wondering if my duct sizing is sufficient

[hddal]

i have read on many sites that improper duct sizing is very common. i was wondering if someone could take a look at what i have and give me some idea where i stand with my a/c. my unit produces very cold air but i think i have an issue with air flow.

i have a 4 ton unit in a 2150 s.f. home. if i set my thermostat at 72, i will get 54 out of a supply vent and 74 in a nearby return vent.

my unit has a GREAT ability to hold its temp.. if i set at 70 at night it will hold that temp all the following day even if it goes up to 100. (almost sounding oversized) But it will not bring down temp or even cycle off on a normal day with normal settings. meaning if i set it at 75 on a 90 degree day, it will run from 2pm until 9pm. here are some measurements that i hope will help evaluate.

supply plenum - 16 x 20 (on unit)
main supply trunk - 11 x 21 (conn to supply plenum)
return plenum - 11 x 25 (on unit)
return ducts - 1 - 16" & 1 - 8" (conn to return plenum)
return plenum attaches to side of the unit and has an opening of 15 x 23 and uses a filter size of 16 x 25.

i seem to feel the return is too small and thereby restricting airflow.

i hope this is enough info to get us started. i thank you in advance for your assist.

tks, dal

[second opinion]

i have read on many sites that improper duct sizing is very common. i was wondering if someone could take a look at what i have and give me some idea where i stand with my a/c. my unit produces very cold air but i think i have an issue with air flow.

i have a 4 ton unit in a 2150 s.f. home. if i set my thermostat at 72, i will get 54 out of a supply vent and 74 in a nearby return vent.

my unit has a GREAT ability to hold its temp.. if i set at 70 at night it will hold that temp all the following day even if it goes up to 100. (almost sounding oversized) But it will not bring down temp or even cycle off on a normal day with normal settings. meaning if i set it at 75 on a 90 degree day, it will run from 2pm until 9pm. here are some measurements that i hope will help evaluate.

supply plenum - 16 x 20 (on unit)
main supply trunk - 11 x 21 (conn to supply plenum)
return plenum - 11 x 25 (on unit)
return ducts - 1 - 16" & 1 - 8" (conn to return plenum)
return plenum attaches to side of the unit and has an opening of 15 x 23 and uses a filter size of 16 x 25.

i seem to feel the return is too small and thereby restricting airflow.

i hope this is enough info to get us started. i thank you in advance for your assist.

tks, dal

Returns are too small. 11x 25 drop is a little small and will deliver the 1600 cfm but velocity is around 900 fpm too high for any filtration set up.

16 and 8 inch are giving you 1160 cfm at 0.06

[hddal]

thank you for your prompt response. so if i understand you correctly. it sounds like system is not "choked" off but a bit too small. can you give reccomended sizes for return plenum and return ducts?

tk u..