Reheat

This checklist identifies Key Commissioning Test Requirements and Key Preparations and Cautions for reheat systems. 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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Typical Problems

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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) With terminal units, the minimum flow rate reflects the actual occupant load rather than a design value.
  • c) To minimize the reheat burden, discharge temperature reset schedules are optimized to maximize AHU discharge temperatures within the constraints of dehumidification and zone cooling requirements.
• ___ 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 reheat control sequence and its integration with the air handling unit sequence and terminal equipment sequence is verified per the design sequence of operations.
• ___ 4. Verify proper staging of electric reheat coils and multi-circuit hot water or steam reheat coils.
• ___ 5. The coil capacity is verified if required by the specifications. Verify capacity tests results 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. Occasionally a reheat coil may be found in the central air handling unit, especially if the cooling coil is being controlled to provide a specific humidity. In this type of application, the reheat coil may not be able to safely handle subfreezing air and may require protection of its own in environments where such exposure could occur. This is not an issue if the reheat coil is located at the zone level.
• 2. 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. These high discharge temperatures can pose the following problems:
  • 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 reheat 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.
• 3. Overly rapid stroking of valves and dampers during a test process can cause air, water, and steam hammer problems in the duct and piping systems serving the reheat element. If any of these conditions arise, it indicates the PID control loop is not tuned properly.
• 4. Functionally testing an electric reheat element during the summer months while the cooling plant is in operation may cause problems if multiple elements are tested simultaneously, including:
  • a) Distribution system load conditions that exceed design and trip the primary switchgear, resulting in unscheduled and unanticipated outages.
  • b) Demand peaks well in excess of those that would normally be encountered during the normal operation of the building due to the demand that the reheat coil places on the system concurrently with the refrigeration equipment.
• 5. Simulating a full reheat load in the field is typically possible under most outdoor ambient conditions by manipulating the other elements of the air handling system as necessary to produce the required flow rates and entering air conditions at the reheat element. Often, this condition can be coordinated with a simultaneous load test of other elements in the unit such as the preheat or the cooling systems although the sensible and latent components of the load may not be correct for a true cooling capacity test.
• 6. 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.
• 7. In efforts to increase the reheat capacity to meet a perimeter heating load, it is common to increase the airflow to the zone. By increasing the airflow, the reheat coil must reheat more air up to the room temperature before the heating load can actually be met. In addition, more air flow at a cooler temperature may be perceived by the occupants as a comfort issue. Supplying less flow of hotter air to the space may result in more comfortable conditions, reduced reheat energy, and reduced fan energy. However, the air flow should not drop below the minimum flow required by the diffuser to maintain an adequate discharge pattern. If the air flow is too low, the air will not be distributed to the zone properly and stratification can occur. Typically, the air flow should be sufficient to maintain the discharge air temperature 10°F to 20°F above the zone ambient temperature.

Page last updated: September 11, 2006