Rooftop Unit Replacement: The Job Is Mostly Logistics

Why the roof changes everything

A packaged unit is the whole system in one cabinet: compressor, coils, blower and heat, bolted to a curb on the roof and fed by two duct openings cut through the deck. Phoenix builders put them up there by the tens of thousands, because a low-slope roof is free real estate and a block-and-stucco tract home has no basement and often no side yard to spare. That decision was made in 1994 by someone who never had to change the unit. Now it is your turn. Replacing a packaged rooftop unit is not a harder job than replacing a split system. It is a different job, and most of what can go wrong is decided before the new unit leaves the ground.

What changes the scope

The cabinet is the smallest variable. The rest is logistics.

• Footprint match. New units are rarely the same dimensions as the one they replace. If the new base does not land on the old curb, you need a curb adapter, and it has to be sized before ordering, not discovered on the roof.
• Crane access. A crane needs a place to set outriggers, clear overhead lines and a swing path from the truck to the curb. A corner lot with a wide driveway is easy. A cul-de-sac with a mature mesquite and a service drop across the front is not.
• Roof condition. Foam, built-up and tile roofs each handle a crew differently. Foam that has baked at 155 degrees for fifteen summers can be brittle around the curb. If a roofer needs to come first, that goes in the schedule.
• Fuel and power. A gas-electric unit needs a gas line up the wall with a drip leg and shutoff. Any unit needs a disconnect, a whip and a breaker sized to the new nameplate, which may differ from the old one. Swapping gas heat for a packaged heat pump changes both pictures; the gas pack replacement guide covers that decision.
• Duct connection. The supply and return plenums under the curb are the most neglected sheet metal in the house. If they are rusted, undersized or leaking into the attic, a new unit inherits the problem.
• Sun exposure. A unit on a west-facing pitch with no parapet runs hotter than one tucked behind a wall. That affects orientation, clearance and how hard the condenser works at four on a July afternoon.

The crane day, step by step

Rooftop replacement is a choreographed morning. The old unit is disconnected, the gas capped, the power locked out and the refrigerant recovered before the crane arrives. The crane lifts the old unit off, sets the new one on the curb or adapter, and leaves. Everything after that is a crew on a roof: sealing the curb, connecting ducts, wiring, gas, condensate drain, thermostat wire, then startup and charge verification. The crane is usually on site for about an hour. The crew is there most of the day. Nobody sets a unit in a monsoon outflow, and crane operators will not swing a load in high wind, so a July afternoon can slip to the next morning.

Curb, ducts, roof and utilities

A curb adapter is a fabricated transition that sits on the existing curb and matches the new unit's base, supply and return openings. Done well, it is airtight and insulated. Done badly, it leaks conditioned air into the attic through the two biggest holes in your ceiling. Ask to see it. Ask whether the plenums below the curb will be inspected and whether the transition will be sealed with mastic rather than tape. This is the moment when duct sealing is cheapest, because the top of the system is already open. Haboob dust finds every gap, and a leaky curb pulls attic air into the return every time the blower runs.

Roof, gas and electrical

The crew walks the roof before quoting. Soft spots, ponding near the curb, cracked foam and old penetrations from an abandoned evaporative cooler all get noted. The gas line is pressure-tested, the disconnect and whip are checked against the new unit's requirements, and the breaker is confirmed. If your panel is at capacity, that is a licensed electrician's task, quoted separately and scheduled ahead of the crane. We will not reuse a corroded disconnect because it is convenient.

Questions to ask any bidder

• Will you measure the existing curb and confirm the new footprint before ordering?
• Who provides the crane, and is it included in the scope?
• Will the supply and return plenums be inspected and sealed?
• Is a load calculation included, or are you matching the old tonnage?
• Does the quote include the disconnect, whip, gas flex, drip leg and condensate drain, or are those "as needed"?
• What happens if the roof needs repair around the curb?
• Is the price flat-rate before work begins?

A bid that arrives without anyone climbing the roof is a guess dressed as a number. The estimates and financing guide explains what a proper proposal contains and how financing on approved credit works.

When not to replace yet

A packaged unit with a failed capacitor, a worn contactor or a leaking drain pan is not a replacement candidate. It is a repair. A unit with a cracked heat exchanger, a dead compressor or R-22 refrigerant is a different conversation, and the repair-vs-replace guide walks that math honestly. A unit that was never maintained will fail younger; seasonal maintenance is the cheapest insurance a rooftop owner can buy, because nobody else is ever going to look at the thing.

Why the scope needs a site visit

A web page cannot size a system, and it certainly cannot see your roof. Curb dimensions, crane access, duct condition and gas and electrical readiness are physical facts, measured on site. Bleuwave's evaluation is a roof walk and a load calculation, quoted flat-rate before any work begins, with nothing ordered until the numbers check.