New regulations from the City of San Antonio with regards to my mechanical permit tripped me up this week. Turns out there are several new codes that took effect January 1, 2010 which affect new central air conditioning systems in remodels. San Antonio is now requiring a duct blast test to be performed by a third party licensed mechanical contractor. After having my new system installed and inspected for rough-in we had to bring in another contractor to test the duct-work for leaks. Since you would never know if there were any leaks until the system was turned on at the time of final inspection they have come up with this cool way to test your contractors work before you hang the drywall. 
A fan is connected to the return air box and all registers are sealed with tape. They then crank up the fan and measure at one of the grills for pressure. The fan measurement vs. its affect on the duct pressure determines efficiency. Any leakage is shown on the computer and would indicate a loose seal somewhere that may need more tape or “pookie.” Before this technology existed we would wait until passing final inspection, turn the a/c on and manually feel around the joints for cool air. Leaks in your ducts cost big bucks on utility bills, especially in hot climates like Texas. If you are remodeling a house its now mandatory to have this test performed, the results are sent to the local power company CPS, and you wont get your electricity turned on without it. If you haven’t checked your system lately it might be worth a trip to the attic, one neighbor to a rehab I did asked for my help last summer. Upon inspecting her system my A/C guy saw that she was losing all the cool air into her attic as the tape from the coil/furnace had come completely unglued. Today we passed the duct blast test as well as rough-in for mechanicals.