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The map of energy flow in HVAC systems

Luis Perez-Lombard a,!, Jose Ortiz b, Ismael R. Maestre c

a Grupo de Termotecnia, Escuela Superior de Ingenieros, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Seville, Spainb Building Research Establishment (BRE), Garston, Watford WD25 9XX, United Kingdomc Grupo de Ingeniería Térmica, Escuela Politécnica Superior de Algeciras, Universidad de Cádiz, Avda. Ramón Puyol s/n, 11202 Algeciras, Spain

a r t i c l e i n f o

Article history:Received 28 January 2011Received in revised form 29 June 2011Accepted 1 July 2011Available online 26 July 2011

Keywords:HVAC energy efficiencyHVAC loadsSankey diagramsHVAC energy use

a b s t r a c t

Heating, ventilation and air conditioning (HVAC) systems are the most energy consuming building ser-vices representing approximately half of the final energy use in the building sector and between onetenth and one fifth of the energy consumption in developed countries. Despite their significant energyuse, there is a lack of a consistent and homogeneous framework to efficiently guide research and energypolicies, mainly due to the complexity and variety of HVAC systems but also to insufficient rigour in theirenergy analysis. This paper reviews energy related aspects of HVAC systems with the aim of establishinga common ground for the analysis of their energy efficiency. The paper focuses on the map of energy flowto deliver thermal comfort: the HVAC energy chain. Our approach deals first with thermal comfort as thefinal service delivered to building occupants. Secondly, conditioned spaces are examined as the systemswhere useful heat (or coolth) is degraded to provide comfort. This is followed by the analysis of HVACsystems as complex energy conversion devices where energy carriers are transformed into useful heatand coolth, and finally, the impact of HVAC energy consumption on energy resources is discussed.

! 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Heating, ventilation and air conditioning (HVAC) systems are asignificant and growing user of energy. The building sector repre-sents 20–40% of the final energy consumption in developed coun-tries [1], surpassing in many cases other large sectors, such asindustry and transport. Within the building sector, HVAC systemsare the most consuming representing approximately 50% of the to-tal, equivalent to 10–20% of the final energy consumption [2]. Inaddition, the HVAC world market has grown dramatically in recentyears as a result of the globalization of the demand for thermalcomfort, particularly in emerging countries [3,4].

The rapidly growing HVAC energy use is an aggravating factorfor the world energy and environmental crisis. Energy savings, en-ergy efficiency improvement and promotion of the use of renew-able energy sources are three key instruments to alleviate thiscrisis. Consequently, on the one hand, energy efficiency of HVACsystems has become a priority objective for energy policies, as indi-cated by the more stringent HVAC energy efficiency requirementsin building energy regulations [5], the development of building en-ergy rating systems and certification schemes [6,7], and the stan-dardization of inspection, operation and maintenance plans ofHVAC systems for the building stock [8–10]. On the other hand,policy guidelines regarding energy use in buildings aim at

encouraging the use of renewable energy as clearly stated by theEuropean Directive on renewable sources [11] and green buildingstandards [12].

A literature review on the field of HVAC efficiency shows manynarrow-scope references looking into particular topics (equipment,control, simulation, etc.) but few global approaches to HVAC en-ergy (or exergy) efficiency analysis [13–15]. Thus, there is a lackof a general framework to guide research and energy policies,due to insufficient scope and rigour in energy analyses and to thecomplexity and diversity of energy conversions in HVAC systems.

This paper proposes a wide-scope analysis of HVAC systems bytracing the energy flow from energy sources to final services andby the examination of the intermediate devices which build upthe links of the ‘global energy chain’. The main stages of the energyconversion chain have been well analysed. The expression ‘path-ways to end-uses’ was introduced as a first attempt to systematizethe conversion of energy [16] by the description of the energychain from resources to end-uses, through intermediate forms ofenergy. Other theoretical approaches can be found in [17,18]. A re-cent contribution by Cullen and Allwood [19] focuses on the defi-ciencies of previous attempts and suggests a new ‘map of energy’based on four technical groupings: energy sources, conversion de-vices, passive systems and final services. Thus, ‘‘the scale of energyflow and the technical potential for efficiency improvement’’ maybe analysed on a new energy map focussing on the specific techni-cal components in each energy chain, from which efficiency gainsare achieved, rather than on broad energy sectors, for which

0306-2619/$ - see front matter ! 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.apenergy.2011.07.003

! Corresponding author.E-mail address: [email protected] (L. Perez-Lombard).

