The "delta T" across the indoor coil should be between 17-20 degrees, however you are not taking the readings at the proper place. The readings are taken at the return and the plenum just past the indoor coil- after the system has run for at least 10 minutes. This is only part of the readings that should be taken. The indoor wet bulb temp should be taken as well.
Depending on if you have a TXV or a piston metering device, the charge (freon psi) should be checked using "superheat" for a piston, or "subcooling" for a TXV. This is done by reading the freon psi, saturation temp to calculate.
Your ducting will pick up heat in the attic as your readings show one duct is picking up as much as 8 degrees. This could mean poor insulation on the duct or a very hot attic or leaking ducts.
If you have an air handler (elect heat or heat pump) it is a good idea to make sure a strip heater hasn't shorted out to ground and heating up a little.
You may need to hire another contractor to check this system out- I have had bad dealings here with home warranty idiots that couldn't fix an a/c if you showed them how.

All in all- I think you still have an a/c problem -and you may also need to make some improvements to the home.
Check the attic insulation, the attic ventilation, windows and doors for tightness and the ducting for leaks.

Hmmm. Well, let me ask this - does it sound like the company who has come to look at the unit has done enough to determine that "everything's ok and it's just an older system that can't handle this heat" or should they actually look in the attic at the ductwork or the furnace (which they didn't do)?

I know I should upgrade the insulation. I looked at the ducts and they looked like they were ok, but with my limited knowledge that basically means they looked like they were connected - no blatantly obvious sags or breaks. Ductwork leaks are covered in the warranty. How are they discovered? When the guy came out initially and pointed the device at the vents, he said there was air flow coming from them. To me, the air flow isn't as good in the warmer parts of the apartment (bedroom, bathroom). I'm not sure if that device just measured temperature or if it measures strength of flow also.

Thank you for your help - it is appreciated!

At a minimum, you have duct problems. As the day heats up, you are no longer able to maintain the split you had at noon. Either poorly insulated ductwork (return and/or supply) or leaks on the return side of the ductwork 'sucking' in hot attic air. or both

No - I wouldn't say the company has done enough. I mean even home inspectors check vent temps. I'd escalate with your home warranty people. Perhaps an independent HVAC person can give them a report (this obviously will cost you).

A furnace is for heating.

I would cut the breaker to the electric heat backup just to see if it helps. Should be a seperate breaker but in 1950 - who knows.

I know delta T is a commonly used measurement but original poster reports supply temps in low 70s. Even for current Hotlanta weather, that is not normal. the fact remains that apartments and/or condos are sized very close and can't stand extreme temps in either direction. I still say that high supply temp is mighty suspicious-ie bad charge or leak. OP may need more CFMs with next size up unit but the high supply temp should be addressed.

a question I would ask OP is this the first summer in this apartment? If not how did apt cool last summer? the weather SE and Atlanta are experiencing is really not out of the ordinary and in fact quite typical.

IMO

Hey tigerdunes - did you mean to say my supply temp was in the low 80s (in the afternoon)? I only ask because I wasn't sure if I had the right terms; the supply vent is that part that blows the cooled air into the house, correct? And yeah, I just moved into this place in May of this year.

Also, I mentioned the furnace because I thought that part was the air handler system that helped circulate the air. But I guess the fact that there is air coming out of all the vents would indicate the furnace is blowing and thus working ok?

check your supply temp at several different supply registers. not the return but the supply. note the location, time of day, temperature and apprx outside temperature.

since you are getting air from the supply registers, then it appears you do not have problem with blower motor.

you are changing your air filters regularly?

do you know if you have furnace or air handler? that's the inside unit usually placed in a closet, attic, or crawl space.

post back.

IMO

When I had checked the temps a while ago, I did so at different return vents - in the living room, bedroom and bathroom. I'm not sure what the temp outside was, but based on what it's been these past few days, it was probably around 88 when I first checked around noon, then 95 when I checked again around 3:30.

I just changed the filter in mid-July; it was the first time I did so since I moved in in May. I know it had been changed back in November (which is a long time, but I don't think it was running too much since no one was there).

And it is indeed a gas-fired furnace. A York Diamond 80, which looks like their cheapest model from the website. I can't see the model number anywhere on the unit. There was one of the yellow stickers on it(probably had the energy efficiency on it), but it's torn off. It's in the attic.

your best bet in this situation is to hire someone yourself to check out the system and give you an honest answer. If you check craigslist, you'll find guys offering to do "check ups" for $50-100 - just make it clear that you want a full inspection with line pressures, subcool/superheat delta-T and static pressure readings documented in writing on the repair order. Tell them before they come out that you aren't interested in a new system, just in getting the unit back to factory specs.

Something is definitely wrong, and it's likely an inexpensive fix if you can get someone to actually break out their tools and check the system properly.

Don't let them talk you into a new system, and don't believe the warranty company's lapdog.

Google the warranty company's name to see what you're dealing with, most have terrible reviews.

Hey zippyhvac - are the things you're talking about having checked (line pressures, etc) things that are all located in the compressor (the outside unit)?

They would need to look at the indoor coil and the blower wheel (to check for blockages/lint/fuzz build up) and also to check the pressure drop across the coil.

Plus, with what you're describing, they would need to look over the duct work carefully for kinks, leaks, etc.

So, no, you can't do a complete inspection without visiting the inside unit.

If that's what they did, they ripped you (and your warranty company) off.

it's likely that your cable guy stomped on a duct in the attic, or there's something blocking the coil, or the refrigerant charge is incorrect - too much or too little can cause the temp split to be off.

This is not likely to be a major or expensive repair.