Greetings. New guy. Great info here. Read back 12 pages picking out topics of interest.
Newer installation. Dec 2005. Outside unit trips. Head pressure issues.
Rheem 3Ton HP (1200cfm max) in a 1129 sq ft house. Have been told it is too large.
I can live with it due to future expansion plans. Maybe. 300 sq ft addition.
Ducting is said to be small. 14" supply and return.
Six 6" supplies and one 4" in the bathroom.
Looking for a (rule of thumb) on the supply drops. Am told that a 6" drop is ~100 cfm. So I'm looking at a ~650 cfm duct system and have 1200 cfm pushing through it. Noisy. Slowed the fan down to 1k. Register temp is 80-85*
Was told that I could replace the 3T with a 2T and all would be well with the ducting and house size.
Was told that I could increase the ducting and keep the 3T. i.e. 8" drops at the six locations, 6" for the bathroom. Increase the return to 16.
I'd like to change the ducting so as to try and make the 3T unit work. maybe even adding two more registers. I seek this generic rule of thumb to help plan the new ducting. It is 95% flex through the attic. The unit is in the garage up against the trusses. Hard 90's into the house then flex from there.
The other option is to remove the HP and replace with a gas furnace and 2T AC
My question is, is there a rule of thumb for cfm/supply drops to help design the ducting?
Thanks for reading, Regis