We have a nice new home that had over 40 HVAC vents full of construction debris / drywall dust. The DuctWorks tech was very professional, and spent an entire day thoroughly cleaning every duct, the furnace, and blower.

Before he left we tested the system (heat only as it was cold outside, and everything looked good). My HVAC pro came out the next day to install a humidifier and electronic air cleaner, and said the furnace looked great.

When he was done we tested everything (heat, fan, cool, etc.). As soon as he tried to turn on the AC, the compressor made a terrible chattering noise, and began to short cycle. This would not quit until we flipped both circuit breakers to the furnace and AC unit. Two hours of troubleshooting later, it turned out that the DuctWorks tech had misswired the AC line to the compressor, with a 120VAC live circuit at the furnace control board (flipped two wires).

End result was a blown furnace control board, and a blown NEST thermostat. Luckily both were still under warranty, but overall this was quite the ordeal, and I had to buy another cheap thermostat to use while NEST sent a replacement.

While in the end everything is OK, it led to me missing some work, and extensively troubleshooting with my HVAC pro (who initially recommended DuctWorks) to figure out what was going on. Not sure if we will use DuctWorks in the future, but I know I will be verifying the wiring before and after any future duct cleaning.