Here's the answer from the Ontario Landlord and Tenant Board website:

...

If a landlord provides heat, the Act requires the landlord to keep the heat to at least 20 degrees Celsius from September 1 to June 15. In addition, many municipalities have their own property standards or bylaws about heat. You should contact you local municipal government to find out if your community has a bylaw that sets minimum standards for heat.

What can a tenant do if their landlord does not turn on the heat?

Heat is a vital service. If the landlord is responsible for providing heat and the landlord does not provide heat to the standards identified in the previous question, the landlord may be committing an offence. You may call the Investigations and Enforcement Unit of the Ministry of Municipal Affairs and Housing at 1-888-772-9277.

Also see the Board’s brochure on Maintenance & Repair for additional remedies available to tenants. Available as a PDF file here.

For more information on this topic and other rental questions view the Ontario Landlord Tenant Board Website.