Applied Energy 88 (2011) 5020–5031

Contents lists available at ScienceDirect

Applied Energy

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statistical data are readily available. Consequently, the map of en-ergy should not stop at the distribution of final energy among sec-tors, but analyse the entire route through which that energy istransformed, by means of end-use conversion devices, into a usefulform of energy, subsequently converted into a final service throughthe passive systems.This paper aims at providing a common doc-trine for easier energy analysis in HVAC field.

Its primary goal is clarifying the energy related aspects of HVACsystems intended to supply thermal comfort service to buildingoccupants. To this end, the paper traces the map of energy flowto deliver thermal comfort. This could be referred to as the HVACenergy chain (Fig. 1). Our analysis proceeds from right to left. Wefirstly examine the final service demanded by the occupants: ther-mal comfort. Secondly, conditioned spaces can be clearly identifiedas the passive systems in which useful energy in the form of heator coolth is degraded in exchange for thermal comfort service.Thirdly, the whole HVAC system could be thought of as a complexconversion device responsible for providing useful heat or coolth toconditioned spaces by the use of a certain amount of final energy.Lastly, the first link in the energy chain looks into the supply sideand deals with the impact of HVAC energy use on energy resources.The main stages of the HVAC energy chain are thoroughly analysedin the following sections.

2. Thermal comfort service

Buildings are complex systems capable of supplying a variety offinal services for their occupants, such as domestic hot water, light-ing, thermal comfort, food preparation, refrigeration, and power forother electrical devices. In particular, HVAC systems are responsi-ble for providing a comfortable indoor environment by keepingthe environmental parameters (air temperature, humidity ratio,mean radiant temperature and air speed) within predefined ranges

and also for assuring an acceptable indoor air quality by control-ling, filtering or diluting air contaminants.

Providing comfort to building occupants is the main goal of thedesigners of HVAC systems, but defining and qualifying thermalcomfort service is a complex subject. It could be defined as thestate of mind which expresses satisfaction with the surroundingenvironment (ASHRAE 55 [20]). According to their ability to pro-vide ventilation and to control temperature and humidity HVACservice can be classified in five service levels (Table 1). Servicequality may vary from SL0 for ventilation systems to SL5 for thosefacilities capable of providing ventilation, cooling, heating, humid-ification and dehumidification. In the residential sector, mostHVAC systems range between types SL0 to SL3, with a majorityproviding only heating without mechanical ventilation. Non-residential buildings usually are of types SL3 or SL4, while classSL5 is addressed to spaces with critical air quality or thermalcomfort requirements such as laboratories, museums, operatingtheatres, etc.

Additionally, different qualities could be defined for each HVACsubservice: ventilation, heating/cooling and humidification/dehu-midification. As for ventilation, standard EN 13779 [21] classifies

Nomenclature

C power consumption (kW)D thermal demand (kJ)DR draught rateh specific enthalpy (kJ/kg)L power losses (kW)_m mass flow rate (kg/s)

PD percentage dissatisfiedPMV predicted mean votePPD predicted percentage dissatisfiedQ thermal load or thermal power (kW)SL service level

Greek symbolsh time (s)

SubscriptsAT air transportAUX boiler auxiliaries

CD condenserCDP condensing pumpCG coolth generationDUCT ductworkEXH exhaust airHG heat generationINF ductwork infiltrationLEAK ductwork leakageMOT motorOA outdoor airPIP pipeworkPRI primaryRA return airSA supply airSP spaceTOW cooling towerTR transmissionWT water transport

Table 1Classification of thermal comfort service quality.

Service level HVAC system service

Ventilation Heating Cooling Humid Dehumid

SL0 !SL1 ! !SL2 ! !SL3 ! ! !SL4 ! ! ! !SL5 ! ! ! ! !

Fig. 1. The map of energy flow to deliver thermal comfort.

L. Perez-Lombard et al. / Applied Energy 88 (2011) 5020–5031 5021


indoor air quality in four IDA levels. Optimum air quality (IDA 1) isnormally required in hospitals, nurseries and laboratories; goodquality (IDA 2) is adequate for offices, museums and classrooms,among others; IDA 3 for shops, theatres, restaurants, hotel rooms,bars, discos, gymnasiums and computer rooms. The lower quality(IDA 4) is allowed in the remaining activities. Thus, most non-residential buildings require IDA 2 or 3, corresponding to ventila-tion levels between 8 and 12.5 l/s per person. Obviously, a changein ventilation rates modifies both service quality and energy con-sumption and hinders comparative analysis of energy efficiency.

