I understand I’m supposed to change the filter- or clean it if it’s a permanent filter- once a month. Our heating cooling system is brand new, but the filter that came with it is so inefficient it might as well be chicken wire! From what we’ve seen so far it hasn’t needed changing in the 5 months we’ve been living in our new house!

We brought with us two portable air filters that do an admirable job, and we clean them regularly.

I replace my filter about every 3 months. I write the install date on the filter so I know when it needs to be tended to.

If you change the air filter appropriately, and use the proper filter, you don’t ever have to clean the rest of the HVAC system.

“Cleaning the ducts” became popular in the late 1990s to generate more revenue for HVAC companies.

If you don’t change the filters; the system has to work harder. I change mine every three months and use a 1700 to 1900 filters

I have never cleaned my system and I replace the filters every 6 months.
Filters are like vacuum cleaner bags. At first they don’t restrict air too much because they’re clean but they also let more dust through. As the filter gets dirty, it filters out a higher percentage of dust.
On Diesel engine trucks, it is not advisable to remove the air filter to inspect it because, in doing so, more dirt gets into the engine, shortening it’s life. So vacuum indicators are now used that measure how much restriction the filters create under load, then lock into position to indicate the inches of vacuum the filters are creating by being dirty. When the service limit is reached, only then is the filter removed and replaced. So a dirty filter in your HVAC can do a better job of cleaning the air if it isn’t replaced too often.