R-value, window type, wall direction [N, S, etc.], etc.) are considered when calculating the heat load and heat loss of a home (or new addition for your scenario). These calculated figures will tell you what size unit you need. When getting quotes, ask the contractor how he/she will calculate the required size. Watch out for answers like: by experience, square footage, similar homes, etc... The answer you're looking for is a load calculation, heat load, load analysis, Manual J, etc. (there are different terms for the calculation). Best of luck to you with the addition.

Great, great advice. Thank you very much, ryan and ky. Important things to look for in HVAC discussions and proposals.

Yes, the sunroom will have a door to the patio/yard, although the main patio is on the other side of the addition (west side, with gas grill). Does that make a difference to the question of whether to have a separate A/C?

Yes, Ryan is right, I was referring to a packaged unit - possibly a window-style unit with a heating coil, or an apartment/motel style unit that's a console on the floor.

I think to adequately answer your question about separate a/c and heat a person would probably have to take a look at the plans and make an assessment. Plus, if you aren't opposed to the idea of a separate unit such as what I described, it would give you a little more flexibility. If you were sitting in this room on a very sunny summer day, it could start to get warm even if designed with plenty of supply vents. Of course, you could close blinds, etc., so a lot just depends on what you want to be able to do.

One advantage of not connecting this to your main system is that in the summer if it was not overly hot, you might want to open the windows in this room and use it as sort of an extension of the adjoining patio.

So there's probably not a right or wrong answer, it just depends on what you want.

Sorry, by the room having a door I meant if the room would have a door to the main floor plan of the home or if it would be open to the main floor plan (i.e. isolated or not). I agree with what Ky114 said above; I think he pretty much nailed it. I'd see what those who visit your home think about your situation.

Now I understand about the door. Yes, there will a 36" wide single door on one wall and a double French door on the other, so there will be considerable air flow in the room. Does that make a difference?

I can't really tell for sure. I was considering the air exchange between the rooms and how it would impact the heat load of the rest of the living space, but I can't tell you how much it would. A neighbor of mine (Maryland area, with reasonable shading as well) has a similar setup with a dedicated small HVAC unit for the space (I think it's for both heating and cooling). Situation would be best addressed by an on-site pro who can do some calculations and assess the situation, in my opinion. Good luck with the new addition and hope all goes well!