Today,the concept of net zero- energy buildings has become a major concern regarding fossil fuels combustion side effects, such as air pollution or global warming in both globaland national aspects.Recently,rising costs of these sources mayalso be another persuasion to replace more suitable alternatives. Nonetheless,for reducing N.G. (Natural Gas) consumption in residential buildingsthe feasibility of solar glazedcollectors’ usage, as an auxiliary energy supplier,is discussed in this paper.Therefore, the second major city in Iran, Mashhad, is selected for this case study owing to great potential of solar irradiance and number of sunny days.Also a residential complex including 136 flat apartments isconsidered to perform this research. Furthermore, to evaluate this potential four scenarios have been taking into account.In the first case monthly D.H.W. (Domestic Hot Water) demand is calculated. Second,two-year N.G. consumptions (2011 &2012)-the sole energy source- in this complex are documented..In thethird step the net energy which can be gain by solar absorbers is calculated .Finally, annual fractional saving, invest return time and CO2 avoided are evaluated respectively.

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