AC Questions

[confusing]

I am looking to install a new unit on my house. I live in AZ. I am trying to decide between a 4 ton and a 5 ton unit, most likely a Trane XL14c, XR14c, or XL16c. I do have what I believe is called a load calc, and it is recommending 4.2 tons. The sq. footage of my house is about 2100. What would be appropriate? I can provide any details that will help, including parts of the load calc sheet (if I could figure out which parts were important).

What are the pros and cons of a 1 or 2 stage compressor?

What are the pros and cons of variable airflow?

[confusing]

After a lot of research, I am thinking that variable airflow is a decent value for comfort if not energy savings. Not sold on the 2 stage compressor yet, would love some input there as I still don't fully understand.

The biggest thing I am concerned about is sizing. I would appreciate any feedback.

[dan sw fl]

http://www.eia.gov/electricity/month...m?t=epmt_5_6_a

- - $0.13/kw-hr

4,560 Cooling Degree Days ... presume 3,400 full operating hours
3.9 tons would be an average load for 'my typical 20 year old, 2,100 Sq foot residence'
with lightly tinted windows at 110'F

The extended system performance data ( de-rating) at > 95'F needs
to be reviewed in determining the system size.

Operating cost =~ $1,180 for 18 SEER
....................=~ $1,520 for 14 SEER

I would probably select a higher SEER unit in AZ
due to
both comfort and operating cost.
Payback might be in the 5 to 8 year range.

Two stage is probably not going to be as great an advantage in
a dry, hot climate versus humid, warm-hot climate.

Two-stage ( larger size - 5 ton) would be a benefit for times when > 10 people are
invited over for a party. However, that seems to be a rather weak
justification for selecting a larger size system in my opinion.
It is a benefit for temperatures > 100'F.

Variable speed would be useful to increase air flow setting.

Decrease the air flow resistance with a 5-inch deep filter set-up.

[confusing]

Thanks for your reply dan. A few points that came to mind:

- My house is closer to 40 years old
- I don't know what you mean by filter set-up, sorry
- I have dark screens on my windows and they are double-paned
- Are you suggesting my needs should be closer to a 4 ton than a 5 ton?
- If I go for a higher seer I pretty much am forced into a two-stage from what I can see
- I know we can't talk price, but it seems that Trane costs quite a bit more in quotes. Should I be considering different brands?

[udarrell]

You should be able to make 4-Ton work well enough to keep you comfortable enough in extreme conditions.

It is easier to properly size the duct system & Return-Air filter areas than with a 5-Ton system.

In your hot dry conditions, you will probably be using 450-CFM of air-flow per-ton, or 1800-CFM with a 4-Ton; it would be 2250-CFM with a 5-Ton, which is seldom design engineered to be achieved in residential systems.

The 4-Ton would normally be much easier to setup better to be much more efficient...that is extremely important to 'actual' operating costs ('actual' EER & SEER) in your climate with its extremely long & hot cooling seasons.