Getting that “animal smell” out of my house

This seemed like the most appropriate place to ask for some tips/advice. We are renting, and the people who lived here before us had their dog/dogs inside the house. The carpets were cleaned, and for the most part the smell isn’t overwhelming. But it still lingers. Especially in the air vents of the house. Every time the heat kicks on the smell becomes overwhelming and then starts to dissipate as they run. I’m at a loss other than moving, which financially isn’t something we can do right now. We have lived here since November and despite us only having a cat the smell is sticking. Any ideas on how I can get the smell from the vents? And tips/tricks? Also, the carpets need to be redone as well...so any tips you have for that would be great as well. Some days you can’t smell it at all and other days (I’m noticing as it starts to heat up outside) it’s more noticeable.

Landlord isn’t much help either, although before long I am going to have to get her involved if I can’t figure it out.