Loans made under the provisions hereof shall be for a period of not exceeding twenty (20) years, payable as to principal and interest as the board of directors may determine, but no obligation of the district issued hereunder shall be sold for less than ninety cents (90¢) of the dollar par value thereof, and shall draw interest not to exceed seven (7) per centum per annum, provided that any such loan may be refunded or renewed, if necessary, for a further period of five (5) years.