As far as heating/cooling are concerned, service quality could beassessed simply by temperature set points, expressed in terms ofspace air or operative temperatures. However, some standardssuch as ASHRAE 55 and ISO 7730 [22] set indoor comfort classes(A–C) depending on comfort indicators (PMV, PPD, DR and PD).An even higher level of definition of thermal environment quality,included in EN 15251 [23], proposes the use of different comfortconditions at building design stage, in energy calculations and forthe verification of thermal comfort in existing buildings.

Humidity controls require special attention. Low humidity, typ-ically in winter dry climates, may cause dry skin, dry eyes and sta-tic electricity, but rarely thermal discomfort. Very high humiditymay cause mould problems and eventually discomfort. Indoortemperatures in most conditioned spaces are controlled, buthumidity is allowed to free float in a wide band (SL3), and therebyonly adding (or extracting) sensible heat to conditioned spaces isrequired. However, air is normally dehumidified in cooling coilsin any case, implying that latent and sensible heats are intrinsicallyinterrelated in HVAC system installations. Cooling coils latentcapacities have a great impact on indoor humidity ratios, HVAC en-ergy use and hygrometric quality. Some standards such as EN15251 set relative humidity set points depending on humiditycategories.

3. Demand of conditioned spaces

According to Cullen and Allwood [24] the building shell couldbe thought as a passive system where thermal comfort service isprovided. In this paper we will use the words ‘conditioned spaces’to refer to the passive systems in the HVAC energy chain. So, oncecomfort service quality has been discussed, the next question is thequantification of the useful heat to be added to (or extracted from)conditioned spaces. The answer to this question lead us to the con-cept of thermal demand. The term ‘energy demand’ can be inter-preted broadly as the amount of energy necessary to provide agiven service. Thermal demand assesses the amount of thermal en-ergy required to provide space comfort.

HVAC terminology commonly refers to heat transfer as thermalload (or simply load), irrespective of whether heat is transferred inthe unit of time (thermal power) or during a certain time period(thermal energy). It is convenient to use different words to referto energy and power to avoid misunderstandings. Consequently,thermal demand (D) and thermal load (Q) are related by a simpledefinite integral extended to a certain time interval (h1, h2):

D "Z h2

h1

Q#h$dh #1$

To precisely define thermal demand one should specify the sys-tem type (ideal or real), the system service (ventilation, heating,cooling, humidifying or dehumidifying) and the time period (daily,monthly or annual).

Under a passive system approach the demand of a conditionedspace is the ‘ideal demand’ that is the amount of thermal energythat a virtual device (‘ideal system’) should theoretically add toor extract from the space in order to maintain certain comfort con-

ditions (temperature, relative humidity, . . .) during a given timeperiod. Ideal demand is also referred to as ‘space energy need’(see definitions in EN 13790 [25]) or ‘room energy demand’ (seeEN 15243 [26]). It may be calculated from a thermal balance ofheat flows (transmission, ventilation, radiation, solar and internalgains) at space level, but may not be measured on-site. Basically,ideal demand depends on climate, internal gains, building shelland comfort conditions (comfort service) and could be thought ofas an indicator of the energy quality of architectural design.

The ideal demand should not be confused with the real demandon the HVAC system that considers heat transfer within actualfacilities responsible of providing useful heat or coolth to condi-tioned spaces. The latter deals with the performance of HVAC sys-tems as conversion devices, it may be calculated via simulation ormeasured on-site and will be discussed further on in this paper.

4. Energy conversion in HVAC systems

Drawing a general scheme to satisfactorily include the hugevariety of components and equipment involved in HVAC systems,together with their possible connections is a complex task. Thedescription of each particular facility is usually provided by a prin-ciple diagram with the details of the equipment (chillers, boilers,pumps, etc.) and their connections (water, air and refrigerant cir-cuits). Fig. 2 shows such a diagram for a typical all-air system withcentral generation of hot and chilled water by means of a boilerand a water-cooled chiller connected to a cooling tower,respectively.

In order to examine the energy conversion processes that takeplace in the whole HVAC system we will proceed as follows. Firstly,we will analyse the ‘thermal energy chain’. Subsequently, we willidentify four subsystems and write the energy balance equationsin order to quantify the energy flows involved. Finally, these flowswill be graphically represented in Sankey diagrams.

