I suspect a leak that was not repaired plus being undersized. What is your current outside temperature? What is your inside supply temperature at several locations? What Is your inside return temperature? Filters are clean?

TD

What makes you think the equipment is undersized? If the equipment is low on refrigerant then even over sized equipment will not be able to provide the proper amount of cooling.

TD, thanks for the input. Outside temp was about 88 yesterday. Filter is less than a month old and all the vents in the house measure between 10 and 11 degree delta. Closest one to the unit is where I measured 11 originally, farthest ones are showing about 10. Return temp is less than a degree warmer than inside ambient temp.

Mike, I've had a couple AC contractors out to look at it for the refrigerant leak and tell me it's undersized. At first I thought they may be trying to sell me a larger unit but I don't think that's the case after doing a little reading. Summer highs are in the upper 90s here. Neighbors who are friends with previous tenants said it never cooled all that well even when it was "working." My house is about 1800 sq ft with 10' ceilings in about 3/4 of the home. Average amount of windows, average insulation, brick veneer, dark gray shingle roof. My unit is 2.5 Tons and they said it should be at least 3 if not 3.5.

A temperature delta of 10-11 degrees is not good. It should be 15-20 degrees. An undersized system would still produce a good temperature delta, but it would struggle to maintain a 75 degree indoor temperature on the hottest days of the year. A 2.5 ton unit for 1800 sq. feet is not that undersized. You probably have a lot of loss if the duct work is in the attic and not well insulated.

How old is the equipment? If it is more than 10 years old then it makes sense to replace it. You need to find someone who will do a load calculation and measure the duct work to see if it could support 3 tons of air flow. The duct work to be inspected and replaced as needed.

Someone who drives up and tells you the equipment is undersized is trying to sell you a replacement job, not diagnose the problem. In my opinion.

Maybe you need new equipment. Call other contractors.

I appreciate the advice guys. Equipment is about 3-4 years old.

"I know the rule of thumb says the air coming out of your vents should be about 20 degrees cooler than the current ambient air temperature. Mine are showing about an 11 degree delta. I've never measured this before. Is this because the unit is undersized, or did they actually not fix the leak/is there something else maybe wrong? "

As Mike says, your delta is low. Delta depends on cooling capacity, airflow and humidity but should be around 15-20 degrees. Low refrigerant, high humidity and high airflow will have a low delta. Low humidity and low airflow will have a higher delta. In your case it sounds like the unit is not performing properly (rather than being undesized).

Try to find a company that installs your brand of AC. This link has companies that have a better chance of not ripping you off (.....than Google):

http://icemeister.net/aop_map.html

I appreciate the advice guys. Equipment is about 3-4 years old.

Is this original equipment? Meaning that the house is 3-4 years old as well?

If so this could be a high efficiency build, it's not uncommon for a home that size to have a smaller capacity unit in a more recent tighter build. Depending on windows and other factors that could be really close to appropriate.

Tighter more recent home builds don't require as much cooling. The other thing is that HVAC contractors are looking for an easy pay day. If they say the unit is undersized --- that's going to lead to a whole replacement. New furnace, new coil --- new outdoor unit.

If you question the size of this unit, you need a on site heat load calculation performed. I would avoid freebie type services that are mostly bent on selling you a bunch of crap you do not need.

In the mean time look for a repair service provider pay for a refrigerant leak check. That refrigerant leak check may lead to replacement of a 'piece' that is leaking. Usually the indoor coil. But it is not a good idea to assume that is where the leak is.

Is the unit a micro channel unit? These are critically charged units, meaning that only a small amount of refrigerant has to leak out until the unit appears to be undersized. In that case it may be appropriate to replace the AC at least. Micro channel is mostly garbage in a hot climate. These unit's primarily benefit the manufacturer. I would avoid them.

Post some pictures of your equipment, including mod # and serial #'s.

Hey guys, I have a home warranty through 210. Since this was previously a rental property I figured that'd be safer. The AC contractor came to check the unit again and found that the previous guys left the service port caps off. I'm not sure that would be the issue, seems like the valve should be holding refrigerant in, not the cap. Next step is to do a leak check. System was 3 lbs low and he refilled it. Boom 20 degree delta now (23 across the evaporator). We'll see how it does in the summer, I have a feeling it leaked pretty quickly in the late summer last year which is why it was struggling. I'll let you guys know how it all shakes out.

@Austin Air Companie - Thanks! My home was built in 2003 and I have some slightly drafty windows which will be fixed before summer. My guess is not a high efficiency build.

Indoor unit: FB4CNF030L00ADAA

Outdoor: 25HCD330A0030060

What is your location? Ideally you get some hot weather soon so you can determine if the unit is undersized.

A 20 degree delta is very good. Keep measuring it on a weekly basis. If it begins to drop then you know the leak has not been fixed. I think the service port cap story was an excuse not to do a leak test. This is the problem when dealing with a company who is paid through a home warranty.

@Austin Air Companie - Thanks! My home was built in 2003 and I have some slightly drafty windows which will be fixed before summer. My guess is not a high efficiency build.

Ok, 2003 era was still a bit on the drafty side... not as bad as 1980ish time frame but no where near today's standard of tightness.

I would say this issue is mostly a refrigerant problem. Once you lose refrigerant, you lose capacity of the system. Easy to think it's a sizing problem when it's probably not.

Who would keep an undersized AC system for 17 years in a hot climate like Florida? (I supposed this might be more likely with a rental home... but even then there would be clues if you could find how frequently the home was being rented and then rented again... sometimes you can see this info on the zillow listing of the property in the history section.)

With that said home warranties are the bane of the industry. It's a waste of time and money playing the home warranty game.

Find a good HVAC contractor who will do a load calc and properly diagnose what's going on. Others, mike and tiger, have knowledge and offer good advice.

If duct sealing and duct leak testing wasn't done when the equipment was replaced (yeah, I know you have no way to know), I'd suspect that in addition to a possible coolant leak. Duct leak testing can be done now, it's not a big deal and can be done from inside the home (but it does cost several hundred bucks) .

Gents, please see previous comments. Equipment is all 3-4 yrs old. House was built in ‘03

Sorry, missed that. I revised my comment (something I don't do without saying so)

Find a good HVAC contractor who will do a load calc and properly diagnose what's going on. Others, mike and tiger, have knowledge and offer good advice.

And the other bad side of a load calc? Whoever does it controls the outcome of it. I've gone behind a major company in my area recommending complete replacement of a system that was only a few years old. (think sales $$)

I repaired the problem (years ago) and it is still running to this day. Sizing is important, but it's not the end all be all.

A properly sized system is actually over sized 90%* of the time. (Follow the path of the sun everyday.)

*exception: Inverter System.

How many systems out there like this one a few years old and you're recommending heat load calcs as a solution?

This is why you should step away from the keyboard.