[LV2ndHome]

Actually we aren't far apart. We both agree that cooling capacity sizing has an affect on operating costs and that SEER has an affect on operating costs. The only difference is that I say they are completely different factors. With terms like "oversized SEER", you seem to be using cooling capacity ratings and SEER rating interchangeably, or at least imply that one is directly related to the other, and they are not. All other things being equal, you always want the highest SEER possible and it is impossible to have a SEER that is too high ("oversized"). You can oversize cooling capacity, but not SEER. But the reality is that all other things aren't equal... namely that the higher SEER units cost more money, so you have to crunch the numbers and figure out that the higher SEER model will save me $xx per month on avg but cost me $xxxx more to install, and decide if it's worth it in your situation.

Originally Posted by tiger08

You're right, if you replace a 2 ton old unit with a 2 ton new unit, it will be cheaper to run but who's to say the old unit wasn't oversized?

Here again instead of addressing the one variable at hand, you are shifting to discussing a completely different, unrelated variable. SEER rating is apples and cooling capacity sizing is oranges. Also note the new 2 ton unit is only cheaper to run than the old 2 ton unit if it has a higher SEER rating, which you didn't specify, but generally speaking the newer units typically have higher SEER ratings.

Originally Posted by tiger08

Now in my case, I went from old 4 ton to new 3 ton and I had a tremendous drop in power bill.

I don't doubt your savings at all, but there is nothing provided to prove those savings are solely attributed to changing to a smaller unit. I happen to know someone that went from either a 3 ton to a 4 ton or 4 ton to a 5 (I can't remember which... it's been awhile) and he reported substantial savings, and likewise this provides nothing to prove the saving came from going to higher capacity.

The problem here is you are attributing all of the savings to a decrease in cooling capacity. That may or may not be the case. You don't provide the SEER rating of each, but again typically newer units have a higher SEER rating. If that is the case in your situation, then certainly part of the savings is attributed to the higher SEER rating, and it's not right to attribute all of the savings to changing to a lower capacity. As another way to illustrate, supposed you went back with another 4 ton unit but with a higher SEER rating. You'd still be oversized, but you'd certainly still see operating savings over the old unit. Yes downsizing is likely contributing additional savings on top of that if you were previously oversized on the compressor unit, a smaller unit is adequate for your needs, and better matches the capacities of the other other components of your system, but that is completely independent of any savings related to a SEER rating change.

Let's go back to a light bulb analogy. Suppose you have a tiny closet that is lit with a 100w incandescent bulb. That is obviously oversized. You can change that to a 40w incandescent bulb for a significant operating savings (which is equivalent to going to a smaller cooling capacity ac unit) and still get adequate lighting for your needs. Alternatively, you can change the 100w incandescent to a 100w equivalent LED bulb. You still have the same oversized amount of light, but the LED light (due to it's higher SEER equivalency) will only use about 15w of power which also provides substantial savings (actually even more savings even though we are still just as oversized on light capacity as we previously were). And as yet another alternative, you can do both by changing to a 40w equivalent LED bulb, which will be about 7w. So you can change the size only, the efficiency only, or both, as they are completely independent and unrelated. But if you do both and drop your power from 100w down to 7w in this case, it's not completely true and fair to say "I dropped my light bill because I downsized that light", which is what I think you are doing. Yes, downsizing the light size was a contributing factor, but changing to a more energy efficient model contributed substantially to the savings as well.

Originally Posted by tiger08

Either way though, an oversized high seer unit will use more power than the same high seer unit that is sized proper. If you don't believe me, go to a pro site and ask a few questions.

I never said otherwise, and this seems to be a pretty simple, intuitively obvious statement, so maybe I'm missing your point. All you're saying is a unit with more cooling capacity, say a 3 ton unit vs a 2 ton unit, assuming all other things equal, will use more power while running. It's also obvious a 3 ton unit will have a higher initial cost, so the 2 ton will have an edge there as well provided it can handle the needs. But keep in mind that the rest of the story is there is a partial trade off in that the 3 ton, while consuming more electricity while running, doesn't have to run as long to provide the same amount of cooling. Thus the operating cost difference, assuming the other components of the system are properly sized to match, is not as great as you seem to be trying to imply. But yes, we do agree it is generally correct to use the smallest unit that will adequately handle the needs.

The common problems with sizing is not only calculating the over all capacity needed to adequately handle the area to be cooled, but also properly sizing the different ac components with each other. What often happens is an installer puts a big enough compressor unit outside, but then cuts corners inside and doesn't have enough vents, or too small of ducts, or too long of runs, or too many ducts on a trunk line, or an inadequately sized trunk line, and so on. Sometimes an owner will have a compressor unit go out, and decide that since it was barely cooling enough, I'm going to use the opportunity to upgrade from a 3 ton to a 4 ton unit, but the problem is they don't change other components of the system to match that new increased compressor unit capacity.

[MediocreButArrogant]

I could be a smartass and claim the higher SEER unit will typically have longer cycle times than the lower SEER unit, given equal cooling capacities. That is because the higher SEER unit will often have two-stage or variable-speed compressors. When run at lower speed, or in the partial-load configuration, the run time will be longer compared to a single-speed compressor. But the reason for this is the cooling capacity of the A/C is reduced when run at the lower speed or partial-load capacity.

I've always thought EER was a better measure than SEER for the Las Vegas climate (SEER varies way more than EER, since high SEER is obtained by running variable-speed compressors at their lower speed). EER is more a measure of how efficient the A/C is when running flat out (a single high outdoor temperature), and I thought that since it reflected the summer conditions in LV, when you're spending the most money on cooling, it was a better measure than the SEER test. We don't have high humidity in LV, so it isn't necessary to run for long period with low cooling capacity in order to remove humidty. Might be interesting for someone with a high SEER unit to weigh in on this and give their opinion on whether SEER or EER reflects their cooling bill better.

[SportyandMisty]

Originally Posted by tiger08

... Too many people here want to slam and go, get paid and move on to the next job.

I'm shocked. SHOCKED, I tell you. Round up the usual suspects.

[SportyandMisty]

Originally Posted by MediocreButArrogant

... I've always thought EER was a better measure than SEER for the Las Vegas climate ...

I'm not a mechanical engineer, but my reading of the literature concurs -- EER is asserted to be a better measure than SEER for a climate such as Las Vegas.

Would more knowledgeable people care to comment?