who is responsible for regular HVAC maintenance?

Read your lease. The responsibility is one whomever the lease says. There are no statutes or judicial decisions establishing who does it or who pays for it. Many leases say that the landlord does it and the tenant pays for it. If you don't have a lease or the lease doesn't say, then lawyers would look to things said during the negotiations, what has been done in the past, and what happens with other tenants in the same project, if there are any. If there really is nothing, then whoever needs and wants it fixed is going to have to get it done and pay for it. Dana  Sack