Heat pump

I have a question and can’t seem to find what I am looking for.
1800 squere foot home with a 3.5 Ton 12 Seer heat pump. There is only 1 return air vent with a filter. No filter on the air handler in the attic.
Where can I find information on the proper supply and return.

What do you want to know?


Is one return and one filter enough for this system.

What are the dimensions of the return trunk?

I am looking back I believe it was 14 inch

14" dia. ?

I don’t size A/C ducts.

If there is only one return for multiple floors, simply note it and recommend a return per floor (minimum). A floor with no return will get stale air.

If you’re adamant on obtaining the proper sizing…This might help a bit


If you want to learn alot about HVAC sizing in 2 days I recommend taking the National Comfort Institute “Introduction to Air Diagnostics” course.


A 14" duct will do 1800 cfm @ .3 in. wg.

.3 in wg is maxed out. It should be more like .1 in wg. (don’t put a filter at the end of it!)
That would be an 18 in duct.

3.5 ton Heat Pump = aprox 1,750 cfm

Is this one story?

If so, one return will work if the house is open inside.

Better be careful before you get into a pissing match with a licensed HVAC company. You may be asked by the seller or buyer to pay the fee for an evaluation of the system after they come out, find nothing wrong with it, and leave a $100 bill behind.

Unless it’s obvious, like 1 return on a 2 story home, I stay away from pointing out suspected design defects.

Exactly correct!

That was a closing thought as I hit the post button.

Analysis of a system is not a requirement of a home inspector. You need more than a Ductulator to determine the proper sizing. You need a Manometer prove that it’s wrong in the first place. Does anyone have these in their toolbox?