Paper ID: ICAE2013-Xxx - DocsLib

Paper ID: ICAE2013-Xxx

International Conference on Applied Energy ICAE 2013, Jul 1-4, 2013, Pretoria, South Africa Paper ID: ICAE2013-xxx ANALYSIS OF THE ENERGY AND COST SAVINGS CAUSED BY USING CONDENSING BOILERS FOR HEATING DWELLINGS IN GREECE

Vasileios Bonaros1, John Gelegenis1, Douglas Harris2, George Giannakidis3, Kostas Zervas1 1Technological Educational Institution (T.E.I.) of Athens, Greece 2Heriot-Watt University, United Kingdom 3Centre for Renewable Energy Sources (C.R.E.S.), Greece

o ABSTRACT TA Ambient temperature ( C) o TD Design temperature ( C) Condensing boilers have been in use for space and domestic o water heating in many countries for some years now, and in TR Water return temperature ( C) o the present work their potential for using them in Greece is TS Water supply temperature ( C) evaluated from both the technical and economic viewpoints. TLC Total heat loss coefficient (W/K) There is a significant market penetration of these devices in α Excess air of combustion Greece, and the economics of their operation vary according to the climate zone in which they are located. The Subscript technology proved to be cost effective for the areas of B Boiler Greece where natural gas is available, with payback periods HM Heating medium ranging between 3 and 5 years. Critical factors affecting the economy of condensing boilers proved to be the total heat loss coefficient of the dwelling, the heating media applied, Superscript the cost of equipment, the energy prices ( for gas and O Nominal conditions electricity) and last but not least the hours of operation of the heating system. These may vary if the use of the boiler is combined with other systems such as solar water heating or a 1. INTRODUCTION heat pump, and the latter system may offer a flexible alternative which can produce optimum running costs Due to the exploitation of the latent heat in the depending on the demand and the price of gas and electricity combustion products, condensing boilers may achieve applying at the time. efficiencies about 10% greater than a high efficiency conventional boiler at the top end of their load cycle, but Keywords: condensing boiler; energy savings calculation; even when not operating in the condensing mode there is dwellings; heating system; cost-effectiveness still some advantage due to their greater heat transfer area [1]. Although applicable with various fuels like light oil, NONMENCLATURE petroleum gases and natural gas (NG), the last is the most interesting due to its higher vapor content in the flue gas Abbreviation (hence, the higher dew point of the flue gas, the easiest to DHW Domestic Hot Water achieve condensation and the greatest energy recovery NG Natural gas potential). The energy benefits are obvious when one TOTEE Technical Guide (T.O.) edited by the Greek considers that e.g. for NG more than 10% of its gross Technical Chamber (T.E.E.) calorific value is released in the form of latent heat which would otherwise escape through the flue. Symbols Apart from the need to neutralize and dispose of the n Boiler efficiency condensate, condensing boilers do not introduce any PL Boiler part load (%) complication or special requirements in the installation,. PSL Boiler standing losses (W) Since their cost is 30-50% higher than that of conventional boilers, it is a matter of monetary benefits for someone to Q Heat output (W) opt for a condensing boiler instead of a high efficiency Paper ID: ICAE2013-xxx conventional unit. This over-cost, which is usually in the The selected boilers are made by the same manufacturer, range of 10-15€/kW [2], is due to their greater heat and have quite similar characteristics. Both boilers are exchange area, to the provision for condensate collection modulating, with a limited capability however (3.5:1 for the and rejection, to the specific material required to withstand condensing, 2.5:1 for the conventional unit), and accept an the acidic condensate and to their more sophisticated external temperature probe to apply compensation (the control requirements in order to sustain the excess air ratio data for the boilers are presented in table 1). and accurately compensate for the various weather conditions. The additional capability for instant production 2.2 Weather data of domestic hot water (DHW) (combi type boilers) increases Greece is separated into four climatic zones, according to the annual fuel savings even more when compared to a the heating degree-days, as shown in table 2. Natural gas is conventional combi boiler. available in areas of zone B (in the capital Athens, and in the Detailed analysis of condensing boiler performance was area of Volos), and zone C (in the broader area of given by Lazzarin and Schibuola [3]. Based on a short Thessaloniki, and in the rest of Thessaly Prefecture where estimating method, the authors found for the Italian Volos also belongs). Typical temperature distributions for territory that the over-cost of a condensing boiler can be zones B and C are simultaneously given in fig. 1 [7]. paid back in between 2/3 and 5 years, depending on the climatic zone. Comakli [4] focused more specifically on Table 1 Operating data of the boilers combi boilers, but the case he studied referred only to Condensing unit Erzurum, Turkey, which is a rather extreme case (4634 K- Max Min days). Although the author concluded a marginal economy Methane input (kW) 23.7 for the complete boiler, an attractive pay-back period Methane flow (m3/h) 2.51 results for the over-cost, in spite of the low value assumed (80-60oC) 22.7 6.5 for the price of NG (0.398$/m3). In a more recent work, Output (kW) (50-30oC) 24.8 7.3 Lazzarin [5] presented history and detailed data of a o relevant case in the city of Treviso (2378K-days), and Flue temp. – air temp. ( C) 67 49 calculated for the complete condensing boiler retrofit a pay- Flue mass capacity (g/s) 12.4 3.1 back period of 8 years. Bonaros [6] found for the cities of CO2 (%) 8.8÷9.1 8.8÷9.1 o Athens and Thessaloniki, Greece, pay-back periods of 4.5 (80-60 C) 96.0 - Boiler efficiency o and 3.5 years, respectively, when the over-cost against a (50-30 C) 104.8 106.9 high-efficiency conventional boiler is considered. Casing loss, burner on/off % 1.4/0.2 2.1/0.2 Due to the volatility of the energy prices and the Flue losses, burner on (%) 2.6 2.2 introduction of new regulations (e.g. the Energy Boiler efficiency at 30% load 109.1 Performance of Buildings Directive of the European DHW capacity (L/min) 13.4 (ΔT=30oC) Council), it is interesting to shed more light on the Maximum consumption (W) 145 parameters that affect the economy of a condensing boiler, and find out circumstances where this equipment is indeed Conventional unit advantageous. This work attempts an analysis of such Max Min parameters by considering the use of a combi condensing Methane input (kW) 25.5 boiler in Greece. Methane flow (m3/h) 2.70 Heat output (kW) 23.77 9.9 2. DATA Flue temp.– air temp. (oC) 98 60 Flue mass capacity (g/s) 12.9 14.0 2.1 Boilers CO2 (%) 7.7 2.9 Two similar combi boilers of 24kW capacity were selected Boiler efficiency rate 93.2 90.5 for comparison, a condensing and a high efficiency Casing loss, burner off ( %) 0.23 0.23 conventional unit, because most units marketed in Greece Flue losses, burner on (%) 6.57 9.27 are of this size. Besides, instantaneous DHW service Boiler efficiency at 30% load 90.2 o requires at least 24kW nominal capacity [5]. A boiler with a DHW capacity (L/min) 11.3 (ΔT=30 C) similar size (26kW) was also examined in [3] but also in [6] Maximum consumption (W) 130 (20kW). Above this size, there are obviously important economies of scale, but of limited usefulness, since this 2.3 Use of natural gas in Greece capacity does already largely overcome the ambient heating Natural gas was introduced in Greece in the mid ‘90s, but peaks. in practical terms started penetrating the market in early

