You may well be entitled to a rent credit or abatement. All rental properties come with what is known as the "implied warranty of habitability," or the obligation imposed by law on the landlord that he/she provide premises which are safely habitable and "fit for their intended purpose"--in this case, residence. A condition, such as lack of heat, which renders premises not fit for their purpose violates this warranty and can entitle the renter to a credit or abatement. If the landlord will not provide such voluntarily, your recourse would be to file a lawsuit and seek the abatement in court.