5.7 Packaged Rooftop Units 2 to 25 Tons: Curbs, Access & Economizers

Packaged Rooftop Units 2 to 25 Tons: Curbs, Access & Economizers

PSI's content outline names the range explicitly — "roof top units from 2-25 tons." That band covers essentially all of light commercial Michigan: strip retail, restaurants, small offices, churches, clinics, and school additions. Everything in this section is testable and is also exactly what a Michigan mechanical inspector will look at on a rooftop final.

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What a Packaged Rooftop Unit Contains

A packaged rooftop unit (RTU) puts the entire system in one weatherproof cabinet on the roof:

• Compressor(s) — often two circuits on units above about 7.5 tons, for stepped capacity.
• Condenser coil and condenser fans.
• Evaporator coil, filter rack, and supply blower (belt-drive or direct-drive ECM).
• Heat section — gas-fired heat exchanger with induced-draft burner (the common Michigan choice), electric resistance heat, or a reverse-cycle heat pump circuit.
• Outdoor-air section — fixed minimum-position damper or a full economizer with outdoor and return dampers and an actuator.
• Controls — a unit controller, often with BACnet or a proprietary building-automation interface.

Nominal capacity in this class steps 2, 3, 4, 5, 6, 7.5, 10, 12.5, 15, 20, and 25 tons. One ton = 12,000 Btu/h, so a 25-ton machine is a 300,000 Btu/h cooling load.

Gas heat sizing check

Rooftop gas heat sections are cataloged by input. Output is input times thermal efficiency (commonly 80% on standard efficiency units). A 10-ton unit with a 200,000 Btu/h input, 80% efficient section delivers:

$200{,}000 \times 0.80 = 160{,}000 \text{ Btu/h output}$

At 4,000 cfm, the temperature rise is:

$\Delta T = \frac{160{,}000}{1.08 \times 4{,}000} = 37^\circ\text{F}$

That must land inside the rating-plate rise range, exactly as on a furnace.

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The Roof Curb

The curb is the single most consequential installation detail on an RTU.

• It is a formed, insulated steel frame that is set on the roof structure, flashed into the roofing system, and topped with a gasket the unit seats on.
• It carries the structural load of the unit. Confirm with the structural documents or the design professional that the roof framing can carry the operating weight plus snow and maintenance loads. Michigan snow loads are not a rounding error.
• It forms the supply and return duct transition, with an internal divider separating supply from return. A curb installed backwards produces an RTU that short-circuits supply into return, and the symptom — low airflow, low ΔT, high head pressure — looks like a dozen other faults.
• It must be set level. A curb out of level drains condensate the wrong way, stresses the cabinet, and can pull the flashing open.
• A curb adapter is used when replacing an old unit with a different footprint. Adapters must be insulated and sealed; an uninsulated adapter in an unconditioned rooftop environment will sweat and drip into the ceiling below.

Set the unit with a properly rigged crane pick, protect the coils, and remove shipping brackets before start-up.

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Access, Working Space, and Guards (MMC Section 306.5)

The Michigan Mechanical Code devotes a specific section to appliances on roofs:

These requirements dovetail with NEC Article 440.14, which requires the equipment disconnecting means to be within sight from and readily accessible from the air-conditioning or refrigerating equipment, and with MIOSHA Part 45 fall-protection rules that apply the moment your crew works at the roof edge.

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Economizers

An air-side economizer modulates outdoor-air, return-air, and relief dampers to cool the building with outdoor air whenever outdoor conditions permit, cutting compressor run time.

Michigan's climate makes economizers very effective. In Climate Zones 5A and 6A, outdoor air is below the supply-air setpoint for a large fraction of the occupied year, so an internally loaded building can run in free cooling for hundreds of hours.

Control strategies

Code and commissioning

The Michigan commercial energy code requires economizers on cooling systems above published capacity thresholds, with defined exceptions. Two compliance points that get missed:

1. The economizer must include a relief path — a barometric relief damper, a power exhaust, or a return fan — so the building does not over-pressurize when the outdoor damper opens wide.
2. The economizer must be functionally tested and documented at commissioning, and the dampers must fail to a defined position on power loss or actuator failure. Economizers have a deserved reputation for failing silently; a stuck-open damper in a Michigan January will freeze a coil, and a stuck-closed damper simply wastes the investment.

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Rooftop Details That Fail Inspections

• Condensate. The drain must be trapped for a draw-through (negative pressure) evaporator section, sized to the cabinet's static pressure, and routed to an approved discharge. Discharging onto the roof membrane is a violation in most jurisdictions and destroys roofing. In Michigan, guard against freeze-up at the trap.
• Gas piping. Rooftop gas piping must be supported above the roof surface on approved supports, protected from physical damage, sized under the 2021 IFGC, and provided with an accessible shutoff valve and a sediment trap at the unit. Do not lay black iron directly on a membrane roof.
• Venting. The gas heat section on a packaged unit is typically factory-vented through a listed cap on the cabinet. Do not field-modify it. Combustion air is drawn through the cabinet's rated openings.
• Electrical. Verify nameplate MCA and MOCPD against the installed conductors and overcurrent device (NEC Article 440), confirm the disconnect is within sight, and confirm phase rotation on three-phase scroll compressors — a scroll run backwards is loud, pumps nothing, and will be destroyed if left running.
• Vibration and clearance. Maintain the manufacturer's clearances around the condenser coil face. Two RTUs set too close recirculate each other's condenser discharge and trip on high head pressure all summer.