Packaged Rooftop Units

What it is

A packaged rooftop unit is a self-contained heating and cooling unit, usually set on a roof curb above the space it serves. Supply and return air pass through the curb into the ductwork. Light commercial RTUs are the workhorse of retail, office and school buildings. Efficiency for commercial units is rated under AHRI 340/360 for most commercial sizes and AHRI 210/240 for the smallest units. Energy codes, including ASHRAE 90.1, set minimum efficiency and economizer requirements.

How to identify

The data plate shows the model, type (gas/electric, heat pump, cooling only with electric heat), nominal tonnage, refrigerant, electrical data and gas input if any. A gas/electric unit has a flue outlet and gas connection. A heat pump RTU has a reversing valve and defrost control. An economizer appears as a hood with dampers on the return or outdoor air side.

RTU types

Economizer basics

An economizer opens an outdoor air damper to cool with outdoor air when it is cool and dry enough, so the compressor runs less. Its controls compare outdoor air with return air or a fixed set point, by dry-bulb temperature, enthalpy or both. ASHRAE 90.1 and local energy codes decide when an economizer is required and which control types are allowed in each climate zone. Many codes also require fault detection and diagnostics on economizers.

Key points

• Commercial efficiency ratings include EER and IEER. IEER weights part-load performance. Check the AHRI certificate for the exact model.
• Common RTU counter items are belts, filters, blower and condenser fan motors, contactors, capacitors, economizer actuators and sensors, ignition controls and gas valves.
• Curb adapters let a new RTU sit on an old curb with a different footprint. Measure the curb and supply/return openings.
• Electrical and gas sizing come from the data plate and the installation manual.

Common mistakes

• Ordering a replacement RTU without checking the existing curb dimensions.
• Bypassing a failed economizer by leaving the outdoor damper wide open.
• Using residential SEER2 language for commercial units rated in EER and IEER.
• Replacing economizer sensors with a type the controller cannot read.

Governing standards: AHRI 340/360 (commercial and industrial unitary equipment); AHRI 210/240 (smaller units); ASHRAE 90.1 (energy standard for buildings).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.