CBECC-Com Nonresidential Compliance Software

Building HVAC Systems - Frequently Asked Questions  (FAQ)

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Q: Does Overall Fan Efficiency include drive losses?
A: No, drive efficiency should be included with the motor efficiency.

Q: My project is using packaged single zone rooftop units. When I enter my systems and attempt a compliance analysis, I get an error message that terminal units are required. My project doesn't have terminal units. What is the problem?
A: Terminal units are required by EnergyPlus as the connection between an air system and a thermal zone. In your case the terminal units would have a type of "Uncontrolled" and corresponds to the grill through which supply air enters the space.

Q: Do two-speed fans work in an air system?
A: No. Two speed fans are only valid for use with four pipe fan coil (FPFC) units. This is a limitation of EnergyPlus.

Q: What is the process for compliance when CBECC-Com determines the HVAC system for the proposed building is undersized?
A: When the simulation has more than 150 hours of unmet load hours, the HVAC system capacities must be manually increased to meet the load. Remember to increase both airflow and coil capacities. There is an exceptional condition that should be marked “Yes” when this is done, and a note will be added to the compliance report.

Q: How do I reduce unmet load hours (UMLHs) in a CBECC-Com model?
A: UMLHs are only calculated for conditioned thermal zones. The following steps can be used to reduce heating and cooling UMLHs for a single zone or multiple zones:
  1. Determine the zone that has excess UMLHs:
    • The UMLHs for each zone can be seen at the bottom of the Energy Use Summary after successful simulation or in the error box that appears if the UMLHs of any zone exceed the maximum allowed.
  2. Increase the rated capacity of the heating coil or cooling coil serving the zone in questions. Make the increase a small % (e.g. 5%).
    • Simulate and check UMLHs. If UMLHs decrease then perform step 2 again. If UMLHs increase or stay the same move on to step 3.
  3. Increase the fan flow at the terminal unit or zone system serving the zone in question. Make the increase a small % (e.g. 5%).
    • Simulate and check UMLHs. If UMLHs decrease then perform step 3 again. If UMLHs increase or stay the same move on to step 4.
    • Typically, maximum air flow is based on cooling loads, so an increase to heating coil capacity does not typically require increased air flow, however, an increase to cooling coil capacity typically does require increased air flow.
    • It is important to balance the air flow of each air system after increasing flow at a fan or terminal unit. This is done by summing all of the maximum primary flows for each terminal unit on an air system and checking that the sum is equal to the system’s supply fan flow capacity.
  4. Increase the design flow rate of any the chiller pumps and cooling tower pumps. Make the increase a small % (e.g. 5%).
    • Simulate and check UMLHs. If UMLHs decrease then perform step 4 again. If UMLHs increase or stay the same move on to step 5.
  5. Increase the rated capacity of chillers and cooling towers. Make the increase a small % (e.g. 5%).
    • Simulate and check UMLHs. If UMLHs decrease then perform step 5 again. If UMLHs increase or stay the same move on to step 6.
    • For heating UMLHs the boiler rated capacity can be increased. For cooling UMLHs the chiller rated capacity can be increased. Additionally, the cooling tower rated capacity, air flow and condenser water flow rate can be increased.
  6. If these steps are not reducing UMLHs as anticipated then the HVAC system could be connected incorrectly or a zone that is set as conditioned does not have primary heating / cooling system reference.
Note: If changes are made to capacities so they do not match the submitted mechanical schedules then the Exceptional Condition, Does the rated capacity ... should be set to Yes.

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