Specifically: would a 2-stage 3-ton condenser be more efficient than a 1-stage 2-ton condenser? Let's assume the same SEER rating for both. What about reliability? - I heard that 2-stage are a lot more complicated, does it mean more maintenance? Finally, is it worth the extra investment if we live in the NorthEast and will be using central air 2 months a year? Please advise.
2 stage equipment is for comfort, not economy. You probably won't make that extra cost back if used 2 months a year and your house leaks AT ALL. (high SEER units are useless in an improperly insulated house) Reliability is doubled in that half the system can be broke and it will still work. Heating in your area is more important I would think, spend that money on an ECM (variable speed) furnace with a modulating gas valve. If they are the same SEER then there is no operating cost difference (right guys?). A 3 ton will just run less often and cost more to do so than a 2 ton. If you want to save money, thermal scan your house and fix those issues. Unless your rich and a green freak, I see no benefit to dual stage systems less than ~10 tons. The second stage is for those months that your first stage can't handle. In your situation it sounds like it would either always run both or never need the second.
ECM's pay for themselves in about 3 years, MGV's 10 or so depending on usage. 18 SEER will never pay for it self unless your house has an astronomical R value.
Quote from HVAC TALK
2 Stage is for COMFORT.
Some people may save some money, from the longer run in first stage because they won't feel that sudden coolness from the short on time that a single stage has at the mild and mid range winter temps. Meaning they don't set the stat as high as they did with their old unit.(comparing 2 stage 90% to single stage 90%) So it looks like the low fire is saving them a lot of money by being more efficient, when its really because they have their stat set lower.
Same with 2 stage A/C. They may set their stat 1 or 2 degrees higher. So they save because they are more comfortable at a higher temp. Which of course, maintaining a higher temp, uses less electric.
Most 2 stage units going in are replacing older lower efficiency single stage units. Moving up from a 80% to a 90%, or 9 SEER to a 15 SEER, so they see the overall savings. And rave about 2 stage savings. Contractors then repeat how much a 2 stage system saved that person.
So its just repeating observations made, without a control, to check it to.
In your case, (not to take money from a contractor, but), I'd just set the lock out lower, and see how it does this winter.
Keep the money you were going to spend on a new system in the bank, and let it make some interest.
This guy "Beenthere" seams to really know his stuff.
I agree with the above comments partially, but there are more pluses to two stage units. Yes, they are for comfort but they also save money by the fact that a one stage unit always runs wide open, where as two stage units will run in single stage mode if there isn't a big enough temperature difference and will save on electricity. On extremely hot or cold days it has the second stage to keep up.
But if I needed a one stage 3 ton and I was thinking about a two stage I would still get a 3 ton and it would save you money over time. If you downsize the unit, then it's just gonna work harder to keep up and it would all be pointless. As an added bonus, most two stage units have dehumidification but as little as you stated you would be using (it two months a year) I agree with these other guys; it's not worth it, and the ECM motors are good in the fact that they ramp themselves up and down according to the CFM but they also have some disadvantages. Such as if the static pressure goes up via dirty filter and / or coil it will ramp itself up to its max rpm's then back itself down.
But after it does this three times, it will lock itself out and a tech will have to come out. Also, the modules on them have been prone to overheating and burning up transistors which is a problem they have about got worked out, unless you bought a Bryant, Carrier, Rheem, Goodman, York, or Lennox unit in 2012.
Which in that case: look out! LOL. But all in all a brilliant design; it uses almost half the watts and electricity as other motors and if a unit is purchased with such motor, by the tax act it's a $1500 break. They're also brushless type motors, which is really good seeing as most motor failures are from worn brushes. ECM's can also be hooked to either 120v or 240v so, all and all worth the money
I think a deeper look at 2-stage can reveal a LOT of 'it depends' on if it's really worth doing vs 1-stage. Remember they are doing to same work, low stage is just taking longer to do it! The variables are so complicated it's really impossible to know if you are going to get better results with 2-stage vs single stage. Partly because estimating your load and it's seasonal variance and selecting exactly the right sized unit is a bit of a crap shoot. If your 2-stage system happens to be a bit oversized, it may run in low so much that it would be similar to a smaller single stage unit. (and the opposite scenario). It's all so complicated and difficult just assuming 2-stage is better is a bit of an assumption. And I've found no easy way to temporarily force a 2-stage to run always on high if that's the response I want. So I will probably have it installed to permanently only run on high anyway. (I selected a 2-stage unit for warranty reasons) Because I prefer the comfort response with it on high. People say the longer cycle times of low-stage can extend the compressor life, maybe. But remember that longer cycles does mean more time spent above target temp on cool, for example..so it can be a comfort issue. And efficiency can be no better, or possibly even worse under some conditions. It depends. There are so many considerations can't list them all here.