4.1. Thermal energy chain: HVAC loops

The examination of the energy conversion in the whole HVACsystem should begin with the analysis of the heat transfer pro-cesses in the ‘thermal energy chain’ to identify HVAC thermal loops[27]. From a conceptual point of view, the operation of the HVACsystem shown in Fig. 2 for the cooling mode can be interpretedas a heat transfer chain that extracts energy from the conditionedspaces and rejects it to the environment via five consecutive loops(Fig. 3):

– Air loop. Air is driven by fans through ductwork from condi-tioned spaces, where heat has been extracted to cooling coilswhere heat is transferred to chilled water. The air loop worksas an open process, since the air mass is altered by ventilation,air exhaustion and ‘parasitic flows’ in ductwork and spaces(leaks, infiltrations and exfiltrations).

– Chilled water loop. Driven by the pumps, the water is heated incooling coils and returns to the chiller evaporator to be cooled.

– Refrigerant loop. By means of a vapour compression cycle, therefrigerant extracts heat from the water in the evaporator andtransfers it to condensing water.

– Condensing water loop. Heat from the condenser is carried bycondensing water to the cooling tower to be cooled andreturned to the condenser.

– Heat rejection loop. Outdoor air gains energy in the coolingtower and rejects heat to the environment.

If the chiller was air-cooled, the cooling tower and the condens-ing water loop could be eliminated and the tower fan be replaced

5022 L. Perez-Lombard et al. / Applied Energy 88 (2011) 5020–5031


Fig. 3. HVAC thermal chain for hydronic systems in the cooling mode. Water-cooled chiller.

Fig. 2. General schematic of a typical all-air HVAC system with chilled and hot water generation.

Fig. 4. HVAC thermal chain for hydronic systems in the heating mode. (a) Hot water boiler. (b) Air to water heat pump.



by that of the chiller condenser. Obviously, in direct expansion sys-tems, the indoor air is directly cooled in an evaporator, with nochilled water loop.

In the heating mode, energy is transferred in the reverse direc-tion and the chain is reduced to three loops (air, hot water andcombustion) when heat is generated in a boiler (Fig. 4a). If a heatpump is used, a refrigerant loop is then added and the other loopsare modified according to its type: air to air (three loops: air, refrig-erant and heat extraction), air to water (four loops: air, hot water,refrigerant and heat extraction, Fig. 4b), water to air (four loops:air, refrigerant, water and heat extraction) and water-water (fiveloops: air, hot water, refrigerant, evaporator water and heat extrac-tion). Consequently, HVAC system types are better understood byanalysing the number and type of loops involved.

Heat exchanging devices linking thermal loops (water coils,evaporators and condensers) do not use energy but destroy exergy(second-law approach) because of the heat transfer across a finitetemperature difference. Increasing transfer area of exchanging de-vices reduces temperature differences, increases exchanging effec-tiveness and improves system performance by using energy moreefficiently. Fluid movers (fans and pumps) and heat generators(chillers, boilers, heat pumps, furnaces, etc.) are the main energyusers since they consume energy (first-law approach).

4.2. Energy balances in HVAC subsystems

The whole HVAC system in Fig. 2 can be simplified to identifyfour subsystems: heat generation, coolth generation, water trans-port and air transport (Fig. 5).

Below, we present a first-law analysis of HVAC energy conver-sion. Power conservation equations in each of these subsystemsare written in terms of final energy, making use of the followingsymbols: thermal loads (Q), positive for the heating mode and neg-ative for the cooling mode; power losses (L), positive when theyleave the control volume and negative if they represent gains;and power consumptions (C), always positive inputs.

– Heat generation. Heat generators use a given amount of energywith the aim of adding heat to water or air. In boilers and fur-naces, energy losses are caused by unburned fuel, flue gasesand through the jacket. In the case of heat pumps, the heat gen-eration losses are negative and represent the energy taken fromthe environment in the evaporator (an energy gain). The energybalance in heat generation subsystem is then:

CHG " Q HG % LHG #2$

– Coolth generation. This subsystem consists of set of equipmentresponsible for the coolth generation. The heat extraction fromthe cold source requires the consumption of a given amount of

energy. The energy rejected to the environment is positive andhigher than the absolute value of heat extraction from the coldsource,

CCG " QCG % LCG #3$

The equipment responsible for heat rejection could be inte-grated into the coolth generation subsystem. Consequently, en-ergy consumption of this subsystem must include not only theenergy used by refrigerant compression, but also the energyconsumption of condensing fans for air-cooled devices and en-ergy consumed in the condensing water loop (pumps, towerfans, water supply, etc.) for water-cooled equipment.

