Make sure your lease arrangement is very clear. In most arrangements, tenants are responsible for repair and maintenance, while landlords are responsible for replacement. This guarantees disputes, unless the conditions are spelt out. The lease agreement must specify that tenants engage qualified companies to perform preventive maintenance, and what ‘qualified’ means. Also specify a threshold dollar amount for above which the HVAC system (or a component) is deemed to need replacement. If tenants need HVAC outside of regular business hours, there must be an agreement on after-hours charges in this situation.