IMO, its economics, which benefit the builder, not the home owner...
But, considering the separation of the two buildings, maybe one unit would be not practical.
We need legislation to force, if necessary, the home builders to reveal all of a homes operating costs.
This should not be that difficult, not now, of course, but in the future..

I don't know about the builder's economics - sometimes they don't consider all the options.

I'll throw out a couple of things. I don't believe that 6 ton units are universally available or at least maybe not 9 years ago. Certainly 7 ton is not available for a reasonable price in residential construction. Cheap requires volume and 6 tons is not a volume item.

Zoning right costs money. Really really big money. You can do it on the cheap but you have real issues.

So you have a 6 ton unit and only the cottage needs heat. So the blower is blowing 6 tons worth of air flow into 3 small ducts and getting return from a single 20x20 grill. That will make one heck of a racket, there will be increased leakage from high pressures and the blower motor won't like it. A variable speed motor helps and so does a 2 stage system but only to a certain extent.

So you can add returns and more ducts but at some point, you have cosmetic issues. I imagine a 225 sq ft cottage with 8 registers and 2 large returns. Even with a staged system, you will have 3 tons going to 225 sq ft, which will short cycle like crazy.

So there are plenty of reasons not to do what you are considering. Are they insurmountable? No. But it is certainly not as simple as 3+3=6.....

And another thing - 58,000 btu is 5 tons. I would think in FL that cooling load would decide tonnage since it does in NC. Maybe you misunderstood something? Also 8 tons of cooling for that square footage, even in South FL, is a bit of overkill. I have 7 tons for 5400 sq ft and it is a bit overkill.

David Cary,

Thank you for your response. You were very exact, so as layman, I will try to be exact.
The attached room (15x15) has three vents: one standard (it looks to be 12 in.X12 in), and then two smaller 12 in.X6 in.(1 in the closet and 1 in the bathroom)
There is 1 return (12X24)
The other room connected to the 3 ton unit is an area 15X35 and that is the master bedroom and master bath. It has 3 vents: two 12in. X 12in. in the bedroom and a 12in. X 6 in. in the bathroom. There is one 18 X 24 return.

The rest of the house (1750 sq. ft with 12 foot ceilings) is on a 5 ton system.

Could or should this be all placed on a 5 ton?

Thanks

First - I am always impressed by the vent sizes in Florida - we never see such large registers.

Returns need to be about a sq ft per ton. So the return in the attached room is good for 2 tons. After that you are looking at duct size and length and type. Register size is an issue but there is not a good rule of thumb for that.

The first issue is that you are downsizing from 8 tons to 5 - right? That is a big change and you may not be able to cool or heat enough. To answer this requires a manual J calculation. Rule of thumb is 500 sq ft per ton but you should know that rule of thumb sizing is a terrible idea. But based on that, 5 tons sounds like almost enough. The real issue for a newer house is number/size of west facing windows and whether they are low-e. Code in Florida or common practice is usually pretty good in this regard, including large overhangs. Now - you mentioned 58,000 btus and if that is cooling load from a manual J, then I guess you are all right. If in North Florida, you might miss the extra 3 tons on a really cold night but the lower cost in the summer will balance that out.

Second, if your ducts aren't redone, you will have pressure problems. That includes more returns. But since you are "just" going for 5 tons into an area designed for three, it isn't as bad of an issue. You could get a staged unit and the "problem" is solved. Low stage is 50-65% of max - so about 3 tons. So basically if you get a 2 stage unit, your ducts issues would probably be minimal.

David Cary,

So by a staged unit you mean a 2 staged compressor?
It looks like we will be going with a Carrier Performance 25HPA6 Heat Pump with 2 staged scroll compressor, and 9 kw with staged heat strips, a Performance variable speed AH, 3 thermostats, and barametric dampers.

How does this sound?

Lenore