13 SEER 410A units have larger coils to begin with (that is one way that they can be more efficient than 10 SEER units). The theory behind having a slightly larger indoor coil is that it can move more heat than one of the smaller nominal size (ie a 3.5 ton can move more heat due to its larger surface area than the 3 ton smaller sized coil under the same conditions). The extra heat removed and transmitted to the condensor is not so much extra that it will adversely affect it. The higher condensing pressure and concurrent higher amp draw on the compressor can be sustained by the outdoor unit without over loading it. In total the slightly larger indoor coil will allegedly make it more efficient.
Technically speaking there is truth in the theory; however, there are other issues to be considered:
1. A unit over sized can cool down a space too quickly. This results in short cycling of the unit - it turns on and off quickly and more times in a day. Similar is the idea of it being easier on a car if you run it longer times on the highway versus stop and go traffic.
2. A unit that starts and stops so quickly only cools the air - removes SENSIBLE heat. That is the heat that we can measure with a normal thermometer. There is also LATENT heat (hidden heat) that cannot be measured with a standard thermometer. That heat is related to the moisture in the air - humidity. An over sized unit will not remove the moisture from the air because it cools too fast. A unit running longer pulls the moisture (latent) load out of the air better as well as the heat (sensible) load.
3. Air filtration is better on a unit that runs longer than one that starts and stops to quickly.
Air conditioners need to be sized according to the heat loss/heat gain of the house to work correctly and give reliable/economical service over the expected service life. HVAC is all about balance (air in vs air out, heat in vs heat out, energy in vs work out). If you have two kids on the play ground on a seesaw and one is the indoor coil and the other is the outdoor coil and they are both matched up correctly - same size and big enough to ride the ride safely - then its a fun day. If one is the skinny kid and too small with the fat kid on the other side, it doesn't work too well.
Yes it may be hilarious to watch but it doesn't work well. Now similarly if this is your machine, the "doesn't work well" part of the last sentence translates into simple English like this:
1. Mismatched equipment is hard to charge correctly and is prone to breakage and failure. Remember balance, the mentality of a little is good so more is great will cost you money in service calls for repairs
2. You will not reach the SEER ratings advertised and lose money year after year to efficiency degradation through mismatched equipment.
3. You are buying comfort (not a machine) and a mismatched unit or improperly sized/installed unit simply will not provide that. The machine will not meet your expectations of service. What the machine will do is follow points #1, #2, #3 and repeat. You will get madder (#1), poorer (#2), and frustrated (#3).
4. If you do have a failure of some part of the unit and warranty work is necessary you run the risk of having a warranty claim denied by the distributor/manufacturer due to mismatched equipment. Your "Free" compressor change out will cost more to repair (now that you are paying the $1,000.00) than it will be if you purchased a new unit on your own (new units ARE NOT warranty options).
Get a ACCA heat load calculation done on your home, get a unit sized to meet that load (correct tonnage and Sensible Heat Ratio capacity), match the indoor and outdoor (no 8 cylinder indoor coils dropped into a Smart Car chassis), choose a manufacturer who's equipment is easy to work on (if its hard, the tech might get lazy and not do maintenance correctly or cut corners), choose a reputable contractor (no my Brother's Sister's Cousin by marriage and no Bubba Joe Butt Crack HVAC LLC), and pick the highest SEER rating you can afford.
Best of luck.
Last edited on Sat Sep 5th, 2009 02:29 am by dkpd1581
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Now let me get this straight: No Joe butt crack HVAC and cousins are recommended? 
