Mismatched AC unit

[calerac]

In what situations would a 3 ton airhandler be installed with a 2.5 compressor? I had started a thread yesterday regarding my parents AC which is set up this way but was wondering if anyone can clarify why a contractor would have done this and if it safe or efficient or technically correct to do.

Also, can it be done the other way? ie.) 2.5 air handler with a 3 ton compressor?

Thanks again for all the advice. This is a great site.

[bornriding]

Using a larger ( by 1/2 ton ) size coil allows manufacturers to rate their equipment at a higher Seer rating. It is done every day.
They take a 10 Seer system - add a TXV controlling device to make it a 12 Seer and then install larger coil to make it a 13 Seer. ( or something like that ).

And no, you don't want to put a 3 ton condenser on a 2.5 ton indoor coil - too much refrigerant for indoor coil.

Hope this helps,
Richard

[udarrell]

Regarding:
And no, you don't want to put a 3 ton condenser on a 2.5 ton indoor coil - too much refrigerant for indoor coil.
Hope this helps, Richard
========================
If you wanted to run a very cold evaporator coil to wring moisture out of the air, you might be able to get away with using a 3-ton condenser on a 2.5-ton evaporator with the following caveats.

You would need a Thermostatic Expansion Valve (TXV) refrigerant metering device to keep the coil sufficiently enough above freezing, along with an optimal heat-load on the evaporator coil to keep the sensible and latent Btu/hr as high as possible.

The amount of refrigerant would be determined by the subcooling along with a counter check of the superheat.

The old condensing units had oversized compressors as compared to the condenser's Btuh rating. A 2.5-ton compressor would have, if I remember correctly, around a 34000-Btuh or larger capacity compressor. Two ton condensers had 27000 to 28000-Btuh compressors.

Of course, the condenser coils would be considerably over-capacity, which probably wouldn't be an operating functional problem.

When the compressor is over capacity, too low a suction-pressure and cooling coil Temperature is the critical factor to be concerned about.

I would like to hear from someone that has tried this combination.

I once replace a 3-ton condenser that was on a 2-ton evaporator. The cooling coil and blower wheel were also loaded with lint, etc., which caused the compressor to fail.
That was back when three-ton condensers had oversized compressors!

Whoever mismatched that application thinking that that would improve cooling, was very ignorant!

I cleaned the coil and blower wheel and installed a new two-ton condenser; --it cooled the home effectively and very efficiently with low utility bills.

[Carnak]

The coil and condenser are going to match up where they balance out, I do not see a TX valve as keeping the suction pressure up to prevent freezing.

[air2spare]

perform a heat load and size the equipment accordingly. Anything else is trouble

[drcustom]

Originally posted by calerac
In what situations would a 3 ton airhandler be installed with a 2.5 compressor? I had started a thread yesterday regarding my parents AC which is set up this way but was wondering if anyone can clarify why a contractor would have done this and if it safe or efficient or technically correct to do.

Also, can it be done the other way? ie.) 2.5 air handler with a 3 ton compressor?

Thanks again for all the advice. This is a great site.
Sometimes manufacturer's, like Rheem/Ruud, rate their residential air handlers in whole number intervals, though each can be tech-modified between two speeds, depending on the heat load/condensing unit. In your case, it might already be set to low speed, or 1000 CFM (high speed option operates at 1200 CFM). If this is the case, then the larger evap. coil supposedly gives you a slight SEER boost, like bornriding said. Are there any complaints about its operation??

[RoBoTeq]

This subject keeps coming up and I will again repeat; INDOOR COILS DO NOT DETERMINE CAPACITY!

There are many instances when a "nominally rated" coil can be over or "UNDER" the capacity of the outdoor unit. If you only use what is rated by the manufacturer of the outdoor unit, you will have a tested and rated system, regardless of the size designation of the indoor coil.

[mark beiser]

There are a lot cases where using an indoor coil that has a model number indicating it is the the same "size" as the outdoor unit would actually result in an unaproved mismatched system...