– Water transport. The subsystem is made up of hydraulic equip-ment (pumps, piping and accessories) intended to drive the pri-mary fluid, usually water, from the generation equipment to thewater coils. Heat added or extracted by generation subsystemsto primary fluid is usually referred to as primary load (positiveand negative for heat and coolth generation, respectively). Thusthe balance equation for the water subsystem is:

QPRI " Q COIL % LWT & CWT #4$

There are two kinds of water transport losses. On one hand,there are thermal losses in the water distribution network. Onthe other, there are losses caused by inefficiencies in the pump-ing equipment: electric motor losses and transmission losses.The difference between water transport electric consumptionand pump losses is thermally degraded in the fluid and can bereferred to as ‘pump heat’,

QWT " CWT & LPUMP " CWT & #LMOT % LTR$ #5$

and primary load can be reformulated as:

QPRI " Q COIL % LPIP & Q WT #6$

If heat fluid is distributed, Eq. (6) reveals how pipework thermallosses increase the need of heat generation, while pump heat(transport thermal effect) represents a gain and reduces heatgeneration needs. For cold fluid distribution, thermal gainsand pump heat both increase the need of coolth generation.

– Air transport1. This subsystem is responsible for air distributionthroughout the building. The only energy use involved is that offans. If fan motor is placed in the air stream, all the consumedenergy is thermally degraded and can be called fan heat

Fig. 5. Energy subsystems in HVAC systems.

1 Some sources consider a transfer subsystem made up of the air-handlingequipment. If energy of condensed water in cooling coils is neglected and thermallosses and air leaks in air-handling equipment are included in the air transportsubsystem, it turns out to be energy irrelevant and could be omitted.



(QAT = CAT). Losses have either a thermal character or are causedby air leaks or infiltrations. From the mass and energy balancesin this subsystem the following equations can be derived:

Q COIL " QSP % Q VEN & QAT % LAT #7$

and

LAT " LDUCT % LLEAK #8$

Q SP " _mSA#hSA & hRA$ #9$

Q VEN " _mOA#hRA & hOA$ #10$

LLEAK " _mEXH#hEXH & hRA$ % _mLEAK#hLEAK & hRA$ & _mINF#hINF & hRA$#11$

4.3. HVAC Sankey diagrams

Sankey diagrams are an important and useful graphical tool formapping the chain of energy flow. Their basic aim is to visualizeenergy flows, energy balances and energy losses of energy systems.The idea is to represent the quantity of energy by the use of arrows,with the arrow width being proportional to energy flow, allowingdominant energy flows to be quickly identified. A detailed reviewof the history, development and applications of Sankey diagramscan be found in [28,29].

Taking into account the loops, subsystems and energy flows al-ready defined in the previous sections, the drawing of Sankey dia-grams for HVAC systems poses no problem and is very informative.A similar exercise has been included in annex K of Standard EN13790 to illustrate the energy flows in HVAC systems.

This paper shows the Sankey diagrams for the typical constantair volume systems installed in office buildings in Spain. Final en-ergy flows are based on average values of engineering ratios takenfrom real HVAC designs. These data were gathered for the designand simulation of HVAC systems in office buildings within the EEC-CAC project [30]. Final energy flows in all the diagrams are normal-ized to represent the percentage of space load.The Sankey diagramin heating design conditions (Fig. 6) help us to arrive to interestingconclusions:

– The system must add heat to conditioned spaces and to outdoorair2. Thermal energy supplied to provide the heating service is ofthe same order of magnitude as that required for spaceventilation.

– Thermal energy exchange in air handling equipment is usuallycalled coil load and comprises the simultaneous effects of theventilation, spaces, fans and air transport losses (Eq. (7)). Conse-quently, the coil is the first element of the transfer chain wheresystem load is transferred.

– The heat added to the water in the heat generator, either a boi-ler or a heat pump, is known as primary load and differs fromcoil load because of pump heat and water transport losses(Eq. (6)).

– Fans consumption is significant (about 30% of space load) andcould be thought as an energy gain because it heats supply airflow. This is also the case of pump heat, although at a lower rate(0.6%).

– Thermal losses in air and water distribution networks are notrelevant (below 4%) if they comply with insulation require-ments in building regulations. The relative weight of these

losses strongly depends on system type. Obviously, in all-airsystems losses tend to be higher in ducts, whereas in all-watersystems the losses in the hydronic network predominate.

– The type of heat generator has no impact on the primary load,but it conditions the final energy use since heat pump electricconsumption is about one third of boiler’s energy use. Obvi-ously, if converted to primary energy, electricity would be mul-tiplied by '3 and the energy use of heat generators would be ofthe same order.

