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  • Alex Mathers

Challenges with Commissioning Package Units (RTUs)


A Package Air Handler Unit or “Package Unit” or “Rooftop Unit (RTU)” for short is a self-contained HVAC system. It is called a “packaged unit” because it includes all the components required for heating, ventilating and air conditioning:

·        Evaporator coil

·        Condenser coil

·        Compressor

·        Gas Heat (if not a heat pump)

·        A Fan (Blower)

·        An intake for outside air

 

Compared to a traditional split system which has an indoor section connected to an outdoor unit by line-sets that run between the two, a packaged system contains the “indoor” and outdoor sections in one self-contained cabinet that is located outdoors.

 

Commissioning a package unit is an essential step to ensure its proper functioning, efficiency, and longevity. However, commissioning can also present several challenges due to the complexity of these units, the expectations of the engineer and sequence of operations and the various factors involved in their proper functioning.

 

Before diving into the commissioning process, it's crucial to understand the basic components and functionalities of a package unit. 

The main components are:

·        Fan/Blower: Moves air through the system.

·        Filter Section: Cleans the incoming air.

·        Coils: Heat exchangers that heat or cool the air.

·        Dampers: Control airflow.

·        Control Panel: Manages the unit operations and communicates with a central system.

 

The typical functions of a package unit are:

·        Air Filtration: Removes impurities from incoming air.

·        Temperature Regulation: Heats or cools the air as required.

·        Humidity Control: Maintains optimal moisture levels.

·        Air Distribution: Circulates conditioned air throughout the space.

 

A typical package unit is anywhere from 5-30 tons, and can occasionally be over 100 tons. These larger ones are often called “Box Cars” because they’re as large as a railroad box car. A typical package unit only has a single supply fan used for single or multi zone spaces with a relief barometric damper to prevent the buildup of building pressure during the economizer mode.


Challenges

 

The first issue with commissioning package units is the disconnect between what the unit is factory programmed to do and the specific application for which the unit is being used.  It is important for designers and engineers to understand that package units are not custom air handlers, and the control points, sequences and integrations are not customizable nor are they vendor agnostic.  A sequence that works for one project line does not necessarily work for all manufacturers. What comes from the factory using their application specific controllers is not easily manipulated to fit a custom sequence.  The integration of package units in a BMS system involves considerable manual effort. Various aspects of automation such as operating modes, sequences, interlocks and alarms have to be reproduced in the field for the visualization and management of the package unit in the BMS.


In other words – package units do what they want to do, not what you specify.

 

Therefore, it is very important to work with MEOR, mechanical contractor, controls contractor, and the package unit vendor to ensure what is indicated in the plans can actually be done. Many times, the boiler plate sequence cannot be accomplished with the selected unit.

 

For example, SAT Control with Heating & Cooling for VAV units can become challenging. Unlike custom air handlers, the internal controls cannot seamlessly switch between heating and cooling to meet SAT.  These units all rely on some form of reference to trigger Heating/Cooling mode. (Sometimes it’s OA Temp, sometimes its RA Temp). Then once in each mode, they control to a certain DAT (Ex: 55 Cooling or 95 Heating).  Also, when a SAT Reset or Duct Static Pressure reset is required per the MEOR, application specific programming from controls is needed for this type of control and should be included in a control’s submittal. These reference temperatures and Supply temperatures will be adjusted based on the current reset SOO. 

 

Package units also need heavy submittal reviews to ensure the proper options are being selected. This includes communication protocol, drive types, pre-wired controls, optimization packages, OA flow measuring, electrical heaters etc.

 

Installation

 

Often, there are issues with controlling sensor placement. This is due to several factors.  One is that the package unit manufacturer is limited to what they can install within the unit. Sometimes SAT sensors are only installed after the cooling coil from factory. Unfortunately, this will not allow for proper control of the heating element even further downstream. In addition, the unit field installation is by the mechanical contractor, not a controls contractor and the Duct Static Pressure sensor tubing is often just thrown into the supply duct from the unit. It is important for the EOR to specify the location of the Static Pressure Sensor pickup for proper control. This goes with the Building Pressure Sensor as well - we always recommend specifying the location of this sensor on the mechanical or control drawings.

 

Startup

 

Once installed and started up, the CxA needs to work with Startup Techs as it’s very hard to determine if the unit is properly configured for the application. Configuration options include Constant Volume, Variable Air Volume, Duct Static Control, VAV Single Zone, VAV Multizone etc. The CxA must ensure on Single Zone VAV or Constant Volume units that the VFDs are hard programmed at maximum speed for their TABed value.

 

TAB

 

TAB verification of airflow is much more difficult with a package unit compared to an air handler.  While an air handler has easy door access to coil and fan sections, a package unit does not.  The OA, supply air and return air sections are not easily accessible and panels may need to be removed or holes drilled to take traverse readings.  This often leads to inaccurate TAB values.

 

Functional Testing

 

Our functional testing of package units have uncovered a number of issues.  Some of these include:

1.      Software interlocks are unreliable. 

2.      The BMS rarely knows the true status of a package unit.

3.      Fan speed command by the BMS is not viable.

4.      Airflow values are not scaled properly.

5.      Boilerplate BMS graphics do not match actual package unit configurations.

6.      Package unit controller loses power when the condensate float switch is tripped.

7.      Fan speed setpoints for various stages of cooling and heating appear to be different and arbitrary across package units.

8.      Alarms from the package unit controller need to be properly programmed into the BMS.

 

Package units are seemingly simple, fully contained, HVAC systems.  However, commissioning a package unit is a meticulous process that involves thorough testing, calibration, and verification to guarantee its optimal performance and efficiency. Following these warnings diligently ensures that the package unit operates as intended, providing reliable and comfortable air conditioning while maximizing energy efficiency and lifespan.

 

Remember, proper commissioning not only ensures immediate functionality but also sets the stage for long-term performance and reliability of the HVAC system. Always refer to manufacturer guidelines and industry standards while commissioning to achieve the best outcomes for your package unit.

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