Which differential value indicates whether an AC is working properly?

My air conditioner is able to keep my condo at 75 degrees when it is 95-100 degrees outside. However, the difference between the supply and return registers is only 4.5 degrees (supply at 70.2 and return at 74.7).

Some articles I've read say that a 15-20 degree difference between outside temperature and inside temperature is the indication that an AC is working properly. But, other articles I've read say that a 15-20 degree difference between supply and return registers is the indication that an AC is working properly. As indicated above, my AC falls within the first suggested limit but not within the second.

So, I'm confused as to whether I have a problem and which measure (outside vs inside or supply vs return) is important. Or are both important? I'd appreciate any comments or advice that you might have. Thanks!

--David

 

With a 4.5 degree delta T, it's not working or you didn't take the temp readings correctly.
There's no way it would cool your condo with 95-100 deg. Outdoor temps.

Make sure the condenser outside is running for at least 5-10 minutes then measure the air temp at the return and at or inside the supply air register.

 

Tom, I decided to try your suggestion, and I measured inside the return register and inside a different supply register than before, for four minutes each, making sure that the condenser had run for at least 10 minutes. This time, the supply was at 64.2 and the return was at 75. (The thermostat is set at 75, and measurements elsewhere in the room indicated that it was 75 degrees.) Obviously, this supply value is better than before, but still not as low as would be preferred.

 

David....when you said this what did you actually mean? What you wrote is 2 sentences saying the same thing.

That temp split is far too low and I'm surprised you are keeping the temp at set point, unless the A/C is running constantly. Is it possible your thermometer is out of whack at the lower end? Try putting it in the fridge. Should read around 40 degrees.

 

Those numbers are more in line with a system that's working, but poorly. You should be blowing cool air in the mid to upper 50's, that's a temp split between 15 & 20 degrees. Numbers like that will indicate the system is operating to it's potential. You might want to have an HVAC company look it over for you.

If you're not able to keep your house 20 degrees cooler then your typical 90 degree summer heat (design temp), but the temp split indicates it's operating correctly, then the system is under sized for the space.

Both are important in terms of operation and efficiency.

 

Oops...never mind about answering about the quote...I wasn't reading it correctly. Thanks to Tomf for jarring my brain gears back into meshing.

 

In Texas, if your air handler is located in the attic, I suggest that you measure the Delta-T twice at different registers. once at 3 PM (while sunny hot outside), once at mid-night. and then take the average.

 

When I see the word Condo I think of a high rise home that may use chill water. Post the model number of your indoor unit.

 

Tom, thanks for the advice. I'll have an HVAC company look over the system as you recommended. (I haven't had the system examined in the 20 months I've owned it, but it's possible that the previous owner might not have had it examined for 7 years more, so there could be all sorts of problems in the system.)

Houston204, the Condo is a four-story building in which my unit is one of several on the third floor. I've been told that each unit has its own air conditioner on the roof. The door to the roof requires an electronic code, which the management company will only give to repairmen, so I'm not able to go up there (personally) to determine what the model number is or to observe the AC unit.

Thanks,
David

 

At only 4 stories it is unlikely to be chill water. You probably have a split system with a condenser on the roof and an air handler in a closet or above the bathroom ceiling or above the ceiling in a closet.

 

The 'indoor' relative humidity is the big determining factor in regards to what the indoor temp-drop at the registers will be...

At around 50%RH INDOORS the INDOOR temp-drop should be between 19 & 21°F.

I.E., a 2-Ton @800-CFM; 75°F indoors; 85°F outdoors; 59¯F wet bulb or around a low 38%RH indoor temp-drop 21°F; @71°F wet bulb or around an 'indoor' 83%RH, temp-drop will be only 11°F.

INDOOR RH% makes a big difference in the indoor temp-drop -- delta T...

Also, take the discharge air temp off the outdoor condenser & compare it to the outdoor temp.
If the split is too high the system may be sucking return air from a hot attic, etc.
If too low a temp-rise 'indoor' airflow may be too low or other refrigerant problems may be present...

Unlike the indoor temp-drop which only indicates the sensible portion; the condenser discharges both latent & sensible Btu...that is helpful when troubleshooting a system...

 

That is very impressive information.

How much would a 98° outdoor temperature and 55% RH indoors affect this calculation since this is probably closer to the actual conditions?

 

At 95 or even 98°F ambient it makes very little difference with the indoor sensible temps, what counts is the indoor conditions, with higher outdoor temps it will have longer runtimes & perhaps a little increase in heatload.

At 95°F outdoors the 'indoor' numbers remain the same except for a little less total Btuh output.

At 105°F ambient the outdoors, the 'indoor' numbers remain the same, except condenser pressure/temp & load goes up & the total Btuh drops some.

The indoor sensible load % goes up to 81% at 105°F ambient over 75% sensible at 85°F ambient.

Well; the indoor 55%RH would lower the indoor temp-drop a little bit & raise the outdoor condenser pressure/temp some, but not much compared to 50% indoor RH.

 

The outdoor relative humidity here this time of year is about 30-35% from noon-8 pm and 40-65% the rest of the time (steadily increasing to a high about 7 am and then going back down).

The indoor relative humidity I'm seeing is about 27%.

 

I think that you may need a new humidity gauge.
27% RH is something that you might see in the winter if you don't have a humidifier.
Do you have icynene insulation?

I am currently reading 32% RH and 95 degrees outside.

 

Houston204, you were right--the humidity gauge I was using apparently was producing incorrect results. I replaced it with a more reliable gauge and wound up with 48% RH and 90 degrees outside and 46% RH and 75 degrees inside, at the present time.

I don't know the answer to the icynene insulation question that you posed.