You can file a free case file with the attorney general of your state not the one they reside in. Then there is a way to go after the installer, even if there was NO paperwork, you can hold him liable. Personal, dab, s corp, doesn't matter. I won all of my cases, statefarm, dodge Chrysler and non paying customers. I received payment in full on all but the residential before actually going to the final court date. The residential took 10 days. Non of them cost me a dime other than my time and filing fees. If you have a case they will represent you , or yank their license. In one case against a landscaper here in town, they showed up at his house, loading equipment seizing everything on his property. He decided to pay on the spot when he raced back home. Little over $10,000.
However if you don't have a case or you loose it only cost you if you hire someone. If your claim is real bogus the judge will rule in their favor " with prejudice " this means you no longer have any rights to sue again pertaining to the items in that claim. If not you can try again with professional legal representation.
This is also how I learned here in Michigan that a verbal agreement is binding, as long as there was a description, price and acceptance stated. Hand shake or not. A working unit and working heating system is a description, your price, and the fact that you didn't call the police during install was a confirmed agreement/acceptance. Now he has to fulfill the working system AND any claimed burn times. Look up your state attorney general and claims, you can do it all online. If it goes to court, an s corp cannot rep themselves, they have to have an attorney by law. So it's costing him not you. The rules change after the first hearing.