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2000, with a steadily increasing rate (e.g. in the broader unchanged in the Country in the last few years, a fact that area of Athens the NG sales increased from 98 mill m3 in seems to be in contradiction to what happens in more 2007, to 231 mill m3 in 2011; in Thessaloniki almost 20,000 mature markets e.g. in the Netherlands where from about new supply contracts have been signed every year since 20€/kW before 2000, dropped to almost half of this value 2004). The great majority of the contracts (≈90%) are for until 2006 [2]. A greater variation however had been independent heating, with the most popular application noticed long ago, in the work of Weber et al. [8] who found being a wall mounted boiler placed on a balcony and an over-cost in selected European countries ranging supplying a high temperature hydronic system with between 260-780€, so it is probable that the referred over- traditional radiators; the same technology is investigated in cost will decrease in the Country in the coming years too. the present work. Table 3 Composition (% v/v) of NG supplied to Greece In this way, NG became a competitor to diesel oil fuel for space heating, as a less expensive, cleaner, and more Constituent Russia Algeria Mixture conveniently supplied (through a network instead of CH4 98.0 91.2 96.3 refillable tanks) fuel. Indicative analysis of NG imported C2H6 0.6 6.5 2.1 from Russia and Algeria to Greece is given in table 3. The C3H8 0.2 1.1 0.4 dew point (for 20% excess air) of the resulting mixture C4H10 0.2 0.2 0.2

