AC PRESSURES VS OUTSIDE TEMP

Reply

  #1  
Old 07-27-01, 09:50 AM
Guest
Visiting Guest
Posts: n/a
Lightbulb

HELLO, I HAVE A 1300 SQUARE FOOT HOME AND HAVE A 3 YEAR
OLD UNIT IN THE ATTIC. THE CONDENSING UNIT IS A TRANE
XE900. IT RUNS LIKE A CLOCK CONSIDERING THE AGE. IT IS
I BELEVE A 2.5 TON UNIT. HERE IN HOUSTON WE HAVE 100%
HUMIDITY AND 100+ DEGREE DAYS. FILTER IS CLEAN AND
COILS ARE CLEAN. MEASURED AT 98 DEGREES OUTSIDE I GET
70PSI SUCTION AND ABOUT 240-250 HIGH SIDE. AT THIS POINTT
THE HOUSE WONT GET BELOW 75 DEGREES AND THE REGISTER
ALLWAYS BLOWS 59-62 DEGREES DAY OR NIGHT. IF I RUN THE STAT
BELOW 75 IT RUNS CONTINIUOUSLY AT THE 98 TEMP. WHAT SHOULD THE PRESSURES BE ? ONLY IN THE AFTERNOON THE UNIT WILL NOT
PULL THE TEMP DOWN BUT MORNINGS AND NIGHTS IT WILL COME
DOWN NICELY(ROOM TEMP). ALSO THE CONDENSING UNIT GETS ALLL
OF THE AFTERNOON SUN. AM I UNDERSIZED ? ANY HEL WOULD
BE GREATLY APPRECIATED.. THANKS
 
Sponsored Links
  #2  
Old 07-27-01, 02:35 PM
PDF
Visiting Guest
Posts: n/a
Pressures sound

okay.Please take a "DELTA T".Measure the air temp into your return with a DIGITAL thermometer.Measure your supply temp at a register.These temps should show a minimum of 15 degrees difference.Do you have an attic exhaust fan to remove heat build?It would help at night to expel the added heat in the attic.ANY AIR LEAKAGE or loose duct in your situation is REALLY hurting you.More so if your return duct in attic is pulling attic air into cooling coil.PDF
 
  #3  
Old 07-27-01, 03:20 PM
lynn comstock
Visiting Guest
Posts: n/a
Sizing:

The design conditions for Houston are 95 degree dry bulb and 77 degree Wet bulb. This is about 45% relative humidity (that happens to FEEL like 100%). 97.5 per cent of the summer weather is below this design temperature. The standard indoor temperature for design is 75 degrees F.

Expect a properly sized unit to run continuously at temperatures above the design temperature. Because the house has a large mass, a few hours above the design conditions will not cause much change in the inside temperature.

ABOUT THE HIGH SIDE PRESSURE:

The gauge needle will show the condensing temperature in the R-22 ring of numbers. At 243 psig the R-22 condenses at 115 degrees F. This is 17 degrees F above the outside temperature (98 degrees F). This is on the low side. I would expect a 20 to 25 degree F difference at design + conditions.

The Suction Pressure corresponds to a boiling temperature of 41 degrees. This is 34 degrees below the indoor temperature of 75 degrees F (about normal for a 10 SEER or better unit).

The supply air temperature is on the warm side. It is about 15 degrees below the indoor temp of 75 degrees. The ductwork may be warming the air before it gets to the grille. Measure the air temperature leaving the indoor coil.

Air volume matters a lot. The manufacturer designs the equipment to deliver an air volume of 400 Cubic Feet per Minute (CFM). Most real world systems deliver significantly less air than that. The LEAST air that the system should move is 350 CFM. (KEEP your indoor coils and filters clean to maximize airflow.)

Nothing that you tell me raises any concern about sizing or functionality of your system. Other people's units may be cycling in mid-afternoon. Their units are OVER-sized. This is the general practice of the industry as it is SAFER and Better Money for the SELLER. You will have no problem finding a contractor to tell you that your unit is too small.

SEE:
http://www.proctoreng.com/articles/bigger.html for an excellent look at sizing air conditioners.

Another view of proper sizing is at http://www.buildingscience.com/501c.PDF
They find that even 80% of Manual J sizing is more than adequate in Tucson, AZ.
 
  #4  
Old 07-28-01, 04:20 AM
PDF
Visiting Guest
Posts: n/a
Lynn

Can you supply some info to handylefty?He owns a ARKLA GAS system.PDF
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: