Over-design (sized) HVAC Systems – Suggestions, Solutions & Recommendations
A review by KHS Engineering Topics
Many HVAC systems installed in commercial buildings have more capacity than is ever required to keep the occupants/tenants comfortable and in order to provide correct conditions for particular building assets. Such over-design HVAC systems can have negative effects on the environment, on occupant comfort and on the economic outcomes (operational efficiency) of the building.
What are the Symptoms of over-design?
The concept of “Design Day” is the standard design practice followed for comfort HVAC applications where the design day conditions that may actually present on average 10-15 days per year. So, a HVAC system design/sized based on above conditions will operate full load or run at full cooling capacity on these 10-15 days only. These full load operation days will further reduce due to load diversity, which is often possible in any commercial building. The key HVAC over-design symptoms, which can be seen commonly, are as below.
- Frequent short cycling issues of chillers (in buildings which have own chilled water plants)
- Unable to maintain of correct delta T (∆T) – relevant for both in-house chilled water plants & District Cooling
- Struggle in maintaining design conditions during period of high Relative Humidity
- Excessive re-heat energy consumption
- Excessive energy/utility bills
What are the Impacts?
All impacts, which are result of an over-sized HVAC system, have a monetary value. Therefore, it can be defined as Impacts pre & post occupancy of a building. Pre-Occupancy Impacts include design, construction & commissioning stages while Post-occupancy Impacts relate to operation & maintenance of the building. These inclusive of high project cost (Capital investment) for higher capacity plant & equipment including accessories, uncontrolled utility consumptions due to inefficient operation, higher capacity charges (relevant only for district cooling) and high maintenance cost also a result of inefficient system operation.
Challenges & Barriers to right-sizing
§ Occupancy loads – The high design guide values for estimating internal heat gains (people, lighting & equipment) are one of the key reason behind an over-sized HVAC system.
§ Internal Temperature Set points / Comfort Zone – A close control of internal temperatures (ex: 22.5 °C ± 1 °C) are sometimes a must requirement from particular clients and which is difficult to maintain in practice for building HVAC systems. Generally, the internal temperatures should be set in the comfort zone (23 °C ± 3 °C). Such requirements will lead to design HVAC systems with excess capacity.
§ Discrete Design Process – The concept design is carried out independently during early stage of a project. The particular designer at that stage usually considered higher margin toward the cooling load estimates. During schematic & detail design stages, these estimates should be reviewed and revised in-line with actual cooling load requirements. Usually no significance is given for such revisions due to paucity of time or budget and which will end up having an over-sized HVAC system.
§ Overshadowing – This is another much likely reason, which is not considering during the design stage. The overshadowing effect from surrounding buildings creates a significant impact on the cooling load calculations especially on peak demand.
§ Contractual Obligations – Sometimes designers feel their reputation would suffer if a building HVAC system fail to maintain internal temperatures on days when ambient conditions go beyond the standard design-day scenarios. Such sensation influences them to add unwanted safety factors to final cooling load. “Design & Build” contracts generally discourage right-sizing approaches. The much competitive tender environment under which such jobs are won creates a little incentive to review and optimize design calculations.
Suggestions, Solutions & Recommendations
Below discussed are the most appropriate and hands-on
recommendations that the building industry should implement & practice to
lessen the barriers listed above.
§ Accuracy of Data – Challenge “rule of thumb” concept for load calculations and “brief requirements”. These mostly based on out of date data or obtain from a previous or a different project specifications.
§ Correct Load Estimation – Ensure of having an accurate load estimation based on established design data, which are specific to project location and type of the building. Review in detail is it crucial, prior to include any safety factors.
§ Dynamic Calculation Methods – The designers are encouraged to use computer-based load estimation platforms, which provide the features for thermal storage and diversification of peak-loads (peak-demand) for individual zones and air handling systems. Dynamic method is recommended because, the “daily diversity” cannot be addressed accurately from Static methods, ultimately which will end up with an over-sized estimate. Furthermore, the impact of introducing innovative concepts (ex: shading schemes) are to be easily quantify from Dynamic method compare to Static.
§ System Approach instead of Component based approach – It is recommended to consider overall system configuration rather than individual component when designing a building HVAC system. In such approach designer is able to define the low load operation strategy applicable to overall system. This approach ensure proper sizing of the chillers (the utmost key component of a HVAC system) as if it not sized properly will influence the overall HVAC system efficiency. Well experience HVAC designer would use the dynamic simulation from system approach to blend & select the optimum chilled water plant control strategies from;
- Chilled or Condenser Water temperature resets
- Primary/Secondary pumping circuits with variable speed control
- Variable flow chillers
- Variable speed control of cooling tower fans
§ Ease of future adaption – An experienced designer make his/her design flexible to handle (up to certain limit/capacity) unknown future add-ons might be from space redesign/renovation and expansion of a building. This is also called designing flexibility into the system but not by over-sizing the entire system. Substantial sizing of piping network and main duct distribution system will provide to add reasonable extra capacity, which can be, used in future requirements.
§ Auxiliary Stand-Alone Systems – A typical commercial building would have significant greater amount of internal heat gaining specialized areas like IT server rooms, trading floors and call centers. In general, the percentage of these areas against total building area is minimal. Recommend to design auxiliary stand-alone system/s only to address extra cooling capacity of these areas. This enable the main HVAC system to operate efficiently throughout the year. However, a system approach needs to be applied while designing these supplementary HVAC systems as the system should efficiently be able to handle a wide diversity in operating capacity.
§ Integrated Design Process – It is highly recommend following of an integrated cooling load & energy-use simulation analysis during the design, which will grant following opportunities & benefits.
- Provide the facility to model & test the proposed design in an integrated manner
- Offers sophisticated insights
- The designer and the team can test the designed system against possible different operational scenarios having various part-load conditions
§ System Commissioning & Maintenance Strategies – The standard HVAC system commissioning (also known as TAB) procedures, which are currently practicing in industry, are purely confirming system’s capability to meet design parameters at simulated design conditions. The energy-efficient operation of a HAVC system is defined by how well it operates steadily at low-load conditions, which cannot be defined and witnessed by traditional commissioning strategies. Therefore, a developed and an innovative commissioning strategy is recommended, which inclusive of following system parameters.
- Testing/tracking of AHU supply & return/exhaust fans at lower speeds
- Test & record of pumping system stability at part load
- Correct functionality of chiller scheduling algorithms
- Correct/fine-tuning of chilled & condenser water temperature reset strategy
Furthermore, the system maintenance regime should be equipped with regular calibration of controls and system parameters to ensure efficient long-term operation at part-load conditions.
§ LCCA (Life Cycle Cost Analysis) – One last recommendation would be implement of a well-equipped LCCA while selecting & finalizing the design solution. Practicing of such strategy will ensure the selection of most optimal design. This strategy can be easily included to Integrated Design Process, which is discussed previously. However, the design firm should be aware that a design process which inclusive of a LCCA will lead to change in both design & procurement processes. Therefore, a better coordination, interactions & agreement is vital within the project stakeholders while introducing of such strategy.
References
§ ASHRAE Fundamentals 2013, Chapter 18, Nonresidential Cooling & Heating Load Calculations
§ ASHRAE Research Project RP 1117 (2013): Experimental Validation of Design Cooling Load Procedures: The Heat Balance Method, D.E. Fisher, D.S.Eldridge
§ Mao, C., Haberl, J., Baltazar, J. (2013): Peak Heating / Cooling Load Design Methods
§ Booten, C., Christensen, C., Winkler, J. (2014): Energy Impacts of Oversized Residential Air Conditioners
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