(Russian NG contributes 75% of the supplied gas) is very C5H12 0.1 - 0.1 o close to the dew point of methane (56.2 C), which can be CO2 0.1 - 0.1 consequently assumed as the unique constituent in NG for N2 0.8 1.0 0.8 the rest of the calculations. Table 2 Climatic zones of Greece, and heating degree-days. Both boilers assumed in the present study have rather similar electric power consumption (130-145W, see table 1). Climatic Heating Degree-days Nevertheless, electricity is also considered here, as a very zone (K-days) popular and competitive energy source both for heating and A 601-1100 DHW production. Electricity is offered at this period at a B 1101-1600 rather competitive price, even for use in direct electric C 1601-2200 heating, and becomes more competitive when time- D 2201-2620 dependent charging is considered. For economic and other practical reasons (independence from a fuel based central heating system, low purchase cost of electrical heaters) there is nowadays some move in the Country from fuel based to electrically driven heating. In table 4, indicative prices for the various energy sources used in dwellings are presented. Zone C Zone B Table 4 Indicative prices of energy sources for use in the Greek domestic sector FUEL Price/unit GCV Price/MWh Diesel Oil 1.30€/L 11.9kWh/L 110 Pellets 0.30€/kg 5kWh/kg 60 Natural gas 11.6kWh/Nm3 82 Electricity 145 Wood 0.12€/kg 4.2 kWh/kg 29 Figure 1 Typical temperature accumulated distribution curves for zones B and C [7] 2.5 Other assumptions The application of a condensing boiler for independent heating of a dwelling is considered. According to Greek 2.4 Economic analysis data statistical data, the mean floor area of single dwellings The suppliers in Greece offer condensing boilers at an increased in recent years from 85m2 (before 1980) to over-cost (in comparison to a similar conventional high 150m2, and the same happened with apartments where the efficiency but non-condensing unit), ranging between 450- mean floor area increased in the same period from 65m2 to 650€, with the lower limit referring to more expensive 110m2, hence a typical floor-area of 120m2 could be equipment. This price difference has remained practically assumed. However, instead of specifying the floor area and

3 Copyright © 2013 by ICAE2013 Paper ID: ICAE2013-xxx construction characteristics of the dwelling, we apply as a The water return temperature depends on the supply parameter the total heat loss coefficient (TLC) of the temperature, which in its turn depends on the ambient dwelling. TLC may reach values higher than 600W/K for temperature and the selected compensation curve for the non-insulated dwellings (built before the Thermal Insulation operation of the boiler. For the condensing boiler, curved Regulation applied in the country in 1979) down to 300W/K lines can be followed, which match better to radiating for an insulated dwelling or even lower for those built after media. For the conventional unit, control allows application the stricter standards resulting from the enactment of the of linear compensation. The return temperature has an European Council Energy Performance of Buildings Directive impact on the flue gas temperature, which actually in 2010. determines the efficiency of combustion. The DHW needs are estimated at 75L/d per bedroom, of The model was calibrated according to the data released water at 45oC (existence of two bedrooms is assumed). In by the manufacturer. For supplies within the European spite of the high instantaneous efficiency of hot water Union, manufacturers provide information according to the production, small flow rates lower the efficiency of combi specific requirements of the Council Directive 92/42/EEC boilers (there is no storage) due to cycling. Hence, in the [10], an extract of which is shown in table 5. place of the nominal efficiencies of 100% for the condensing The model was additionally tested for the accurate and 93% for the conventional unit, seasonal efficiencies of reproduction of other available data (given also in table 1),

