How can I tell if my (h-vac) system needs professional duct cleaning?

There are a few things you can check yourself:

1. Check the air filter. Whatever debris is on the filter may only be a small portion of what could actually be in your return air duct. If the filter on your furnace is dirty, that’s an indication that duct cleaning is required.
2. Check inside the return air ducts. Generally the return air ducts are the larger rectangular vents located on the bottom of a wall. If the air ducts are located on the floor, remove the vent cover and look inside with a flashlight. If the ducts are attached to the wall, they are usually held in place with screws which are quite easily removed.
3. The return duct system draws the air from inside the house, then brings it back to the furnace where it is filtered and re-conditioned before recirculating inside the home. The return ducts usually contain the most dust and debris.
4. Check your supply/heating vents as well. These are almost always located on the floor. Inspect the interior of these ducts to see how much debris is accumulating. If you find them dirty, then duct cleaning should also be considered.