Regarding the Sankey diagram in cooling design conditions (Fig. 7)the following considerations can be made:

– The system must extract energy from conditioned spaces andfrom outdoor air flow when outdoor air temperature is higherthan that of the return air flow. Usually, in cooling design con-ditions, the absolute value of ventilation load is below that inheating design conditions because of the lower differencesbetween outdoor and indoor temperatures.

– If during the provision of a cooling service at conditionedspaces, the outdoor air temperature falls below that of thereturn air flow, ventilation load would reduce space load, pro-viding a ‘free’ portion of the necessary cooling and reducing oreven cancelling cooling coil load. Under such conditions, theenergies to provide cooling and ventilation services have oppo-site signs and the coil must meet the ‘net load’ (Fig. 8).

– The thermal equivalent of fans consumption represents anenergy loss, since it increases cooling coil load significantly.To a lesser extent, this is also true for pump heat, whichincreases cooling load on primary equipment.

– Thermal gains in air and water distribution networks are con-sidered as ‘energy losses’ since they increase cooling load oncoils and primary equipment.

– The type of condensation has no impact on cooling generationload, but it is determinant for its electricity consumption, whichis higher for air-cooled chillers.

5. HVAC energy resources

Building services are end-users of energy, or stated in anotherway ‘final energy is consumed’ by buildings. This energy flow couldbe simply referred as final energy use. If we knew the processesand conversions that energy resources have undergone fromextraction to consumption, the final energy use could be translatedto primary energy. Analogously, the environmental impact of en-ergy use could also be estimated in terms of greenhouse gas emis-sions in the extraction to consumption path.

As for HVAC systems, energy consumption is necessary to oper-ate end-use conversion devices, mainly thermal generators andfluid movers. Below we briefly comment on energy supply sidefor HVAC systems.

Electrical power, natural gas, oil derivatives, coal and renewablesources are the most widely used energy carriers3 in HVAC systems.Fuels are the most frequently heat source, although the proliferationof heat pumps has increased heating electrical consumption in re-cent years. In coolth generation, electricity is almost the only coolthsource, given the small market share of gas engine driven chillers,gas-driven air conditioners and absorption refrigerating machines.Transport subsystems and auxiliary equipment use electrical energy.

In Europe, recent work of CEN/BT WG 173 [31] has significantlyincreased the complexity of the supply side of buildings and conse-quently of HVAC systems. This is due to the introduction of theconcepts of standard ISO 13600 [32] for the analysis of energy

2 Situations in which conditioned spaces must be heated (QSP > 0) and outdoortemperature is above indoor temperature (QVEN < 0) are very infrequent (start-up inhigh thermal inertia buildings).

3 In district heating and cooling systems, imported thermal energy must beconsidered an energy carrier.



systems within the building field. This has generated additionalconfusion with the application of many ‘energy types’ for buildingenergy analysis: delivered energy, exported energy, net deliveredenergy, cogenerated energy and self-generated energy. In orderto clarify this topic, let us review the schematic in Fig. 9. Theamount of energy delivered to building site by energy companiesis referred to as delivered energy or site energy. For those buildingsexporting self-generated (or cogenerated) energy, net delivered en-ergy can be defined as:

Net delivery energy " Delivered energy& Exported energy#12$

Almost every building is equipped with thermal energy gener-ation for its own use, but rarely part of this energy is exported.Electricity generation and cogeneration of heat and power inbuildings have become not uncommon, either for their own useor to be exported, especially since energy policies provide incen-tives or require feasibility studies for this kind of facilities, asthose laid down by the European Directive on the energy perfor-mance of buildings [33] for new buildings with a floor area above1000 m2.

The most controversial subject is the use of renewable energytaken directly from the environment (site-renewable energy). Thisenergy flow can encompass: daylighting, natural ventilation,

Fig. 6. Sankey diagrams of a constant air volume system in winter design conditions. a) Hot water boiler. b) Air to water heat pump.



free-cooling, passive cooling and heating systems, heat pumps forspace heating and self-generation of thermal or electric energyfrom renewable sources. The use of these free and inexhaustibleenergy sources reduces the need for importing energy and therebydelivered energy.

6. Discussion

In this section, the links of the HVAC systems energy chain arediscussed and the ways to reduce energy use, improve energy effi-ciency and promote the use of renewable energy are examined.

Fig. 7. Sankey diagrams of a constant air volume system in summer design conditions. (a) Air-cooled chiller, (b) Water-cooled chiller.