88% [5] and 80% [4] are respectively assumed. namely flue gas flow rate and CO2 content, water return temperature and flue gas exit temperature. Afterwards, the 3. THEORY AND CALCULATIONS model was used to further predict the efficiency of the boilers for cases where these data were not available. For the condensing boiler, the resulting performance 3.1 Modeling the performance of the boilers curves are shown in fig. 2. As expected, the efficiency is Estimation of boiler perfromance incorporates two tasks slightly increased with the decrease of the load (due to the (i) the estimation of the boiler efficiency for the various reduction of the flue gas mass flow rate), but more loads within its nominal range of operation, for various importantly with the lowering of the water return return temperatures of the heating medium (water), and (ii) temperature (flue gas exit temperature is consequently the calculation of the cyclic efficiency of the boiler, when reduced). the load is below its minimum capacity. The boiler efficiency was approximated by developing a 1.10 o simple model, similar to that proposed by Che et al. [9], TR=30 C based on the combustion reaction, and the change of load 1.05 o TR=45 C and heating media operating temperatures with the ) V

C 1.00 ambient temperature. The combustion reaction has the o N

( TR=60 C

general form: y c 0.95 n e → i CH4+2∙(1+α)O2+7.5∙(1+α)N2 CO2+2H2O+7.5∙(1+α)N2+2∙αO2 c i

f 0.90 f The return TR and supply temperature TS follow the E relations: 0.85

o o 1/m TS +TR - TD-TA 0.80 TR=TD+ ∙ - { TD }{ } 2 TD-Tmin 0 20 40 60 80 100 o o TS -TR TD-TA - ∙ { 2 }{ TD-Tmin } Part load (%)

o o TS -TR TD-TA TS=TR + ∙ Figure 2 Variation of condensing boiler efficiency with the { }{ } o 2 TD-Tmin load and the return temperature (30/45/60 C) where m specifies the behavior of the heating media, For the conventional boiler we found that the efficiency according to the exponential relation: increases with the decrease of return temperature, as m o THM-TD shown in fig. 3, where the two lines correspond to the QHM=QHM o { THM -TD } maximum heat flow (upper line) and to the minimum heat flow. Nevertheless, the factor which finally determines the and depends on the heating media used (e.g. for efficiency is the output of the boiler, and due to modulation conventional radiators m=1.33). of gas flow without the respective adjustment of air supply (to sustain air/fuel ratio), a decrease of output leads to

4 Copyright © 2013 by ICAE2013 Paper ID: ICAE2013-xxx increase of excess air and hence to a slight decrease in the the casing are very low, affecting in this way the cyclic efficiency, as indicated by the dashed line in the same efficiency at very low loads where condensing boilers figure. Hence, the boiler keeps rather steady efficiency already have high efficiency due to condensation. without substantailly gaining from the lowering Based on the intermittency of burner operation, temperatures. For instance, at maximum gas flow, decrease Malmstrom et al. [11] analysed the various losses of boilers of flue gas temperature from 100oC to 60oC would increase and remarked that a linear relationship between the load the efficiency from 93 to 96%, decrease of flow at the same and the energy consumed is a good approximation for temperature of 100oC would lower the efficiency to 86%, domestic units. This linear relationship, as well as a steady and the combined effect finally leads to a slight decrease of state model, proved adequate for calculations for efficency to 90% (dashed line in fig. 3). conventional boilers, with variation in the prediction of energy consumption below 3% [12]. In the same context, Park and Liu [13] found experimentally that the efficiency of a boiler, when cycling at various partial loads, can be estimated from its maximum capacity and its standing losses. Hence, a relationship of the following form arises between the nominal efficiency nO and the cyclic efficiency

nC: nO nC = O PSL 100 1- O { 1- } QB PL Rosa and Tosato [12] noticed that the same linear relationship may be valid for condensing boilers, too, but only over a narrow range of loads, otherwise the variation of losses should be also considered. Actually, condensing boilers have modulating capabilities, hence cycling starts Figure 3 Variation of conventional boiler efficiency with flue below the bottom end of their operation range where the gas/water return temperature, for maximum (upper curve), energy demand is in any case very limited, and this has a minimum (lower curve) and variable gas flow (dashed line). minor impact on the accuracy of the whole calculation.

