A Study of Over-Sized Building Systems

A Study?of Over-Sized Building Systems:

This article aims to provide a detailed explanation on ;

1. Signs of Over-Sizing
2. Impression of Oversizing
3. Challenges to right-sizing
4. Recommendations and Remedies for an efficient system design

An oversized Building HVAC System Negatively impacts building occupants. It leads MEP systems in the buildings to "sick building syndrome". It also creates a negative impact on the environment which results in excessive carbon footprint. Mainly, It Increases building operation costs and maintenance costs.

1. Signs of Over-Sizing:

• The system will never run on a full load
• Frequent short cycling of individual chillers
• A nightmare to maintain correct conditions during high humidity
• Excessive re-heat energy consumption
• Difficult to maintain correct CHW system delta T

2. Impression of Oversizing:

Component oversizing;

like Chillers, cooling towers, heat exchangers, pipes, pumps, valves, Terminal units, and Ductwork

Incorrect calculation of building heat load (generally due to inappropriate design margins with a lack of consideration to load diversity) leads to oversized plant and equipment selections for the building HVAC system.

Derive a system based on design day conditions rather than deciding system capacity based on a design day, diversity, and redundancy strategy.

Inefficient Operation;

?The entire system is oversized to handle peak design day conditions. Hence all installed plant and equipment are operating at low part load factors for the majority of their total operation time. Intermittent plant operation tends to cool the plant and is unable to turn?down to low?load levels(when the diversified loads during off-peak conditions) that allow the plant components to operate stably, It leads to

• Frequent restart operation.
• Shorter mean-time failures
• Higher maintenance requirement

Oversized fans with VFD operate at the lower speed possible and work as a virtual constant speed system. Such situations create no control of airflow.

In the cases where the lowest airflow (as per the VFD speed setting) still to be high; excessive re-heat energy has to be used.

Cost Impact:

Capital cost penalties on system components

Higher allocation for capacity charge(applicable only for District Cooling Supply)

Higher operation and maintenance costs due to an inefficient operation

Impact on Thermal Comfort:

• Poor Temperature?and Relative Humidity Control within different areas/Zones.
• Various Indoor Air Quality issues
• Light Level issues
• Duct Noise and Downdraft from supply air diffusers

3. Challenges to right-sizing:

1. Occupancy and other internal loads, High design guide values for estimating internal heat gains (occupancy, lighting, and equipment) are the one key reason behind oversized HVAC systems in buildings. Non-availability of exact internal loads at the design stage also ends up having an oversized system.

2.?Temperature set points, Close control of internal temperature(indoor temperatures) is difficult to maintain in practice and which can lead to excess capacity. (ex: Set temp. +-0.5 or 1). Wider thermostat settings should be considered which improve the stability of operation due to a larger “dead-band” provision. (ex: Set temp. +-3)

3. Discrete Design Process, Concept design for each?discipline is carried out independently. The HVAC designer may consider overly estimated heat load for the cooling system at this stage. if an overly estimated heat load is not revised at a later?stage, might be due to a paucity of time or budget constraints. It will end up having an oversized HVAC System.

4. Overshadowing, The impact of surrounding buildings(existing or planned for the near future) should be considered while doing the cooling load calculations as they have a considerable impact on peak demand.

5. Contractual obligations; Generally, HVAC designers are conscious that building use changes frequently and trust the design of the HVAC systems to be able to cope with such changes by oversizing or introducing extra margins. The cost of design is based on very competitive tenders and mostly does not allow for iterative or integrated design solutions. Design and construct (Design and Build) contracts commonly discourage right sizing approaches due to competitive tender situations which barricades the option for review and optimize design calculations.

4. Recommendations and Remedies:

• Rules of thumb must be challenged in the design, which may be out of date and may be copied from a previous project specification.
• The best practice is to obtain current equipment load data/details that match the intended use at actual operation.
• Accurate Load Estimation, Use established design data(ex: ASHRAE) to load estimate. Resist the temptation to apply safety factors due to forecast various expectations.
• Do not use W/sq.m loading or other approximate methods for equipment sizing.
• Dynamic Calculation Methods, Practice of computer-based load estimation programs which facilitate for thermal storage and diversification of peak load for each zone.
• ?Encourage estimating air-handling systems for each relevant zones.
• Dynamic methods confirm proper accounting for the daily diversity (ex. Behavior of cooling load between East & West Zones) compare to static methods which will lead to a final over-sized block load estimation.
• The dynamic methods are equipped for the impact of innovative shading schemes., that are difficult to quantify using static methods.
• Systematic Approach Should practice a systematic approach while designing the HVAC system rather than a component-based approach
• Should consider overall system arrangement and design a low-load operation strategy.
• Ensure the system is compatible for good part load performance.
• Dynamic simulation analysis with a systematic approach will provide an experienced designer to select & optimize an approximate mix of equipment and control strategies to obtain a cost-effective and energy-efficient design.
• Various Configurations and control options like Chilled/condenser water temperature resets. Primary/secondary control circuits with variable speed control. Chillers that can handle variable flow or have inbuilt variable speed control. Variable speed control for cooling tower fans.
• Designing Flexibility, Ensure designing flexibility into the system for future unknown loads.
• But, be alert not to oversize the entire system. (Ex.- Substantial sizing/designing of distribution pipework and main ductwork will ensure available capacity to be re-distributed in the future when required.
• Separation of high-load areas, During the design process, identify the greater amount of internal heat gain areas, such as; IT rooms, Data centers,
• In general, these types of areas are a small percentage of the total area of a building.
• Adding auxiliary stand-alone systems is one of the best economical designs for such areas.
• A system approach needs to be applied in designing these supplementary systems considering efficient operation while handling the wide diversity in operating capacity.
• Integrated?Design Process, Highly recommend using an integrated load and energy-use simulation analysis.
• Because, validated whole building simulation platforms provide the opportunity to model and test the purpose design in an integrated manner and offer sophisticated insights, which other methods are unable to provide.
• Moreover, these models can represent the building and its interactions in far greater detail and also allow the description and testing of different operational scenarios.
• Life cycle cost analysis(LCCA) it is good to use a comprehensive LCCA selection of the most optimal design solutions.
• An integrated design process can be used to test the impact of a change in one system across the whole building
• A detailed simulation model will allow these interactions to be tested reasonably quickly and the associated costing to be viewed
• A design process that LCCA will lead to a change in the design and procurement process.
• Lastly, there can be significant benefits to applying LCCA during the integrated design process.
• Commissioning and Maintenance Strategies, The standard commissioning procedures concentrate on confirming system design parameters at stimulated design conditions. However, energy-efficient operation means, how well the system responds and operates stably at low load conditions.
• The commissioning strategy must include, calibrating and testing the tracking of fan speeds at lower speeds.
• Stable part load operation of pumping systems
• Correct functioning of chiller scheduling algorithms
• Correct implementation of chilled/condenser water temperature reset strategies.
• Maintenance Strategy must include, in addition to general maintenance regimes, calibration of controls, and system parameters to ensure long-term effective operation during part load conditions.