Twenty-one degrees, all year
Section 1.0 defines the standard directly: "adequate and suitable heat" means "that the minimum temperature of the air in the dwelling unit shall be 21 degrees Celsius (21 °C)".
Two things make Waterloo unusual together. The figure is 21 rather than the 20 that most Ontario municipalities use, and there is no season — the bylaw names no months anywhere.
Combined, that is a stricter standing obligation than most of the province. A landlord operating in both Waterloo and a 20-degree seasonal municipality cannot run one thermostat policy across the portfolio.
The definition speaks to the air in the dwelling unit rather than to equipment capability, which puts Waterloo with Toronto and Oshawa on the operating side of the line rather than with Barrie and Guelph on the capability side.
How it is enforced
Section 2.0 authorises every officer to enforce the bylaw and to "enter on land at any reasonable time for the purpose of carrying out an inspection to determine whether the provisions of this Bylaw are being complied with".
Entry to a dwelling unit itself is expressly made subject to section 437 of the Municipal Act, 2001, which is the provision governing entry to a dwelling — so the bylaw does not give an officer a free right of entry into the home.
An officer may also require information from any person concerning a matter related to the inspection, under section 436 of the same Act.
A note on the source
Waterloo publishes this bylaw as a web page rather than a PDF, and the page states that it is a consolidated digital version rather than an exact reproduction of the official legal document.
That is worth knowing before relying on it in a dispute: for a hearing you would want the certified copy from the City Clerk, not the web consolidation.
For day-to-day operations the web version is the practical reference, and it is the one linked below.