Table 5 Minimum requirements for boiler efficiency, according to 92/42/EEC European Council Directive [10] 4. RESULTS - DISCUSSION Low temperature Gas condensing boilers boilers 4.1 Application of condensing boiler for continuous Efficiency heating and DHW production

at rated ≥ 87.5+1.5·log(Pn) ≥ 91+log(Pn) We applied the model of the previous section to estimate output* the annual fuel consumption for heating and DHW supply in

Efficiency ≥ 87.5+1.5·log(Pn) ≥ 97+log(Pn) a typical dwelling, the potential gas savings by the use of a at 30% average water- temperature of condensing boiler and the relevant economy of the system. part load boiler temperature boiler water In fig. 4 we present the annual gas savings by the use of a 40oC supply 30oC condensing instead of a high efficiency conventional boiler * for average boiler-water temperature of 70oC (in kWh per year) for the two climatic zones B and C of Greece, together with the corresponding pay-back periods for the over-cost. Continuous heating (24-hours daily) and a 3.2 Cyclic efficiency of the boilers balance temperature of 18oC were assumed, which led to When the boiler is cycling, a mean (cyclic) efficiency is rather optimistic estimations concerning the economy of defined to incorporate all losses namely with the flue gas the boiler. (sensible and latent heat), through the chimney (ventilation Regarding typical TLC values, in the range of 300-400 losses) and through the casing (surface losses). In the forced W/K, pay-back periods between 3 to 5 years arise for the air-circulating burners, switching off the boiler (and the fan) two zones. As expected, the less insulated a dwelling is, the temporarily halts any draft losses through the chimney. most profitable a condensing boiler becomes. Since there is Moreover, in modern high efficiency boilers, losses through great room for savings, the short pay-back renders condensing boilers in these cases a first priority measure.

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Although it is rational to upgrade the envelope before equivalent to 2,8 GWth of installed thermal power. changing the equipment e.g. to avoid later penalties due to According to a recent law in the Country, all new buildings oversizing, it seems that there is not such a problem with should cover at least 60% of their DHW needs with solar modulating condensing boilers as they demonstrate high energy . Consequently, an increased rate of installation of efficiency in the complete range of their operation. new solar hot water systems, with an impact on other alternative DHW production systems such as combi boilers, is expected. Considering that the assumed combi boiler will cover up to 40% of DHW needs, then the annual gas savings are consequently decreased by at least 120 kWh-gas and 130 kWh-gas for zones B and C, respectively. Table 6 Gas consumption for DHW for the two types of boilers, and consequent potential savings Gas consumption for DHW Savings -200kWh/yr Condensing Conventional (kWh/yr) Zone B 1966 2163 197 Zone C 2124 2336 212

4.3 Use of condensing boiler for intermittent heating The results shown in fig. 4 assume continuous heating of Figure 4 Annual fuel savings and pay-back period for the the dwelling. However, intermittent heating is more usually over-cost of using condensing instead of conventional boiler applied, following the occupancy pattern, while during the (continuous lines refer to zone C, dashed lines to zone B. night set-back may be applied or no heating supplied at all. In this context, the assumption of 8-hours heating per day is The above assumed TLC range reflects a sufficiently quite reasonable. The respective consumption can be 2 insulated dwelling of 120m floor area. The consumption approximated by the use of an appropriate reduction was estimated at 100 and 155 kWh/yr for zones B and C, coefficient αred, which for 8-hours daily heating (7d/w) and respectively, which are quite close to the actual mean for a thermally medium-weight construction gives the values [14]. The seasonal efficiencies for the condensing approximate value of 0.70 (Comakli [4] applied accordingly boiler are 98.6% and 99.2%, and for the conventional boiler a coefficient of 0.67). Diagrams of fig. 4 may be still of use,

