It is hereby declared to be the policy of this Commonwealth that each housing authority shall manage and operate its housing projects in an efficient manner so as to enable it to fix the rentals for dwelling accommodations at the lowest possible rates consistent with such authority providing decent, safe and sanitary dwelling accommodations, and that no housing authority shall construct or operate any such project for profit, or as a source of revenue to the city or the county. To this end an authority shall fix the rentals for dwellings in its projects at no higher rates than it shall find to be necessary in order to produce revenues which (together with all other available moneys, revenues, income and receipts of the authority from whatever sources derived) will be sufficient (a) to meet the cost of, and to provide for, maintaining and operating the projects (including the cost of any insurance) and the administrative expenses of the authority; (b) to pay, as the same become due, the principal and interest on the bonds of the authority; (c) to provide a margin of safety for making such payments of principal and interest; and (d) to create and maintain a reserve sufficient to ensure the authority can pay the principal of and the interest on the bonds of the authority as the same shall come due.