Question

The architectural and HVAC plans for the Uytengsu Building on campus are provided. The details of the equipment installed are provided. From this information determine the estimated overall building heating and cooling requirements and compare to the "rule of thumb" information given in the first chapter. Briefly, write up your findings in a technical memorandum showing your calculations stating any assumptions. Upload your findings in a pdf.

Answer 1

INTRODUCTION

The most important aspect of an efficient heating system is getting the size right. It was once common practice to oversize boiler plant, to ‘over-ensure’ that heating demand was met. In addition, improvements in building fabric and an increase in internal heat gains, such as from IT equipment and change of use/occupancy, means that where a boiler has not been replaced for many years, the heating load of the building may have changed significantly. It is therefore important to identify and understand core business requirements and use this to inform any proposed improvements to an HVAC system. Typical questions you should consider are

• What is the current internal temperature of the building?

• Are employees happy with the internal environment?

• Are there any hot or cold spots within the building?

• Are there any areas of the building where temperature is critical?

• When is the building occupied.

CALCULATING HEAT LOAD/DEMAND

There are five factors that determine the energy load of a heating system:

• The design, layout and operation of the building – this affects how the external environment impacts on internal temperatures and humidity.

• The required indoor temperatures and air quality

• The heat generated internally by lighting, equipment and people – all of these have an impact on how warm your building is;

• The type, design and efficiency of the heating plant;

• Building usage patterns. Using ‘rules of thumb’ or the Simple Sizing Calculation is often sufficient to determine approximate plant requirements and sizes at the concept stage.

RULES OF THUMB

Rules of Thumb are general principles derived from practice and experience rather than precise theory. They can be useful for approximately calculating values, setting outline targets and rapidly comparing different options.

GROSS INTERNAL AREA (GIA):

The floor area contained within the building measured to the internal face of the external walls No boiler is 100% efficient. Heat is lost via the flue gases and through the main body of the boiler itself. Therefore, this value needs to be adjusted to correctly size heating plant according to the plant efficiency.

Boiler ‘seasonal efficiency’ values should be used rather than manufacturers quoted instantaneous efficiencies, as this takes into account the actual operation of the boiler or its practical use, measured at full and part load. It is a weighted average of a defined number of hours of full and part load operation which represents a full year of operation. Note that the boiler efficiency is also affected by the heating system type.

SIMPLE SIZING CALCULATION

This calculation method works by determining the primary heat losses in your building, and then working out the level of heating that would be required to achieve the building’s desired temperature. Begin by determining the Total Heat Loss, QT:

QT = QF + QV (Watts)

QF is fabric heat losses; QV is ventilation & Infiltration heat losses.

Watts (W) are units of power, the rate at which energy is generated or used and are often quoted in kilowatts, 1000W = 1kW.

QF = U × A × ΔT (W)

U is the u-value for each surface material (W/m2ºC)

A is the surface area of each building component (m2)

ΔT is the difference between desired internal temperature and external temperature (°C)

QV = ⅓ × N × V × ΔT (W)

N = number of air changes per hour

V = volume of building (in m3).

There are industry-accepted values for N based on building type – the reader should refer to CIBSE Guide A for examples.

To calculate the Annual Energy Load, Eannual use:

Eannual = [[QT ÷ (Ti-To)] x DD x 24] ÷ 1000 (kWh)

Ti is the internal design temperature;

To is the outside design temperature.

DD is the Degree Days for your region, the number of days hotter or colder than a certain reference temperature (often 15 C).

As before actual energy load is dependent on efficiency of system

Etotal = [[QT ÷ (Ti-To)] × DD × 24 x (1 ÷ η)] ÷ 1000 (kWh)

η is the system seasonal efficiency.

Total annual consumption can then be calculated from:

Eannual = Etotal × heating hours in year

Once this has been estimated, compare this value to how much energy the building has actually used over the last year to determine whether your system is sized correctly. It can also be used to see how much difference a new more efficient boiler will make and the resulting payback.

Finally your consumption and load can benchmarked against buildings of similar types. CIBSE Guide F and their TM46 provide detailed benchmarks for a number of building types, based on m2 . Comparing to benchmarks provides an indication of the overall opportunity to improve and reduce consumption.

However, some of these benchmarks are out of date, and so should be taken as a ‘minimum’ rather than a ‘maximum’ indication for potential improvement.

MULTIPLE & MODULATING BOILERS

Low Temperature Hot Water (LTHW) boilers give optimum efficiency at a particular load point (standard boilers at full load, condensing boilers at part load) so, it makes sense to have a series of boilers operating at around their peak efficiency loads and together matching the range of heating demands that may be experienced in a commercial building.

