We are a plumbing & AC/Heating company- can we file liens on customers if they do not pay us?

The short answer is, yes. 

In Ohio, mechanic's liens are a remedy that can be used by any person "who performs work or labor . . . or furnishes material in furtherance of any improvement undertaken by . . . contract . . . with the owner . . . of any interest in real estate." Assuming you are under contract with the owner, then plumbing work looks to constitute an "improvement" to the property, meaning a mechanic's lien could be placed on the property in the event of non-payment.