air conditioner advice needed

[rvCharlie]

Hi,

I need to mine the HVAC brain trust here about my home A/C; hoping it's not inappropriate to ask here.

My Trane central units are around 25 years old. They were pretty high efficiency for the time of mfgr, and have required minimal maintenance considering their 25 year age (a couple of capacitors and the air flow motor in the compressor, control boards in the air handlers at different times, and one pressure sensor for the heater section).

This summer, the primary unit *seems* to be under-performing, but because conditions have changed in the house, I'm uncertain whether to blame the A/C or the changing conditions.

We normally close off about 40% of the house, and only run the primary unit. Last year, we opened up one of the bedrooms & a bath that had been in the '40% zone', but just let ambient air from the main part of the house cool it. The HVAC vents in the room are plugged.

This summer, when it became obvious that there were performance issues, I started investigating. Both the evaporator coil and the condenser coils were quite dirty; they've been cleaned.

I've checked return air/register air using a cheap indoor/outdoor thermometer. Before cleaning, temp delta after a short check was ~11-12 degrees F. After cleaning, and letting the system run for quite a while, delta finally got to ~18 degrees F. Right now, it's 17 degrees. It seems to take a long time (several minutes minimum) for the delta to get that wide. Not sure whether that's an A/C issue, or slow response of the cheap thermometer.

It's very humid in MS, and I've had problems with moisture condensing and dripping as air leaves the registers near the air handler both before and after cleaning the coils. The condensate drain is clear (verified when I cleaned the coils). The air handler is under the house; the vents blowing water are near the ceiling. They're a good 10' above the evaporator coil.

A couple of very curious details; at least to me: When I cleaned the evaporator coils (typical A frame shape, laid on its side in the air handler), I found a piece of sheet metal covering over half of the top side of the 'A', and wedged into the metal flange at the apex of the 'A'. The other odd seeming thing is that when I started to clean the evaporator, I got interrupted, and didn't get back to cleaning it for about 1 1/2 hours. When I got back to cleaning, the coil itself, pulled about half way out of the housing before I was interrupted, was *still* cold and condensing moisture out of the air.

We normally turn up the thermostat to ~75-76 in the daytime, and down to ~69-70 at night. It's taking a couple of hours to bring temps down from 75 to 70. This seems to be excessive, with daytime outside temps in the mid/high 90s and evening temps in the mid 80s. Mitigating that is the fact that we added about 250 sq ft to the previous ~1400 sq feet that the unit was handling.

I have not done any refrigerant pressure measurements; that's a skillset & tool set I have not yet acquired. Sorry for the long post, but I was trying to include as many details as I could.

Thoughts from those experienced with refrigeration systems and HVAC?

I can find the money to replace the system, but I don't want to needlessly replace it if it's simply a situation where we're getting near the system's capacity.

Thanks,

Charlie