The Problem With A/C Nominal Sizes
I remember, for example, one job in Arkansas that I went out to look at many years ago, which just didn’t get the house temperature down to accepted nominal conditions, although the total load had been carefully calculated - and re-calculated - and what I thought to be a 3-ton system had been selected and installed. And of course, I (as the distributor’s service manager) went out to look at the job to find out what had gone wrong, suspecting that the contractor had made some sort of a mechanical mistake.
So, I carefully examined the ductwork and the refrigerant charge; I checked the refrigerant pressures and for air in the refrigerant; I checked the refrigerant superheats and the subcooling; then I went back and re-checked the load calculations, the airflow, and the entering and leaving air wet-bulb temperatures (plotting a psychometric chart); and found absolutely nothing wrong!
And I’ll admit, that coming from the service side of the business, this job had me stymied! I was looking for a mechanical problem, and it took me a long time to get around to looking at the equipment matchup. After all, both the outdoor unit and the air handler had 036 in their model numbers, so I knew that it was a 3-ton system (12,000-Btu/ton into 36,000-Btu = 3 tons)… but it wasn’t! As it turned out, the particular matchup only provided 34,000-Btus, which under maximum-load conditions (Arkansas on a sunny, hot July day) just didn’t get the house cool enough.
Do you think that your customers ever run into this problem? Well, listen to their service guys when they come in to buy a replacement condensing unit. You’ll hear them saying “I need a 3-ton condensing unit.” Never mind what output the unit they buy actually produces when matched with the existing airhandler and coil!
So, many contractors, to make sure that they don’t undersize the equipment, specify a half-ton (or more) larger than their loads show is actually needed. And what does this do? Yes, their customer can get the system to run cool enough, but on milder days the house is humid and uncomfortable, because the system isn’t running long enough to properly dehumidify the house! Why, I’ve seen molded wallpaper, molded shoes, and even water dripping down the insides of windows!
So, remember (and advise your customers) that one of many good reasons for replacing both the indoor and outdoor units at the same time (rather than just replacing the condensing unit by itself or the airhandler by itself) is to make sure that the matchup will meet the existing load conditions.
And when it comes to their choosing whether to oversize a system (for a little CYA), you might suggest that they ask their customers whether their first priority is getting the house cool enough, or if they want better humidity control. You’d be surprised to find that such a simple question clearly shows consumers the options they have. And if they want both, then an upgrade to a variable-speed indoor fan should be suggested.