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Commercial Rooftop Units

Commercial Rooftop Units

Planning a Rooftop Unit Changeout, From Nameplate to Crane Day

Most rooftop units sold to commercial contractors are replacements, and a replacement lives or dies on planning. The unit itself is only one line on the job. The curb, the electrical service, the gas connection, the crane, the roof penetrations, the controls, and the permit all have to line up on the same day, usually with a tenant who wants the space open again by the afternoon. This category holds packaged rooftop units from 3 to 25 tons from Daikin, Rheem, and Comfortstar, organized into three subcategories: Gas/Electric Rooftop Units, Heat Pump Rooftop Units, and Air Conditioner Rooftop Units. Below is the planning sequence we see work on real changeouts.

Step 1: Document What Is on the Roof

Before you price anything, get the following from the existing unit. Photos of each are better than notes.

  • Nameplate: model number, serial number, nominal cooling capacity, heating input if gas, voltage, phase, minimum circuit ampacity, and maximum overcurrent protection.
  • Curb: outside dimensions, height, and the location of the supply and return openings. Measure the curb itself, not the unit, because the unit may overhang.
  • Airflow configuration: downflow through the curb or horizontal with ductwork entering the side of the unit.
  • Gas: pipe size at the unit, where it enters the cabinet, and the presence of a shutoff and sediment trap.
  • Electrical: disconnect size and rating, conductor size, conduit route, and whether there is a service receptacle.
  • Accessories: economizer or barometric relief, smoke detectors, thermostat or building automation connection, condensate routing, and any hail guards or coil guards.

The model number on most commercial units encodes tonnage, heat input, and voltage. If you cannot decode it, send it to us with the serial number and we will help identify it.

Step 2: Size It, Then Check the Sizing

A like-for-like tonnage replacement is reasonable when the building use has not changed. If the space was repurposed, for example an office turned into a gym or a retail bay turned into a restaurant, a load calculation is worth the time. Oversizing a rooftop unit tends to cause short cycling and poor humidity control; undersizing leaves the space uncomfortable on design days. The table below shows nominal capacities and what this category currently carries at each size.

Nominal tonsNominal cooling (Btu/h)Efficiency rating usedHeating types listed hereVoltages listed here
336,000SEER2 / EER2Gas/electric, heat pump, cooling only208/230V-1, 208/230V-3, 460V-3, 575V-3
448,000SEER2 / EER2Gas/electric, heat pump, cooling only208/230V-1, 208/230V-3, 460V-3, 575V-3
560,000SEER2 / EER2Gas/electric, heat pump, cooling only208/230V-1, 208/230V-3, 460V-3, 575V-3
672,000EER / IEERGas/electric, heat pump, cooling only208/230V-3, 460V-3, 575V-3
7.590,000EER / IEERGas/electric, heat pump, cooling only208/230V-3, 460V-3, 575V-3
8.5102,000EER / IEERGas/electric, heat pump, cooling only208/230V-3, 460V-3, 575V-3
10120,000EER / IEERGas/electric, heat pump, cooling only208/230V-3, 460V-3, 575V-3
12.5150,000EER / IEERGas/electric, cooling only208/230V-3, 460V-3, 575V-3
15180,000EER / IEERGas/electric, cooling only208/230V-3, 460V-3, 575V-3
20240,000EER / IEERGas/electric, cooling only208/230V-3, 460V-3, 575V-3
25300,000EER / IEERGas/electric, cooling only208/230V-3, 460V-3, 575V-3

Not every brand is offered at every size and voltage. For example, 575V options here come from Daikin, and single-phase options stop at 5 tons. Use the filters or check the subcategory to see exactly what is in stock for your size.

Step 3: Curb Fit, Adapter, or New Curb

This is where changeouts get expensive if nobody checks ahead. You have three paths:

  1. Matching footprint. Some current units are designed to sit on the same curb as the manufacturer's earlier models of the same size. When the replacement is from the same family and the curb is sound, this is the cheapest and fastest option. Confirm with the manufacturer's curb dimension drawings, not just the tonnage.
  2. Curb adapter. When the new unit's footprint or duct openings do not line up, an adapter sits on top of the existing curb and transitions to the new unit. It avoids cutting the roof and reflashing, but it raises the unit height, which can affect clearances, screen walls, and gas and electrical connections. Adapters are specific to the old unit and the new unit, so you need both model numbers to order one.
  3. New curb. If the existing curb is rotted, undersized, out of level, or the roof is being replaced anyway, a new curb is the right call. It involves the roofer and usually adds a day.

