re: New Construction HVAC question

Who is your HVAC contractor?

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Carrier infinity system. Look it up.

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10 years ago we installed 2, 21 seer Carrier infinity 3 tons units with spray foam insulation. They cool the house very well.

Your ceilings are tall but 1 ton per 500 sq/ft is a lot for a new house. That typically would be in line with an older home, did they run the specs from your plans through an estimating program to properly size the unit?

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That was another concern, although they have since quoted others...Goodman, Rheem, Tempstar, Amana.

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Stay away from Lennox, Goodman, Tempstar, and Mitsubishi minisplit.

Trane is the way to go. Carrier would be the 2nd best option regardless of what kinda “deal” you can get. There is a reason people get good “deals” on Lennox and Goodman units. Those Mitsubishi units are finicky and can be more trouble than they are worth and are notorious for Freon leaks.

Yep. Did that already.

I am a building contractor. I had a home I built for myself that had a similar set up. I had 3200 sqft and foam insulation. It had the Lennox 5 ton 16 seer var speed unit with a 2 stage condenser. The 1st story was on one zone and the 2nd on the other. I was there 5 years and the house cooled great. I cant remember what series it was, but it was the high series, maybe signature series? It was compatible with their best thermostat. It never broke, needed service, etc. I paid about the same as 2 of the lower seer/non efficient models. It was more about trusting my HVAC guy and he recommended it for the spray foam house being multi stage and var speed. He ran a J Load and I would def have that done.
If it were me, I'd probably go with 2 units if you have regular insulation set up and 1 unit that has all the bells and whistles if you go spray foam.
Only one complaint on the Lennox, it had a loud clunk when the condenser shut off. They said it is a relief on the fan motor. It was pretty loud and sounded funny. I've worked with Carrier, American Standard, Goodman, and Traine. I would use Lennox again.

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in South LA too large of a unit is worse than too small.

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Why is that? Just curious.

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An AC's job isn't really to lower air temperature; Its job is to remove humidity and ventilate the conditioned space. The heat contained in the water vapor in the air is typically the majority of the heat contained in the air, by a LARGE margin. The AC's job is to get rid of this heat by chilling the air so the water condenses out. To do this effectively, it needs to run for a time to allow adequate condensation to occur.

Oversized systems can turn on and very quickly chill the air, causing the thermostat to shut the system back off after only a few minutes. This is called "short cycling". When this happens, the AC does a great job of dehumidifying the air that actually passes through it, but does a bad job of dehumidifying, mixing, and circulating the bulk air in the conditioned space because it just doesn't run long enough to really move enough air through the AC to ensure that the air in the conditioned space is well mixed and dehumidified.

Oversized systems may result in a house with hot spots and cold spots because there isn't adequate mixing and circulation. Houses with grossly oversized AC systems can also exhibit problems normally associated with poor moisture control and poor ventilation, like mold and mildew, due to the inadequate dehumidification because of short cycling.

Thanks. Owner of hvac says he did manual J calculation, which I specifically asked him. I trust that he did. I'm leaning towards 2 separate systems.

Just as a Q on condenser units. They are a compressor, most are now scroll type, and a condensing coil. Basically a simple unit.

How can they all be so different in efficiency ?

Coil types, size, compressor efficiency ?

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An AC's job isn't really to lower air temperature; Its job is to remove humidity and ventilate the conditioned space. The heat contained in the water vapor in the air is typically the majority of the heat contained in the air, by a LARGE margin. The AC's job is to get rid of this heat by chilling the air so the water condenses out. To do this effectively, it needs to run for a time to allow adequate condensation to occur. Oversized systems will turn on and very quickly chill the air, causing the thermostat to shut the system back off. This is called "short cycling". When this happens, the AC does a great job of dehumidifying the air that actually passes through it, but does a bad job of dehumidifying, mixing, and circulating the bulk air in the conditioned space because it just doesn't run long enough to really move enough air through the AC to ensure that the air in the conditioned space is well mixed and dehumidified. Oversized systems may result in a house with hot spots and cold spots because there isn't adequate mixing and circulation. Houses with grossly oversized AC systems can also exhibit problems normally associated with poor moisture control and poor ventilation, like mold and mildew, due to the inadequate dehumidification because of short cycling.

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this shite right here is why we need a Housing/Real Estate Board.