''Water conservation device'', for all showers, shower stalls, shower compartments or shower baths, a low-flow showerhead which shall have a maximum flow rate not exceeding 2 1/2 gallons of water per minute, for all faucets a maximum flow rate not exceeding 2 and 2/10 gallons of water per minute and for all water closets, ultra-low-flush water closets not exceeding 1 and 6/10 gallons of water per flush, contained within a dwelling unit.
(b) A landlord may cause to be installed by a plumber licensed in the commonwealth, at the expense of such landlord, submetering equipment in the landlord's building to measure the quantity of water provided for the exclusive use of each dwelling unit, provided that such equipment meets the standards of accuracy and testing of the American Water Works Association or a similar accredited association; and provided further, that a submeter is installed for each dwelling unit in the building and for the common areas of the building, so that all water used in a building is measured by both a primary meter and a submeter.