We’ve all heard our customers describe how the air conditioning system works well in the morning, but doesn’t deliver any cooling in the afternoon. We send out a service tech, he services the unit and reports that it’s working pretty well. End of story, right? Well, not for the customer. He or she is still uncomfortable every afternoon until the weather cools down in the fall.

Why does this happen? The most likely reason is duct loss.
Ideally, the temperature of the air leaving the equipment should be the same as the air leaving the farthest supply grille. But if the duct is running through a 140F attic, it’s not uncommon for supply air to lose up to half of its Btus of cooling.

Let’s take a look and discover how a simple five minute test can quantify the duct loss and help us solve a problem that may have been overlooked for years. But you won’t find the answer to this question at the unit. You’ve got to pull your head out of the box and check the duct system.

Check the Delta T
Delta T is the difference between any two related temperatures. Checking it is easy and quick: simply take two temperature readings, and subtract one from the other.


The test is simple. First, drill a 3/8-in. hole in the supply plenum (not too close to the coil; that hissing noise can really mess up a nice day). Insert a good quality temperature probe such as a fast-acting thermocouple into the duct near the discharge of the equipment. Read and record the temperature.

Second, insert the temperature probe in one of the farthest supply registers. Allow the temperature reading to stabilize, and then record the supply register air temperature.

Finally, subtract the equipment discharge air temperature from the supply register temperature. Here’s an example:

Supply air temperature (60F) - Equipment discharge temperature (55F) = Duct temperature loss (5F)

Calculate Duct Loss
Five degrees may not sound like much, but a couple more quick calculations will bring it into perspective. (Trust me, these calculations are simple, so even if you don’t like math you can still look like a genius.)
What we want to do is divide the duct temperature loss (the 5F we calculated above) into the temperature drop over the coil.

Measure the temperature drop over the coil like this:

Air temperature entering the coil (75F) - air temperature exiting the coil (55F) = temperature change through the coil (20F).

Now just divide the 5F temperature loss through the duct into the 20F temperature change to identify the percentage of Btu loss through the duct. Here’s how the math looks:

Duct temperature loss (5F) / air temperature change through the coil (20F) = percent of Btu loss through the duct (25%).

That’s it. Few rooms can take a 25% Btu loss during the hot days of summer. Even more important is the 13 SEER equipment is now delivering the performance of a 9.75 SEER. If you perform these tests regularly, you’ll find the average uncomfortable room has a duct loss of one-third to more than one-half of its design Btus.

The Solution
In a fantasy land, the answer would be to get the ducts out of the 140F attic, but that’s usually not an immediate option. The realistic solution is to add a blanket of insulation over the ducts. However, be careful not to add insulation with another vapor barrier, or you’ll be dealing with a moisture problem from the water that condenses between the two vapor barriers.

The best fixes are made by installing 4-in.-wide blankets of R-19 insulation. This is available from commercial metal building vendors. It sounds pretty extreme, but think about it: Code requires R-30 or better insulation in attics; why not spend a few extra dollars insulating the part of the building carrying the most expensive air? You’ll find R-4.2 and even R-6 duct insulation inadequate on long duct runs in attic areas.

The Hard Part
Improve the level of service you provide to your customers by asking comfort questions, and then take the time to test duct loss and identify system problems outside of the equipment. Look at the duct system. It takes a strong commitment to improve your work and delight your customers with superior effort and results. However, your commitment — and some simple calculations — will move you ahead of the rest of the industry.

To support you in this, I’ve prepared a free one-page report that contains the tests described here and is suitable to show your customers. Given a little education, they’ll allow you to improve the performance of their systems and increase their comfort during the hot days of summer — when they need all the comfort they can get.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring and improving HVAC system performance. If you're an HVAC contractor or technician interested in a no cost one page test report and procedure on how to measure duct loss, contact Doc at robf@nationalcomfortinstitute.com or call him at 800-633-7058.