Something I get asked to do on a regular basis is take a look at a duct layout for someone’s house and see if it “looks OK”.  Here is a good example.  Suppose someone showed you this sketch of a 3.5 ton system and said, “Does this look OK to you?”  You might look each trunk/branch combination and say, “a 10″ serving two 8″ ducts, that seem OK.  A 16” return duct on a 3.5 ton, that seems reasonable.  Each run might look reasonable and there are registers in all of the rooms, but the main question should be, “Will ALL of the ducts handle ALL of the air?”

In about 10 minutes I can tell you if there are any serious problems related to duct sizing. Here is how I do it.

Table 1 – Duct Size vs. Airflow at a Friction Rate of 0.1

1. Figure out how much air is the system is supposed to handle. I usually use 400 cfm per ton (condenser tonnage) as a minimum.  If the designer tells you a higher number, use that.
2. Are there enough supply trunks to handle this much air? List the diameters of all the start collars coming off of the supply plenum. Use the airflow table, right. This table represents how much air a certain size duct should handle in a “reasonably well-designed” system (friction rate = 0.10 iwc/100ft).  Add up all of the flows – they should be greater than the target flow (from #1).
3. Are there enough supply branch runouts to handle this much air? Repeat step 2 for the supply branches (ducts that serve a single supply register).
4. Are there enough return ducts to handle this much air? Repeat step 2 for the return ducts.

This test will not tell you if the equipment is over or undersized, nor will it tell you how well the system is balanced – whether the air is going to rooms in the right amounts, relative to other rooms, but it will quickly identify one of the most common problems: undersized ducts that impact overall airflow to the system.

Let’s do it for this sketch: The target airflow would be 3.5 x 400 =1400 cfm.  The four main trunks add up to 1200 cfm – NOT GOOD.  The supply branch runouts add up to 1270 cfm – NOT GOOD.  I frequently see 16″ returns on 3.5 and even 4 ton systems.  A 16″ duct should only handle about 1050 cfm – VERY BAD.  This system would run at a much higher static pressure and probably would not pass the minimum air flow and maximum fan watt draw test of CA’s Title 24 energy code (350 cfm/ton and 0.58 watts/cfm) and those are not meant to be hard numbers to beat, but far too few installers know how to properly size ducts.  Some even take the time to use Wrightsoft or Elite Manual J/S/D software but then have some hidden setting in them that messes everything up. (See my article about why we need a simpler Design Methodology).  Most installers just use old rules of thumb that result in these substandard systems.  Kudos to those who actually do a good job.  Their numbers are growing, but far too slowly.

If they had used a better design methodology they might have come up with this revised version of the same plan.  The revised layout passes this quick test.  The four main trunks add up to 1800 cfm, which is good. The supply branch runouts add up to 1480 cfm, which is close, but OK. The return is a 20″, which can handle 1875 cfm, which is very good. Note that an 18″ duct would barely work.  The intermediate “trunks” can be checked independently in a similar manner.  This layout will probably work just fine, at least in terms of delivering 1400 cfm.  Something that I have found to be true in many cases is that very good overall airflow will forgive a lot of sins, including minor balance issues and even minor over or under-sizing of equipment.  It’s not that hard to do, folks!