The first thing you should do is speak to the main floor tenants and ask them to keep the thermostat at an acceptable level, you should also follow-up with a letter. If they do not comply then this can be grounds for you to issue a notice based on the reason that they are interfering with the reasonable enjoyment of the premises by the other tenants. You would issue this notice using the Form N5, which can be obtained from the Landlord and Tenant Board’s website at http://www.sjto.gov.on.ca/ltb/forms/.