Preheat

This checklist identifies Key Commissioning Test Requirements and Key Preparations and Cautions for an air handler’s preheat section. When writing a test, use this checklist to help ensure that these key areas have been covered. The buttons following the checklist items link to supporting information within the Functional Testing Guide and the Control System Design Guide.

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Control System Design Guide

Key Commissioning Test Requirements

• ___ 1. During design review, the following items were addressed to avoid issues during functional testing:
  • a) The reheat control valve range matches the requirements of the control sequence and does not overlap the range of any other elements served by the same signal.
  • b) To prevent unintended simultaneous heating and cooling, the control element range matches the requirements of the control sequence and does not overlap the range of any other elements served by the same signal.
  • c) The functionality of the design features to ensure that the coil can safely and reliably deal with subfreezing air are verified.
• ___ 2. The following "prefunctional" checks were completed and verified prior to performing a functional test (note that this is not a comprehensive list of all prefunctional checks):
  • a) Control valve leakage testing reveals no detectable leakage. Valves meet maximum leakage tolerances set in the specifications.
  • b) The coil is flushed and pressure tested (required in some instances).
  • c) All safeties, interlocks, and alarms are programmed (or hard-wired, if applicable) and function correctly.
• ___ 3. The preheat control is integrated with the air handling unit per the design sequence of operations. In particular, correct integration with the economizer to prevent simultaneous heating and cooling and the minimum outdoor air flow rate to minimize the potential for heating unnecessary volumes of ventilation air.
• ___ 4. Correct shut down sequence for a dedicated heating source that serves preheat coils during warm weather is verified.
• ___ 5. The coil capacity is verified if required by the specifications. Capacity test results are evaluated in the light of the accuracy of instrumentation and the actual conditions at the time of the test.

Key Preparations and Cautions

• 1. It is important that the design and test plan recognize the difference between preheat, reheat, heating, and warm-up elements and functions.
• 2. Testing preheat coils with subfreezing entering air conditions subjects the coil to the danger of freezing if it has not been properly applied and set up. If there is a problem during the test that causes the preheat element to fail to perform, the system and building can be subject to freezing conditions. Therefore, testing should proceed in a logical sequence that verifies primary interlocks and safety systems prior to verifying more complex control processes, integrated control functions, and tuning loops.
• 3. Some test procedures, either by design or by failure of the element under test to perform as intended, can cause air handling system discharge temperatures to become significantly elevated above normal. This can pose several problems including:
  • a) Occupant discomfort
  • b) Disruption of the process served by the system and potential damage to product
  • c) Inadvertent activation of fire dampers, heat detectors, and/or rate of rise detectors. Fire dampers may shut as well as trigger false fire alarm and building evacuation. This is of greater concern with steam or electric preheat devices.
• Test plans should provide for these contingencies by taking steps such as disabling key fire detection elements for the test and ensuring that fusible links have been selected to tolerate any temperature that can be produced in the system.
• 4. Overly rapid stroking of valves during a test process can cause water or, steam hammer problems in the piping systems serving the preheat element. If these conditions arise, it indicates the PID control loop is not tuned properly.
• 5. Functionally testing a large electric preheat element during the summer months while the cooling plant is in operation can cause several significant problems including:
  • a) Distribution system load conditions that exceed design that can trigger trips in the primary switchgear resulting in unscheduled and unanticipated outages
  • b) Demand peaks well in excess of those that would normally be encountered during normal operation due to the demand that the coil places on the system concurrently with the refrigeration equipment.
• 6. Simulating a real preheat load in the field is a practical impossibility. In most instances where capacity verification is required, it will be verified based on achieving a target temperature rise above the current ambient conditions, or based on documenting coil performance under the given conditions and then modeling the coil under those same conditions. A capacity test may be limited by the elevation of the actual inlet temperature above the design inlet temperature, the temperature of the heat supply source, and the length of time and conditions under which is it possible to operate the system with an elevated discharge temperature. Often, this condition can be used to simultaneously load test the cooling system although the load is a sensible load rather than a sensible and latent load. An example of one of the more limiting test situations would be load testing a preheat coil on a hot day with a coil supplied by a low temperature water system. Regardless of when an initial test is performed, a preheat coil should always be checked for proper operation under normal seasonal conditions.
• 7. Valve leakage tests and tests that are targeted at verifying valve stroke, spring range, and sequencing should be conducted with the pumping system operating at its peak differential pressure. The differential pressure across the valve plug can have a significant impact on the close-off rating and shift the operating spring range of the valve.

Page last updated: September 11, 2006