6.1. Final service

Usually, the higher the service quality demanded by the users,the higher the need of energy to supply it satisfactorily. Conse-quently, a careful balance between energy use and the amenitiesdelivered using that energy is mandatory to avoid unnecessarywaste [34]. The large impact of final services on the global energychain should be examined by researchers, practitioners, policymakers and final users in their search of saving energy in buildings.

Ventilation rates and indoor comfort conditions are two crucialissues to be tackled, regarding HVAC service. HVAC energy policyshould set limits on both issues to avoid unnecessary over-ventilation and bring winter and summer space temperatureslower and higher, respectively. These measures could be thoughtas a first step to reduce building energy use, since they affect thefinal service in the HVAC energy chain.

Minimum ventilation rates in non-residential buildings causecontinuous controversy, as clearly shown by national standardsfigures (Table 2). For example, for office buildings Spanish regula-tions specify 12.5 l/s per person, well above other countries. Addi-tionally, the same standard may include different calculationmethods leading to different ventilation rates. The use of catego-ries to specify ventilation service quality may also cause confusion.In Europe, standards EN 13779 and EN 15251 recommend differentventilation levels to reach indoor air classes (IDA 1 to 4) or catego-ries (I–IV), respectively. Anyway, minimum ventilation rates inbuilding regulations should be considered as a maximum recom-mendation, since ventilation normally increases energy consump-tion, electrical peak demand and operation costs of HVAC systems.Only compelling reasons should lead the designer to specify higherminimum outside air rates than the minimum requirements in thestandards. Indeed, there are building regulations [39] that requireHVAC systems to be adjustable, so that ventilation rates can be re-duced in the future to the minimum mandatory requirement toprevent over-ventilation.

As for indoor comfort conditions, there is a considerable trendto relax comfort range, specially in buildings without mechanicalcooling due to the different expectations and adaptation capacityof building occupants [40,41]. Currently energy regulations setrequirements on maximum heating and minimum cooling temper-ature set points for occupation periods and require a minimumdead band within which the supply of heating and cooling to thespace is shutoff or reduced to a minimum.

6.2. Passive system

The impact of building shell on ideal demand has been widelyanalysed and large savings potential can be achieved by designingbuilding envelope to low-energy standards (such as Passivhaus inGermany [42] and R-2000 in Canada [43]) using high insulation,good air tightness and heat recovery ventilation systems. Thesekind of measures should be thought as a second step to reducebuilding energy use, since they affect the passive system in theHVAC energy chain.

The significant impact of building shell on HVAC (and lighting)energy use is usually addressed in energy policy instruments. In-deed, most energy regulations include a section about the buildingenvelope, while other nations introduce an independent standardfor the limitation of ideal thermal demand of buildings.

6.3. Conversion device

The analysis of HVAC systems as energy conversion devices al-lows us to identify three types of energy flows: thermal loads, en-ergy consumptions and energy losses. Thermal loads quantify heattransfers: at conditioned spaces, to ventilation air flow, at coils orat primary equipment. Consumptions refer to energy end use ofconversion devices, mainly thermal generators and fluid movers.Energy losses are due to wasted energy or equipment inefficien-cies. Reducing HVAC loads and HVAC losses would be the third stepto reduce energy use in HVAC systems. Below we comment onthese flows.

The word ‘load’ should be clarified in the HVAC field to avoidcommon confusions. Space load (Eq. (9)) can be calculated fromsupply air flow rate and the enthalpies of supply and return airstreams. Ventilation load (Eq. (10)) depends only on climate andon HVAC design air quality requirements. Coil or secondary load(Eq. (7)) quantifies the net load due to conditioned spaces, ventila-tion, air transport thermal equivalent and air transport losses, ex-cept for dedicated outdoor air systems (DOAS) in which outdoorair flow is handled independently. Primary load (Eq. (6)) is esti-mated from secondary loads, water transport losses and thermaleffect of pumps.

HVAC losses can be classified as transportation losses, genera-tion losses and ventilation losses. Reducing these losses automati-cally reduces energy consumption and improves energy efficiency.

Fig. 8. Sankey diagram of a constant air volume system in the cooling mode when outdoor air temperature is below space temperature. Air-cooled chiller.



Transportation losses are due to thermal losses in fluid distribu-tion network, losses due to leaks and infiltrations in ducts, andlosses caused by inefficiencies of fluid movers. Building energy reg-ulations usually limit distribution thermal losses through

minimum insulation requirements, while the reduction of air leaksand infiltrations in air systems is achieved by a sealing require-ment. Pumps and fans losses should be reduced by the use of highefficient equipment.