Curb adapters and manufacturer curbs are listed in the Commercial HVAC category. Order them with the unit so everything arrives on the same truck.

Step 4: Electrical Service Check

Compare the new unit's minimum circuit ampacity and maximum overcurrent protection to the existing breaker and conductors. A higher-efficiency unit may draw less, but adding electric heat to a cooling-only unit or switching to a heat pump with backup heat can push the circuit well past what is there. Measure the actual voltage at the disconnect. A building on 208V service needs equipment rated for 208/230V, and a unit rated 460V will not run on it without a transformer. Note whether the disconnect is fused or non-fused and whether it is rated for the new load. If the electrician has to upsize the feeder, find out before crane day.

Step 5: Gas, Condensate, and Controls

For gas/electric units, compare the new heat input to the old one and confirm that the gas pipe size and supply pressure can carry it. If the new unit's gas connection is in a different spot, plan the repipe. Condensate drains need a trap per the manufacturer's instructions. On the controls side, decide whether you are reusing the existing thermostat, installing a new one, or tying into a building automation system, and check whether the new unit needs a different control interface. Smoke detectors and fire alarm shutdown wiring must be reconnected and tested per local requirements.

Step 6: Economizer and Code Items

Energy codes based on ASHRAE 90.1 and the IECC require air economizers on many cooling units above a certain capacity, depending on climate zone, and some jurisdictions apply this at replacement. Ventilation requirements also apply. Check the locally adopted code edition and ask the inspector if you are unsure. Economizers are model-specific and are listed in Commercial HVAC by the cabinet family they fit.

Refrigerant is the other code item. Current Daikin units here use R-32 and Rheem units use R-454B, both A2L refrigerants. Make sure the crew has A2L-rated recovery and evacuation equipment and leak detectors, and follow the manufacturer's installation instructions on leak detection and clearances. Older stock that does not list a refrigerant in its title should be checked on the spec sheet before you order.

Step 7: Crane, Rigging, and the Roof

  • Get the shipping and operating weights from the spec sheet. Crane size depends on the unit weight and the reach from the set point to the curb, not just the unit size.
  • Use the manufacturer's lifting points and spreader bars as shown in the installation manual. Many cabinets will rack or dent if lifted with chokers.
  • Confirm whether the crane can set up on the street or in a parking lot, and whether a lane closure or permit is needed. Early morning lifts with lots blocked off are common in retail.
  • If the new unit is heavier than the old one, or if the roof structure is questionable, involve a structural engineer.
  • Protect the roof membrane around the curb during demolition and setting, and plan for a roofer if flashing is disturbed.

Step 8: Lead Time and Ordering

The 3 to 10 ton range at 208/230V three-phase is generally the easiest to source. Larger tonnages, 460V and 575V configurations, and units with factory options such as stainless steel heat exchangers, coil guards, or specific drive packages can take longer. Lock in the unit and every accessory before you book the crane. Commercial units ship by freight, so arrange for a forklift or liftgate and inspect the unit before signing for it.

Which Subcategory Fits

Gas/Electric Rooftop Units are the choice when the existing unit burns natural gas or propane for heat. Heat Pump Rooftop Units fit all-electric buildings and owners moving off gas, typically paired with electric backup heat. Air Conditioner Rooftop Units are cooling-only units that can take an optional electric heat kit, common in warm climates and in buildings where heat comes from another source.

Frequently Asked Questions

Will a new rooftop unit fit my existing curb?

Sometimes. Compare the manufacturer's curb drawings for the new unit with measurements of the existing curb, including duct opening locations. If they do not match, a curb adapter made for that specific old and new combination is usually the answer.

What information do I need to order a curb adapter?

The full model numbers of the existing unit and the new unit, plus measurements and photos of the existing curb. Adapters are not universal.

Can I replace a 7.5 ton unit with a 6 ton unit to save money?

Only if a load calculation supports it. Commercial loads change with occupancy and lighting, so some buildings are oversized, but cutting capacity without checking risks an uncomfortable space on design days.

Do I need a crane for a 3 ton rooftop unit?

Usually, unless the roof is low and there is safe access for another lifting method. Check the unit weight on the spec sheet and your company's rigging policy.

Can I reuse the existing disconnect?

Only if it is rated for the new unit's voltage and current and is in good condition. Compare its rating with the new unit's maximum overcurrent protection.

How far ahead should I order?

Check availability as soon as the unit is selected, and order before booking the crane. Larger tonnages and 460V or 575V units can take longer than common sizes.

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