Marty,

Duct size is not the only issue. The fittings and equivalent length of the installed duct has a lot to do with it, too. The duct sizing could be just fine and have bad fittings. Example would be an elbow in the main duct without turning vanes. Or worse yet, tapping the supply plenum from the blower right into the bottom of the supply trunk duct.

I've seen duct installations that make me cringe when it comes to equivalent length (on houses we were installing vacuum systems in, not the duct). One had a 5 ton A/C unit (oversized anyway) that had 24" x 12" duct, both ways from the furnace. This duct size should be adequate for 5 tons. However, because of the fittings they used and their equivalent length (EL), the furthest 3 registers came up to 650' EL, 665' EL, and 685' EL. I haven't been back to talk to the home owners, but I'm guessing they don't get much air to those rooms.

The contractor that did this continues to do it. If he'd just not tap into the bottom of the supply duct with his plenum (not recommended in any application), use turning vanes, and use top takeoffs instead of just using elbows, his systems would be much better. Unfortunately, he just continues, assuming that because it supposedly worked last time, it must work this time, etc....

Anyway, as far as existing duct capabilities, the most accurate ways to measure the duct capability are:

- measure each register's cfm and add them up.
- using the electric furnace (I'm assuming that's what you mean when you say electric A/C), find out the following and plug it in to the equation:
- temperature difference between the supply air and the return air
- kW size of the strip heat
with this info, BTU output = (kW size) X (3413)
Then, CFM = (BTU) / (1.08 X Temp. Difference)

With the CFM, one should be able to look up/find the blower data for the blower you have (brand of furnace) and see what static the duct/blower is performing at. Should be able to then apply a bigger blower and see what that does to static, etc... You'd need someone knowledgeable at this point to weigh higher statics vs. blower power performance, etc.

Hope this isn't too vague...

There's one more way to get cfm... If you have the blower data for the blower, measure the static pressure difference between supply and return. Look in the blower data charts and see how much cfm is there.

Sorry for any confusion- I have a natural gas furnace and an electric air conditioner. The furnace is a 110,000 BTU 60% efficiency furnace.

Thanks for the detailed reply. When you say cfm, do you mean cubit feet per minute of air flow? How would you measure this? I don't think I have any data for the blower, but I'll look and see if i can find the model number and look it up online. Supply and return air temperatures will be easy enough. In your equation, is CFM only dependant on the temperatures and the BTU? Or is there a volume flow component as well?

I have two main duct outgoing duct lines (with accompanying returns). The furnace is on my lower level of a split level house. The front half of the house is a single floor above a concrete crawl space. The back half is two levels- half a flight down to the lower level and half a flight up to the upper level. One line goes up from the furnace to the ceiling, and then takes a 90 degree turn through the ceiling to the front half of the house. All of this duct work is in the ceiling of the crawl space below the main level. One main duct runs the length of the first floor with smaller circular ducts running out at 90 degrees to the various registers. No wacky turns or cuts that I can see. The second outgoing duct line goes up to the second floor, but it's in the walls so I can't see how it's run.

Tanks-

Marty

Can do the heat load if you'd like to PM me. Duct capacity depends on lots of things as clark mentioned.
Based on what you told us, I'd guess the 5 ton guy is mistaken.
j