87.5% and 88.6%, for zones B and C, respectively. Hence, provided that the product (αred·TLC) is introduced in the the application of a condensing boiler may lead to 10.5-11% place of TLC. gas savings. As these are maximum achievable savings, Nevertheless, a critical factor in intermittent heating is further consideration of the factors that may have an the preheating period that is needed until the dwelling impact on the economy of condensing boilers is necessary. again reaches the set-point temperature. During this period, it is reasonable for the heating system to operate at full 4.2 Combined use with a solar hot water system load, because (a) if occupied then it should reach the The consumption of gas for the production of DHW and desired temperature -and establish thermal comfort- the the expected savings by using a condensing boiler are sooner and (b) if this takes place just before occupation, a presented in table 6. If the DHW production capability of shorter preheating period restricts the heat losses that take the boiler was not exploited (in the case of a solar hot water place out of the occupancy period. system also being installed) then the diagram of fig. 4 would For a typical dwelling in Athens (with floor area 120m2, still be applicable for the economic evaluation of the boiler, TLC 360W/K, effective thermal capacity 260kJ/m2-K) we by appropriately correcting the expected savings, which for found that for 8-hours heating per day, which is quite usual both zones are almost 200 kWh/yr less. The grey broken in the country, the energy demand for pre-heating (full load line in the same diagram demonstrates the graphical operation) exceeds 85% of all needs, as depicted in table 7, estimation of the expected pay-back time extension. For which means that the boiler supplies most of the heat instance, the arrows reveal the extension of the payback output operating in non-condensing mode. At full load period from 6.4 to 7.7 years in a dwelling of zone B, with operation (60/80oC) the condensing boiler achieves almost TLC=250W/K, when the boiler is not used for DHW 3% higher efficiency than the conventional unit (table 1). production. Hence intermittent heating will finally lead to about 4% Solar hot water systems are very popular in the Country, savings (instead of the expected 11%), and the pay-back with a total installed surface area of 4,087,200m2 which is

6 Copyright © 2013 by ICAE2013 Paper ID: ICAE2013-xxx period will be more than double that estimated in fig. 4, 4.4 Use with a heat-pump in the form of a hybrid rendering the economy of condensing boilers marginal. system In fig. 5 the variation of dwelling temperature when applying 8-hours heating in a typical day in January is Condensing boilers may quite often serve the heating needs shown. Although pre-heating and heating periods seem to of a dwelling in tandem with a heat-pump, since most be equal in duration, the heat supplied during preheating is modern dwellings are equipped with such units to cope much more, as most of it is additionally stored in the with the cooling loads. Indeed, the use of air-conditioners envelope of the dwelling. (AC) has dramatically increased in the Country, with the total number of units now approaching 7 million pieces . Table 7 Variation of monthly heating energy demand in a ACs initially penetrated the market to cover cooling needs typical dwelling in Athens. Intermittent and continuous only, but for the last two decades, units with both cooling heating are assumed for comparison. and heating capabilities have been marketed. These units Monthly energy demand, kWh/mo have become more popular in the Country, since they offer Intermitent heating, 8 h/d Continuous an economically viable route to independence from a fuel Full load Part load TOTAL heating based central heating system, due to the mild climate of the Nov 518 52 570 867 country and the high COP they consequently achieve. Dec 1116 199 1315 1855 Due to their interchangable role for heating, the Jan 1116 227 1343 1916 operating costs of condensing boilers must be compared Feb 1210 190 1399 1985 with those of heat pumps for various outside temperatures, since both the boiler efficiency and the heat pump COP are Mar 1027 176 1203 1717 ambient temperature-dependent. This is shown in fig. 6 for Apr 432 20 452 789 the city of Thesaloniki, together with the accumulated TOTAL 5419 864 6282 9128 distribution of gas savings when a condensing boiler is used in place of a conventional one. From this figure it becomes Although continuous heating may lead to 11% savings by apparent that almost half of the savings take place when using a condensing boiler, it is not reasonable to apply this the ambient temperature is above 7oC, where the cost of pattern of heating if not necessary (actually there are useful heat from the heat-pump is less than half that from tenants who apply such a practice, believing that end-use the gas boiler. Hence, the heat pump-only system has just consumption is the same as that with intermitent heating). over half the operating expenses and also lower primary This is because intermitent heating e.g. by 8-hours daily energy consumption, as shown in table 8, while the higher results in almost three times the above savings, as also equivalent CO2 emisssions are mainly attributed to the very shown in table 7. high emission coefficient for electricity adopted by the

Greek regulations (0.989 kg-CO2 per kWhe).

