C&Ped from the Massachusetts Office of Consumer Affairs, here:
Duty to Provide Habitable Premises
You must provide habitable apartments and common areas for the entire tenancy in accordance with the minimum standards of the State Sanitary Code which seeks to protect the health, safety, and well-being of your tenants and the general public.
Heat: Landlords must provide a heating system for each apartment or one system that services all apartments in good working order. The landlord must pay for the fuel to provide heat and hot water and electricity unless the written rental agreement states that the tenant must pay for these. The heating season runs from September 16 through June 14th, during which every room must be heated to between 68˚F and not more than 78˚F between 7:00 a.m. to 11:00 p.m., and at least 64˚F at all other hours.
[Sounds like you've agreed to pay the heat, but that doesn't change the code definition of "habitable" as able to maintain that level of heat. Skipping ahead.]
Tenants' rights
Rent Withholding
If you fail to maintain the premises during the entire tenancy, in habitable condition, your tenants may rightfully withhold part of the rent from the date you have notice of breach of the Warranty of Habitability, if:
They complained to you of defects or problems or the Board of Health cited the apartment or building for Code violations; The tenant was not in arrears in rent before you knew of the conditions complained of; You do not show that the complained of conditions were caused by the tenant or occupant;
[and goes on to say that also if it's uninhabitable, you can pay to make the repairs yourself and deduct it from your own rent, and the landlord can't say boo about it. You could withhold paying a full four months rent and they couldn't retaliate (evict you, refuse to renew a lease, etc.)]
no subject
Duty to Provide Habitable Premises
You must provide habitable apartments and common areas for the entire tenancy in accordance with the minimum standards of the State Sanitary Code which seeks to protect the health, safety, and well-being of your tenants and the general public.
Heat: Landlords must provide a heating system for each apartment or one system that services all apartments in good working order. The landlord must pay for the fuel to provide heat and hot water and electricity unless the written rental agreement states that the tenant must pay for these. The heating season runs from September 16 through June 14th, during which every room must be heated to between 68˚F and not more than 78˚F between 7:00 a.m. to 11:00 p.m., and at least 64˚F at all other hours.
[Sounds like you've agreed to pay the heat, but that doesn't change the code definition of "habitable" as able to maintain that level of heat. Skipping ahead.]
Tenants' rights
Rent Withholding
If you fail to maintain the premises during the entire tenancy, in habitable condition, your tenants may rightfully withhold part of the rent from the date you have notice of breach of the Warranty of Habitability, if:
They complained to you of defects or problems or the Board of Health cited the apartment or building for Code violations;
The tenant was not in arrears in rent before you knew of the conditions complained of;
You do not show that the complained of conditions were caused by the tenant or occupant;
[and goes on to say that also if it's uninhabitable, you can pay to make the repairs yourself and deduct it from your own rent, and the landlord can't say boo about it. You could withhold paying a full four months rent and they couldn't retaliate (evict you, refuse to renew a lease, etc.)]