Audit the HEATING AND COOLING demand in your building

Performing a heating/cooling energy audit before you begin other actions means you’ll understand where energy is being consumed, and allow you to more effectively identify opportunities for reducing demand primarily before needing to consider heating upgrade/replacement options. Some easy wins can be identified through this process, such as identifying areas that are overheated as a result of incorrect thermostat or BMS settings; identifying areas where heating and cooling are in direct competition because dead bands have not been set, or that the heating and cooling services are not fully understood by the occupants. One very common misperception that heating/cooling capacity is inadequate is often undermined at this point too.

An energy audit is an inspection of the energy dynamics within a building i.e. Where, when and how energy is used and identifying opportunities to reduce this consumption. The audit may also assess the efficiency, physical condition, and programming of mechanical systems such as the heating, ventilation, air conditioning equipment, and thermostat. Audits can range for a simplistic, helicopter view walk around a site, down to in-depth, complex measurements of services.

Implement a heating & ventilation strategy

Rethinking the overall provision of and need for heating and cooling within your building should be the priority. Passive Design and Heat Recover are two key measures to consider to reducing overall heat demand identified in an audit:

Passive design

There are often opportunities within existing buildings to incorporate elements of passive design, by switching off unnecessary heating and developing a ventilation strategy to distribute and even out temperature variations within the building. More generally, passive design uses the sun along with ventilation so that nature provides the majority of fresh air and temperature requirements. As simple as it sounds, natural ventilation relies on air flow through openings of a room or building, preferably from opposite sides. It also applies to rising hot air being replaced with cooler air sucked in through windows or vents from a lower level. Obviously, passive design and ventilation are best considered at building design stage, but don’t rule out using them instead of HVAC.

Some examples of passive design heating systems

• Window designed to allow more heat from the sun and the collection of solar energy through south-facing windows

• Storing this heat in "thermal mass," comprised of building materials with high heat capacity such as concrete slabs, brick walls, or tile floors.

• The natural distribution of the stored solar energy back when required, through the mechanisms of natural convection and radiation

• The use of trombe walls, a sun-facing wall separated from the outdoors by glass and an air space, which absorbs heat from the sun and releases it selectively towards the interior at night; and solar corridors/conservatories.

Natural Ventilation is simple and very cost effective

Making the most of natural ventilation is a simple and cost-effective way of achieving big savings, and can significantly reduce heating and cooling loads:

• Passive solar heating should be considered for use in circulation spaces such as lobbies and atria, hallways, break rooms, and other types of spaces with low internal heat gain that afford occupants the flexibility to move out of the sun.

• There are likely to be opportunities for cross-ventilation whereby windows or vents can be closed in hot spot areas, and opened in cooler/higher level areas to enhance air movement.

• Finally, reduce internal heat gains, for example by sourcing it provision into a separate, sealed area, and introducing thin client technology for computer systems.

Heating & cooling recovery

Heat recovery is the collection and re-use of heat (or cooling) arising from any process that would otherwise be lost. The addition of heat recovery means that some of the heat contained within the extract air can be recovered. The heat energy is passed into the incoming fresh air effectively pre-heating it and meaning the boiler needs to add less heat. The two air streams need not mix directly to allow the transfer of heat. This is often well suited to ventilation systems bringing cool fresh air into a building using fans in air handling units (AHUs), or for pre-heating in boiler or hot water heating circuits. It is often low grade heat recovered, so cannot be used as a primary heat provision mechanisms (also visit section A1.6 for rules on hot water and legionella).

DISTRIBUTION OF HEATING AND COOLING

A review of opportunities for improving the existing heating and cooling distribution and storage network should include the following areas:

• Ensure that all pipes, ducts and vessels are adequately insulated. This includes valves and couplings, which are often overlooked, and insulation on refrigerant pipework as poor condition will affect the temperature of the refrigerant flowing through the system and thus consume more energy in maintaining the required temperature.

• Ensure that all ahu filters are maintained and cleaned regularly. It may also be worth considering the use of low energy air filters, and fitting pressure gauges to indicate when replacement of filters is required.

• Identify opportunities for either decentralising heating/cooling and hot water provision, or combining distribution options. Large legacy calorifiers could be replaced with point of use hot water systems if hot water demand is restricted to taps. Consideration, however, needs to be given in this instance to the change of fuel and impact that has upon costs and carbon emissions.

