Have the contractor invoice you separately for the water heater and the installation. Better yet, buy the water heater yourself, then pay a plumber to install it. Then you have two separate invoices that you can deduct the under $2500 expense.
De Minimis Safe Harbor Up to $2500. You may use the safe harbor to deduct amounts up to $2,500 per invoice or item (as substantiated by invoice). https://www.irs.gov/businesses/small-businesses-self-employed/tangible-property-final-regulations
