The heat should be turned on whenever you feel that your home is uncomfortably cold. The World Health Organization lists the ideal indoor temperature as between 18 and 21 degrees Celsius. Therefore, when your thermostat falls below 18 degrees, it might be time to turn on your home heating system. It’s also worth noting that according to a City of Toronto bylaw, landlords are required to maintain a minimum temperature of 21 degrees Celsius in all residential dwellings between September 15 and June 1 each year. However, assuming you’re a Toronto homeowner and not a landlord, it is completely up to you to decide when to turn on the heat.