On a one to four unit residential appraisal the value comes out of the sales comparison approach, so nothing you spend gets credited on the basis of what it cost. Your receipts have no line in the grid. The appraiser picks closed comparable sales and adjusts for differences, and your $18k of mechanicals only shows up if it produces a difference the grid can express.
Which means you answered your own question in the setup. If every comp already has a modern panel and copper, your mechanicals buy you comparability and nothing more. They keep the appraiser from adjusting you downward or from rating the condition lower, which is worth real money in the negative direction, and they add nothing on top. That work belongs in your scope because the alternative is a C5 condition rating or a lender who won't lend against galvanized supply, not because it lifts the number.
What does move the grid is anything with its own adjustment line. Gross living area, bedroom and bathroom count, garage, basement finish, and a broad condition and quality rating that jumps in steps. Two examples that behave differently: adding a legally permitted third bedroom in a comp set of three-bedroom houses can move the number materially, while going from a good kitchen to a great kitchen usually moves the condition rating not at all.
The ceiling is your comp set. Spend past the best comp and the appraiser has nothing to compare it to, so the money sits in the property with no financing against it.
One thing to watch given the mechanical scope: added square footage or converted space that isn't permitted often gets excluded from gross living area, and permitting rules and what counts as habitable area vary by jurisdiction. If any of the $18k triggers a permit, pull it and keep the final inspection card. An underwriter who spots unpermitted work can kill the refinance outright, which is a worse outcome than a flat appraisal.
