(n) Upon receipt of a bill for water usage from the landlord and within the time allowed for paying the bill, a tenant may request that a person or entity with expertise in the installation and operation of water submeters and with no financial or other relationship with the landlord, test the submeter for the dwelling unit leased by the tenant to determine whether it is accurately measuring the water being used in the dwelling unit. If the submeter is found to be measuring more water than is being used in the dwelling unit, the landlord shall install a new submeter at his own expense and shall also pay for the cost of the test. In addition, the person or entity conducting the test shall determine as accurately as possible the amount of water that was improperly measured by the submeter in both the prior and current billing periods. The landlord shall calculate the amount the tenant was overcharged for the prior billing period and reduce the bill by that amount, or, if the tenant has already paid the bill, give the tenant a rebate in that amount. Upon receipt from the water company of the bill for the current billing period, the landlord shall calculate the amount of the bill attributable to the excessive measurement by the submeter and reduce the bill to the tenant by that amount prior to sending it to the tenant. If the submeter is found to be measuring no more water than is being used in the dwelling unit, the tenant shall pay for the cost of the test; provided, however, that if the tenant does not pay for the cost of the test, the landlord may add such cost to the next bill sent to the tenant and such cost shall be considered a part of the bill for purposes of paragraph (f) and clause (i) of subsection (4) of section 15B of chapter 186.