The authority may from time to time sell and issue its refunding bonds without limit as to principal amount for the purpose of refunding any bonds of the authority at the time outstanding, paying the expenses of issuance thereof and paying any premiums necessary to be paid to redeem any bonds so to be refunded; provided, however, that no refunding bonds (other than refunding bonds issued to refund those bonds of the authority designated Building Renovation Revenue Bonds, Series 1990, originally issued in the aggregate principal amount of $29,500,000) shall be issued unless the present value of all debt service on the refunding bonds (computed with a discount rate equal to the true interest rate of the refunding bonds and taking into account all underwriting discount and other issuance expenses) shall not be greater than 99 percent of the present value of all debt service on the bonds to be refunded (computed using the same discount rate and taking into account the underwriting discount and other issuance expenses originally applicable to such bonds) determined as if such bonds to be refunded were paid and retired in accordance with the schedule of maturities (considering mandatory redemption as a scheduled maturity) provided at the time of their issuance. Provided further that the average maturity of the refunding bonds, as measured from the date of issuance of such refunding bonds (other than refunding bonds issued to refund those bonds of the authority designated Building Renovation Revenue Bonds, Series 1990, originally issued in the aggregate principal amount of $29,500,000), shall not exceed by more than five years the average maturity of the bonds to be refunded, as also measured from such date of issuance, with the average maturity of any principal amount of bonds to be determined by multiplying the principal of each maturity by the number of years (including any fractional part of a year) intervening between such date of issuance and each such maturity, taking the sum of all such products, and then dividing such sum by the aggregate principal amount of bonds for which the average maturity is to be determined. All refunding bonds shall be subrogated to the bonds which are refunded thereby.