Drops per run of supply air duct

Just wondering how many do a visual on the # of drops per supply air trunk line within an attic while inspecting the attic.

I’ll have to say no, because I don’t have a clue what you’re even talking about :smiley:

Hmmmm I thought I was speaking English guess not, lets back up a Drop is the ceiling register how many outlets are on one trunk line from the supply air plenum, does that make more sense

I don’t Charley. Is there something in particular you look for? Does it depend on length of the run and size of the ducts?

What should we be looking for?

Got it. Nope, never have considered it, but I’m ready to learn.

What’s the rule(s)?

Do not forget to mention in relation to spacepaks as well.:slight_smile:


While I do not count the number of drops off of a trunk line or plenum I do look for remote plenums (either triangular or box type). I would have to do a load calculation and then measure each room’s air flow to see if all supply diffusers were getting enough flow. Do you count each drop to give yourself an initial idea of much airflow each room may be getting?

No, I don’t.
And I concur, that unless you are doing a load calc, or airflow measurement, you can’t just “look at the number of drops” and make any definitive conclusions,
short of saying something like " that looks like too many, or not enough"
The load calc and airflow measurements are outside the SOP for a home inspection.

I agree.

Charley might be talking about a “comfort balance”, which is basically whether or not the room feels comfortable.
May be as easy as walking in a room and see if it feels comfortably conditioned.
Using your hand to feel the air movement may help to establish adequacy. It may involve adjusting/moving registers, installing restrictors/dampers after the fact, or just insuring the air doesn’t blow directly on an occupant. Many occupant complaints are associated with air circulation: too much and too little.

No specific rules I don’t do load calculation at the register but my chest has all the hair beat off from thumping my chest on the roofing thread:p

I like people with open minds all I do is just make a mental note if the duct is in the attic the size and the amount of drops. Supply air duct is all about Blower CFM and static pressure within the duct. A 12 inch trunk line normally will have 8 inch drops to the register a perfectly balanced 12 inch duct would normally have no more than three 8 inch drops Ocassionally I would install an additional 6 inch drop for a total of 4 drops on one 12 inch trunk. When one sees 5 or more drops on on 12 inch trunk that static pressure at the end of the trunk is going to be low and this is what creates hot and or cold rooms.

A 10 inch trunk should have no more than two 8 inch drops and possibily sneak in one 6 inch to a bathroom without static problems.

The very top end of the supply air plenum should have no ducts connected without a damper for balancing.

I say again I am telling no one to exceed your precious SOP. I am supplying information to help you detect hot and or cold rooms in a house due to a lack of static pressure and to many drops on one trunk line.

My statement is and always will be to the client it is what it is and this is what you can expect out of the duct system.

Guys you are being paid for an opinion why not provide one