To begin with, if you are reading this, you are not a regular person. Either you're a geek, or a HVAC specialist, or recently had, are having or about to have a problem with your HVAC unit. Other people are treating HVAC units as appliances - and I bet that they pay much more attention when they select a refrigerator or even a kitchen faucet than when they select a HVAC unit. If they even acknowledge its existence, that is - usually they blame all problems on the thermostat.
That's too bad, because a proper selection may make your life a bless, and improper one will make it a misery.
HVACs are notoriously difficult to find information about. And, they're complicated (when your HVAC contractor tells you it's rocket science, you better believe it). So complicated, not all HVAC contractors comprehend the complexity. Just ask them what superheat and subcooling is when they are charging your system - just make sure you are able to comprehend the explanations, and keep your bs detector handy. By the way, don't try to fish the answer out of Wikipedia - it doesn't have one for the HVAC subject area, at least as of moment of writing.
To add insult to injury, HVAC system is not even the place you should start at. Heat loss and gain calculation is. But, it is usually very difficult to get. That is, unless you are willing to apply some elbow grease. Don't forget to investigate the sensible vs. latent capacity while you are at that.
But suppose you know the capacity of the HVAC unit you need already. In this case, you're forced to make a decision that basically boils down to this: how much money you are willing to lose.
You see, the reality is a stubborn thing, and face it, ROI is not looking good. High end unit that is able to sustain a house about 2000 sq. ft (about 186 m^2) will cost you somewhere around $8,000..$10,000 (installation included), and even if the installation is perfect (caveat emptor - specs say "up to X SEER", not "X SEER") and the unit never breaks on your dime, you may wait up to ten years until you actually see the first cent of profit.
And make no mistake, all the money that you've invested into a nice air conditioning and/or heating setup is wasted when (not if) you sell the house - for the reason above, normal people care more about a fancy set of door knobs you bought at a garage sale for $5 a piece than for the state of art home comfort system.
Moral of the story?
- Getting a new HVAC unit will hurt.
- The less prepared you are, the more it will hurt (actually, the opposite is true as well because the way potential buyers are treated is guaranteed to drive you raving mad if you have a minimal clue about the problem; it's just the kind of pain that is different).
- The more time you have at your disposal, the better prepared you are, the better your task is defined, the less chance of falling prey to unscrupulous salespeople you have.
- The less you have to lose (30 year old Goettl, anyone?), the more you have to gain.
- Or just bite the bullet and buy a house with a good HVAC in it. Actually, if you do have a 30 year old unit, it may be a good time to get out of the house anyway... Unless it is 150 years old.