2 stage A/C questions

attofaradSeptember 29, 2011

My most likely installer has quoted a Carrier system with options for either a 5 ton 24ABB compressor or a 5 ton 2-stage 24ANB760A003 compressor (for $950 more). In either case, I would be combining with a 120kBTU Carrier Infinity ICS 58MVC120F120, or maybe the 100kBTU version, Infinity controllers, etc.

How do 2-stage compressors work? Are they sorta like the furnaces, where they run a lower amps, BTUs and lower blower speed? I can't find specs in the A/C for output on low on the Carrier site, like that I can find for the furnace, or any description of system operation.

The system will be zoned, so I'm hoping to be able to cool only 1-2 zones while running on low A/C, and have the blower running at low speed to match.

I've lived in this house 22 years with no A/C and no huge need, but my new wife runs hot, and likes to work out. Since I'm doing a total furnace/duct tear out and relocate, it is now or never. Also, an extra 0.5% in overall house price to have A/C will maybe make it easier to sell, and cost be recouped.

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Okay, I found the info in the installation manual. The 2-stage scroll compressor has solenoid control of valves. When the valves are open, some of the gas which would be compressed is dumped from the first compression pocket back to the low side of the compressor, thereby decreasing the displacement and capacity.

Which brings me to a related question. When the A/C compressor is on Low, the expansion coil is effectively quite oversized. The Infinity controller should slow down the blower to match the load. Generally speaking, what is the disadvantage of having a coil larger than necessary?

    Bookmark   September 30, 2011 at 3:09PM
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An oversized evap coil (such as 3 ton with a 2.5 ton condenser) will have a lower temperature than the smaller coil. This favors sensible cooling (temperature) at the expense of latent (humidity) removal.

In your case, when the staging is low, the airflow should be adjusted to provide the same (approx) sensible heat ratio (in effect, adjusting the coil temperature).

Intentionally using an oversized coil to increase SEER without considering the sensible and latent cooling needs (by way of a heat calc) seems amateurish. Consult the mfg's product specifications.


    Bookmark   October 1, 2011 at 9:16AM
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It is very common for Carrier legal combinations to allow an oversized coil. I have this in my house. I have a 2 ton condenser with a 3 ton coil, and a 3 ton condenser with a 4 ton coil.

If you choose a Carrier Infinity 2-stage condenser you will still have good humidity control with an oversized coil.

    Bookmark   October 1, 2011 at 10:59AM
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Mike, do you know if your blower speed is set up for 350 CFM per ton, rather than 400? I had an Infinity quote which was specified with an oversized coil and contractor noted that speed was 350/ton.

It sounded counterproductive to me. Can any PROs respond?


    Bookmark   October 1, 2011 at 11:59AM
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According to the Carrier product data sheet for my condenser (24ana7) the blower speed in the high stage is 350CFM per ton in the high stage, and about 235CFM for the low stage. The bulk of the dehumifying will be done in the low stage.

The Infinity furnace or air handler has a variable speed electronically controlled motor. I don't think the installer sets any speed. The controller sets the air flow based on the equipment connected to the system. In addition the fan will increase the RPMs in order to reach the correct CFMs for high or low stage.

    Bookmark   October 3, 2011 at 4:49PM
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