Table 8 Comparison of energy consumption, costs and Dwelling equivalent emissions of a gas boiler and a heat pump temperature (results per W/K of TLC). Full Part load load Condensing Heat pump Ambient boiler

temperature Annual consumption 42.8 kWh-gas 13.1 kWhe Energy costs (€/yr) 3.51 1.90

Equivalent CO2 Heating 8.39 12.96 emissions (kg/yr) Primary energy 44.94 37.99 consumption (kWh/yr)

Figure 5 Variation of dwelling temperature, when 8-hours intermittent heating is applied (ambient temperature data of Athens, for a typical day of January)

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promising. The high solar energy potential and a relevant legislative obligation for the use of solar water heaters to cover at least 60% of DHW needs, restricts the field of application of combi boilers. Besides, the popularity of ACs units, due to the warm climate of the country, which may perform as a heat pump (in the form of central or split Gas units), arises as a very competitive alternative to gas heating. Lastly, intermittent heating cancels to a certain Electricity degree the potential benefits of condensing boilers, restricting their low temperature (and so highly efficient) part load operation. Unfortunately condensing boilers are marketed in the country either at a high purchase cost or at high over-cost, in comparison to low temperature conventional boilers, hence the additional investment is not expected to be paid- back in a short time. A promising alternative seems to be a hybrid system including a gas boiler with a heat pump, but Figure 6 Distribution of gas savings to ambient in this case there is no need to apply a condensing boiler. temperatures, and respective costs of useful heat generated This notwithstanding, installation of a condensing boiler by a gas fired condensing boiler or a heat pump may still be a very important and economical measure for non-insulated dwellings, where there is great scope for fuel The two systems may be considered as alternatives for savings. This measure could be of first priority, because any heating purposes, as a high temperature heat pump has envelope upgrade which may be attempted afterwards, been considered here (to supply traditional radiators at does not cancel the benefits of the condensing boiler 80oC). In spite of the obvious benefits of the heat pump, (penalties for oversizing) due to its modulating capability, its there are still reasons for this to be combined with a gas limited stand-by losses and its consequent high efficiency at boiler: part load operation.  To cope with extreme weather conditions  To install a smaller capacity heat pump, with obvious ACKNOWLEDGEMENT economy This research has been co-funded by the European Union  To obtain benefits of time dependent invoices, (European Social Fund) and Greek national resources under switching from one energy source to the other, as the framework of the “Archimedes III: Funding of Research appropriate Groups in TEI of Athens” project of the “Education &  To avoid very high electrical consumption, which Lifelong Learning” Operational Programme. would disproportioanlly increase the costs due to scaled invoicing of electricity. REFERENCES In this context, a hybrid system including a gas boiler (to [1] Harris D.J. A guide to energy management in buildings. cover peak loads) in combination with a heat pump is a Spon Press:London;2012. promising alternative, but in this situation it is not [2] Weiss M, Dittmar L, Junginger M, Patel M, Blok K. necessary for the gas boiler to be a condensing unit. Market diffusion, technological learning, and cost-benefit dynamics of condensing gas boilers in the Netherlands. 5. CONCLUSIONS Energy Policy 2009;37:2962-76. Condensing boilers have many advantages compared with [3] Lazzarin R, Schibuola L. Performance analysis of heating conventional high efficiency low temperature boilers, such plants equipped with condensing boilers. Heat Recovery as their greater heat exchange area, their higher efficiencies Syst 1986;6:269-76. in the complete range of operation, and the similarily [4] Comakli K. Economic and environmental comparison of efficient production of DHW. Seasonal efficiency for the natural gas fired conventional and condensing combi Greek territory was estimated for the condensing boiler at boilers. J of the Energy Inst 2008;81:242-6. 99% while for the conventional it was 88%, which means [5] Lazzarin R. Condensing boilers in buildings and plants that the relevant pay-back period for the over-cost could be refurbishment. Energy and Build 2012;47:61-7. below 5 years. [6] Bonaros V. Prospects and limitations for the use of gas In spite of this, the perspectives for the penetration of fired condensing boilers in Greece. MSc Thesis. Heriot-Watt condensing boilers in the Greek market are not so University. Athens;2008.

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