re: Pay this bill or naw?

Talk to owner. Ask why he doesn't stand bruins his work. Six months old is bullshite. If you have to pay, make sure you give a non-biased account t on the web. Let him know your intent beforehand.

per trane website for ex.

WHAT COMPANY WILL DO: Company may request proof of Product purchase and/or installation in order to provide Product parts under this limited warranty. As Company’s only responsibility and Purchaser’s only remedy under this limited warranty, Company will furnish a replacement part to the licensed HVAC service provider, without charge for the part only, to replace any Product part that fails due to a manufacturing defect under normal use and maintenance. The Purchaser must pay for any and all shipping and handling charges and other costs of warranty service for the replacement part. If a Product part is not available, Company will, at its option, provide a free suitable substitute part or provide a credit in the amount of the then factory selling price for a new suitable substitute part to be used by the Purchaser towards the retail purchase price of a new Company product. Any new Product purchase shall be at Purchaser’s sole cost and expense including, but not limited to, all shipping, removal, and installation costs and expenses.

Scan a copy and tell them to get fricked. (If this hasn't been said already)

Probably not