The highest standard in the region
Section 2(a) of the Adequate Heat By-law defines the term: "adequate and suitable heat" means "a minimum air temperature of at least 22 degrees Celsius (72.1ºF)".
Section 3 then makes the failure an offence: it is an offence for the landlord or owner of every rented or leased dwelling "which is normally heated by or at the expense of the landlord" to fail to maintain adequate and suitable heat "at all times between September 1st of each year and June 15th of the following year". That end date was set by amending By-law 98-2019.
Set against Toronto's 21 degrees from October 1 to May 15, Oshawa is one degree warmer and roughly six weeks longer at each end. A landlord operating in both cities cannot run one policy.
"At all times" is doing work in that sentence. It is not an average over a day, and a unit that drops overnight is not compliant because it recovers by lunchtime.
Why it is not where you would look for it
Oshawa's property standards bylaw is By-law 1-2002, and the clause where a heating standard would normally sit, section 7.1.2, reads "Intentionally Deleted". A landlord who checks only the property standards bylaw will conclude, wrongly, that Oshawa has no municipal heat requirement.
The requirement was moved into a standalone instrument, the Adequate Heat By-law. This split is common outside Toronto — several municipalities keep heat in a vital services or adequate heat bylaw rather than in property standards — and it is the single most common reason a landlord misses a municipal obligation.
Inspection on request
Section 4 lets an officer, "upon the request of any tenant, sub-tenant, lessee or occupant", enter, inspect and examine the premises at any time to determine whether adequate and suitable heat is being provided.
That is a municipal power, and it does not require an application to the Landlord and Tenant Board, a filing fee, or a hearing date. A tenant complaint can put an officer in the unit quickly.
The practical consequence is the same as in Toronto: what you can evidence about your response time matters more than the eventual repair. Log the report, log the dispatch, log the temperature reading.