Experts weigh in: Compressor vs. coil size


Old 06-12-03, 11:10 PM
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Experts weigh in: Compressor vs. coil size

Experts weigh in:
I have a 5 ton AC that needs a new compressor.
I was thinking of replacing it with a 3 1/2 ton compressor, because the system is somewhat oversized for the load anyway.

Let me say in advance that I know how important it is to match components, but is it acceptable to a point (in theory) to run a smaller compressor on higher rated coils?

Any insight on this is appreciated!
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Old 06-13-03, 02:03 AM
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I'm no expert but before you do something like that you need to have a professional come and do a load calculation and see what size you really need. You CAN'T just guess and downsize it. Going from a 5 to a 3.5 ton is a pretty steep drop so you better make sure it's right before you just drop it. If you replace the outside unit do the indoor coil as well. With a 3.5 ton outdoor and a 5 ton indoor I would think humidity removal would suffer. I'm no expert but I've always been told the sizes should match and if not only a .5 ton difference. I've heard of people putting in an indoor coil that is a .5-1 ton smaller than the outside and supposedly improving humidity removal but I've heard others advise against that.

Match the sizes and get a load calculation done before you think about changing sizes. Also, you can't go by square footage in determining what size you need. Get someone out who will do a thorough load calculation. This will involve carefully measuring each and every room, window, door.etc. It will also consider R-value of your insulation in the ceiling and walls and what type of windows you have. Size of outside walls will also be considered and type of siding, slab or basement will be factored in as well. If someone comes and just looks at the house and guesses or just measures rooms and doesn't look at the other things I mentioned then call someone else.
Old 06-13-03, 04:52 AM
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indoor and outdoor sections

MUST be matched, and i agree have a load calculation done.
Old 06-13-03, 11:35 AM
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This is a freestanding unit for a shop space, so no load calculation would apply in this case.

Thanks for the responses!
Old 06-20-03, 11:54 PM
lynn comstock
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Matchup for coil and compressor.

Manufactures often list and approve coil matchups with coils that are bigger or smaller than the same-size matchup. In Houston, a smaller coil may be good for extra dehumidification. In Arizona, the oversized coil is good design because in Yuma or design humidity is ZERO.

The smaller compressor with the 5 ton condensor coil will make the 3.5 ton system more efficient (using less power). I have done this often in Arizona when the systems were oversized (as most of them are).

We don't know enough about your needs/load or climate to nail the answer. Your suggestion is possibly a good one. You can slow the fan down if you need to remove more humidity (by making the coil colder).
Old 06-21-03, 07:43 AM
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I've heard recently that some of the higher SEER 14 like units are using larger coils to both absorb and reject larger amounts of heat, as a result the compressors are built with a different compression ratio, and many are being returned as a bad compressor seeing the low side high and the high side low it APPEARS as though the valves are worn out, although they are OK. The refrigerant going through the coil would easily absorb the heat as long as it has a TXV with an external equalizer. The valve may hunt alot though. The problem is, the suction side would be too low and like a loss of refrigerant and freeze up the coil unless you added a Hot gas bypass valve, to maintain the pressure....It sounds over engineered now, to me !

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