I have not seen clarification from the IRS on repair or replacement vs original installation, although to be honest I have not looked for such clarification.
The auditor is supposed to use the audit manual, but might have their own opinions. The audit manual is supposed to follow the regulations, but we know of cases where the tax court has ruled that auditors were wrong. The regulations are supposed to follow the law, but we know of cases where courts have ruled that the law means something different than what the IRS wanted it to me. So if someone claimed a credit for a replacement or a repair, and was audited, they might need to pursue the matter to a higher court to get a favorable answer.
