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Heating & Air Conditioning

2 stage A/C questions

12 years ago

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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Comments (5)

attofarad
Original Author
12 years ago
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?
veesubotee
12 years ago
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.

V
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Those prices are high. When you pay that much for HVAC, it really makes sense to superinsulate and then just use electric baseboards. Even 1/2 of $17k would get the structure to a level where you could fully heat with electric baseboards for what you want for $100 a year. If you need a/c, do a minisplit for $4000 installed and that $100 goes to $30. By superinsulate - sprayfoam is not it. In your climate, you want R40 walls which is achieved by 12 inch thick walls with cellulose/fiberglass or 3-4 inches of rigid foam on top of 2x4 wall. Sprayfoam has its place but filling a cavity with it is not what it is best for. Are you really looking at R20 walls? Is that even code? Or is there rigid foam that you have also? There are no drafts in a modern wall - fiberglass or not. $8k for sprayfoam is a waste. If it is 2 inches of rigid foam on the outside - then it is not. My walls in NC are zip R3 over fiberglass bibs in 2x6. The small amount of rigid foam just helps with the thermal bridging of the framing. The bibs fill voids much better than batts. It is blown in. We have foam in small areas that bibs would not work with. I think $10k did our 3800 sqft house (not counting the zip R which was $3k upcharge over zip). I would think you would want R-80 in the roof which is probably best achieved by rigid foam over the sheathing given the rafter space needed. Or you could just do code minimum and minisplit with electric baseboard back up it since it isn't full time use.
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mike_home
12 years ago
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.
veesubotee
12 years ago
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?

V
mike_home
12 years ago
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.

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