• Ensure condensing and evaporating devices are clean and well maintained. Check condensers are not obstructed, for example by equipment or vegetation.

• Any constant volume AHUs should be identified and considered for retrofit to a variable air volume (VAV) system.

• Check the configuration of each hot water valve on each heating coil (includes air handling units, fan coils, etc.). If three-way valves and constant volume pumps are installed, convert the valves to two-way and install variable frequency drives on hot water pumps.

• Perform combustion efficiency analysis and install automated o2 trim systems to adjust the air-to-fuel ratio linkages feeding the boiler burner, so that they are burning most efficiently.

• In commercial or industrial buildings with warm air heaters and high ceilings, de-stratification fans can reduce energy use by 20% by blowing warm air down to ground level where it's needed.

• Consider purchasing a new energy-efficient burner if your existing burner is cycling on and off rapidly.

Decentralization means changing from a centrally provided heating &/or cooling system to a local provision i.e. Smaller boilers or heaters for individual areas.

Condensers are usually located on the outside of buildings and reject heat that has been removed from inside the building by the cooling system.

Variable air volume (VAV) systems maintain the air flow at a constant temperature, but supplies varying quantities of conditioned air in different parts of the building according to the heating and cooling needs.

MOTORS, PUMPS, DRIVES, AND FANS

Motors are used extensively throughout many HVAC systems. Specifying high efficiency motors when replacing can result in good savings in terms of electric power with little additional capital cost, but can also significantly improve heat distribution. Consideration should also be given to soft starts on motors if not already fitted, this generally reduces the wear and tear on the motors and reduces the need to replace.

In addition, fitting Variable Speed Drives (VSDs) or purchasing motors with integral VSDs, can also reduce speeds and deliver accurate flow rates of hot/chilled water as and when required. High efficiency motors and vsds generally have one of the best paybacks within energy management, as the power to energy ratio is cubed. For example by matching air volume in ahus to actual heating/cooling loads, the use of vsds with variable air volume (vav) fans cuts energy consumption by up to 60% versus constant air volume systems. Many HVAC systems also have a variety of pumps and fans – consider direct drive pumps and fans which are more efficient than those that are belt driven.

Soft start: a device that can temporarily reduce the load and torque in the motor during start-up. This reduces the mechanical stress on the motor and shaft, extending the lifespan of the system.

Variable Speed Drives (vsd) also commonly known as variable frequency drives or inverters – are used to control the speed of AC induction motors. Energy use can be reduced considerably if the speed of the motor varies in response to the changing process conditions.

Direct drive: takes the power coming from a motor without any reductions (such as a gearbox).

VSDs, soft starts and high efficiency pumps reduce energy use Retrofitting vsds, soft starts and replacing with high-efficiency pumps can save up to 60% of energy consumed by fans, pumps, motors and drives where there are variable conditions of operation. The upfront capital costs also tend to be fairly low, and paybacks can be very quick.

BUILDING INFORMATION MODELLING

A Building Information Model is a computer model of the physical elements of a building and how they function, and aim to help share and manage knowledge on a building through it’s lifecycle, from conception to demolition. Building Information Modelling (BIM) responds to the current demands for improving building performance rapidly and cost-effectively, by going beyond the planning and design phase of a project. It extends throughout the building life cycle to include such processes as cost management, construction management, project management and facility operation.

BIM presents a vast range of opportunities by providing better and more integrated tools, with more opportunity for collaborations, working through an integrated project delivery team model. BIM can quickly estimate design energy performance, enabling an understanding of how to achieve cost effective, low energy and zero carbon buildings, whilst assessing return on investments for those buildings. Specifically BIM analysis tools help analyze heating and cooling requirements, identifying major building equipment that may reduce energy use. Additionally, they incorporate local weather and electric grid data to estimate building energy consumption and carbon emissions.

Metering, Monitoring and Targeting (MM&T), is the management information system that supports energy management. It is an on-going cyclical process from data collection to taking action, via data analysis and communicating the resulting insights, as shown above.

MM&T will give you:

• Timely, relevant information on energy use the ability to investigate the energy performance of buildings and processes

• The ability to take action to rectify exceptions in performance and to improve energy performance over time

• Energy reports to support accountability for energy use

The ability to verify savings made following project implementation. Meters and/or sub-meters should be installed for major energy consuming equipment as these enable action to identify and prevent excessive energy use. Measurements of temperature and relative humidity can assist in monitoring occupant comfort. This information also allows the efficient allocation of all budgets involved in operating the building and the ability to set energy and emissions reduction targets.