Generation losses refer to inefficiencies related to heat and cool-th generators. Losses in coolth generators are mainly due to theheat rejected to the environment in the refrigeration unit. This heatcould be recovered to heat sanitary water or to meet simultaneous(or non-simultaneous, with the use of heat storage) HVAC heatingloads. For heat generators, it is wise to distinguish between heatpumps and fuel burning devices. In the first case, the energy takenfrom the environment in the evaporator constitutes an energy gainand thereby heat pumping could be equivalent to using renewableenergy from the environment [11]. Losses in fuel burning devicesare caused by unburned fuel, exhaust gases or through the jacket.

Fig. 9. Building boundary and energy flows.

Table 2Minimum ventilation rates in building regulations. (Office space. Occupancy density10 m2/per).

Country Energy code Outdoor air rate (l/s/per)

USA ASHRAE 62 [35] 5.5China EEPB [36] 8.3UK Part F [37] 10.0Spain RITE [38] 12.5



In boilers, it is possible to recover part of the energy in combustiongases (flue losses) to preheat inlet air or water flows. By means ofcondensing boilers, latent heat recovery (condensation of water influe gases) can also be achieved to increase energy efficiency.

Thermal energy in exhaust air flow is an energy loss that couldalso be recovered. The aim is to reduce coil load, either by the useof heat exchanging devices to transfer heat from exhaust to out-door air flow (passive recovery), or by the use of a heat pump totransfer heat from exhaust air to mixed air flow (active recovery).The operation of a heat recovery system may increase fan energyuse, save heat and coolth generation energy and reduce the poten-tial of free cooling, so these factors should be carefully balanced toreach global energy savings.

6.4. Energy resources

Energy efficiency measures affecting the supply side of HVACsystems could be thought as a fourth step to lower building energyuse, since they may reduce the need to import energy and alleviateshortage and environmental concerns.

In our opinion, the use of daylighting, free-cooling or heatpumps is as renewable as the installation of solar panels. However,some authors make distinctions between active and passiverenewable sources. Those technologies allowing on-site thermalor electric generation from solar, wind or water energy are referredto as active, and they are usually favoured in energy accounts, sincethey are thought to increase renewable share without reducingdelivered energy use. The rest of the technologies are referred toas passive and are treated as energy saving measures not countedas renewables by energy information organisations. Site-renewables should be considered all the same and energy policiesshould provide incentives for all of them, avoiding the current pre-dominance of active renewables to the detriment of passive ones.

7. Conclusions

The map of energy flow to provide thermal comfort has beentraced to clarify the four basic stages in the HVAC energy chain:(1) thermal comfort as the final service delivered to building occu-pants, (2) conditioned spaces as passive systems where useful heator coolth is degraded to provide comfort service, (3) HVAC systemsas the complex conversion devices where energy carriers are trans-formed into useful heat, coolth and fluid motion, and finally, (4)HVAC energy use and its impact on energy resources. Each of thesestages brings significant opportunities to save energy and thereby,they should be exploited by designers and policy makers in orderto reduce HVAC energy use and its impact on the environment.

A rational use of energy should first focus on the service de-manded by the users. Comfort service quality depends on theHVAC system ability to control indoor air quality, basically throughventilation and indoor comfort conditions. Ventilation rates and in-door comfort conditions are key elements for energy conservation,and consequently, both users and public administrations shouldtake measures to avoid over-ventilation, and to bring winter andcooling setpoints lower and higher, respectively, since an accept-able thermal comfort service can be provided with wider deadbands and lower ventilation rates. These measures could bethought as the first step to reduce HVAC energy use.

The amount of theoretically necessary useful heat or coolth toprovide thermal comfort could be referred to as the ‘ideal demand’,and could be thought of as an indicator of the energy quality ofarchitectural designs. Ideal demand deals with the passive systemin the HVAC energy chain and may be significantly reduced byimproving building shell design with more stringent envelope

requirements in building energy regulations. This should be thesecond step to reduce HVAC energy use.

Two helpful concepts have been introduced for the analysis ofHVAC systems as conversion devices: HVAC loops and HVAC sub-systems. The first concept arises from the examination of the heattransfer chain. Energy balances in the HVAC subsystems allowsrepresenting Sankey diagrams in which HVAC loads and losseshave been identified to guide the improvement of energy effi-ciency. This should be a third way to reduce HVAC energy use.

Lastly, energy efficiency measures affecting the supply side ofHVAC systems (site-renewables, self-generation, etc.) could bethought as a fourth path to lower building energy consumption,since they may reduce the need to import energy and alleviateshortage and environmental problems.

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