When I talk to them about it I let them know they will get better AC/heat if they change the filters. Since they are responsible for the utilities they usually make sure to do it.

There are some renters who have a hard time computing that information in a simple way. Anytime I inspect a property I check the filters and remind them. If there is some maintenance I need to do I check the filters even if my maintenance is not for the HVAC unit.

It doesn’t take me long to check and I note if I need to go buy a filter. I charge the tenant since it is in the lease it is up to them and that I charge for it when I have to do it.