You have entered into the contract with the builder.
This is who you have the right to sue.
You need to get a report form the installers to confirm what they have said and how they believe it happened.
Then you can sue the builder that you contracted with.
You do not have a contract with the aircon installers.
If you wish a call, I am sorry but you will have to elect to have a call and it is a further fee to what you have already paid.
I am happy to continue here though if you wish.
Once you have the evidence from the aircon people then you can seek to get the builder to replace it or sue them for the cost.
You can also lodge a complaint with the Department of Commerce.
If you wish to have a phone call to discuss this in more detail please elect a call or I can send you a request.
I hope this makes sense and is of assistance. If you have found my answer to be helpful, please rate me five stars so I can get paid for my services to you. You rating my work 5 stars allows me to continue to provide a service to those who require it.
I am happy for you to ask further questions on anything you wish more detail on.
Let me know if I have answered your question today.