Seller And Hvac Company Failed To Honor Repair Rider. Who Do I Target?

Location: Massachussets

I am looking for some legal direction on determining who you would send a demand letter too in the following situation:

I purchased the top floor of a two-unit condo conversion in Massachusetts in April 2025. During the inspection, we found the HVAC system was failing. The seller and I signed a contract rider stating the seller would fix the heating system prior to closing.

At the final walkthrough, the seller provided no proof of repairs and used the same company that did the original (faulty) install. By December, the system was failing. I’ve since had three independent HVAC companies inspect it; all agree the unit is undersized, the ductwork is insufficient, and the original install was botched. Quotes for a fix are $12k–$18k.

Fast forward to December, I looked into three different companies that all said the same thing, the unit is undersized, needs new ductwork and is not running efficiently. Quotes ranged between 12-18K

I have tried to work with the HVAC company on multiple occassions and they are horrible. However, the most recent interaction they agreed on (which I have in writting) doing some repairs that should of been done as part of the closing. But thye have since ghosted me and now wont respond to me.

I want to send a demand letter before bringing someone to court.

My Questions:

1. Who is the primary target for the demand letter? The seller (for breach of contract/rider), the HVAC company (for deceptive trade practices), or both?
2. Do you have any other advice or suggestions?

Thank you all so much!