This may scare you…
I recently got certified to do energy audits and boy, did I learn a lot about a/c that I didn’t know! I’ll share some of it with you today.
The four most significant facts I wasn’t aware of is that virtually ALL systems are oversized (because houses are so leaky), work at only 60-80% of their design efficiency (due to poor installation or service), have average duct air leakage of 32% and most return air grilles are undersized (also reducing efficiency) ! And people wonder why their bills are so high!
From what I see on a daily basis I can tell you that most filters are dirty and a lot of the outside units need cleaning (the coils or “fins” suck air in continually and get clogged with dust) with some brands worse than others (Rheem and Ruud in particular). Most people never have their unit serviced until it stops cooling! Some units DO keep the space cool but are running constantly during hot weather (the unit should cycle on and off even on very hot days if it‘s working as it should).
Some people use the wrong type filter too (so called “allergy” or “electrostatic” filters). These DO filter well BUT they restrict the airflow thru the equipment, making it run longer and hotter than it should, shortening it‘s life & raising your electric bill. The best ones are the pleated paper type and they last longer too. They filter well but don’t restrict airflow.
If your indoor unit is noisy, I can just about guarantee that your return air grille is too small. The absolute bare bones minimum is 1 square foot of grille for every ton of a/c and 2 square feet is much preferred. Your return air cannot be too big! We often see 3 ton systems with a 12X24 grille which equals 2 square feet (33% smaller than even the minimum needed!). No wonder they make such a loud sucking sound! So if you have a 2400sf house with a 4 ton a/c, you should have AT LEAST a 4sf return air grille with 8sf being preferable (you can always add a 2nd grille). (This calculation doesn’t take into account the area of the grille louvers themselves which reduce the area of the intake so in actuality you should have a little more than 1sf per ton minimum…)
Something you can do yourself is seal your ducts including the return air space. Go buy some duct mastic (consistency of pudding) from an a/c supply house (may be sold at Lowes or Home Depot), put on some latex gloves and smear it onto all the duct joints. This is best done during the cooler months. Caulk the duct boxes to the ceiling, this is where most air leakage occurs. You just prevented 32% (on average) of the air you pay to cool from blowing into the attic! Companies charge a LOT of money to do this (I was recently quoted $700-$1300 for a 1300sf house) but it’s really very simple.
PS: the class instructor said to NOT get your ducts cleaned unless they’re round metal (which there are very few of out there…).