A duty, then a hole cut in it
Section 3: "A Property Owner and the Landlord of rental residential premises shall provide a supply of adequate and suitable heat to habitable spaces within the premises or parts thereof and no Property Owner or Landlord shall cause or permit the lack of adequate and suitable heat to habitable spaces within the premises or parts thereof." No dates appear in it.
Section 4 supplies them by subtraction: section 3 "does not apply (a) between the dates of May 15 and September 15 in a single calendar year, both dates inclusive".
The arithmetic gives a heat duty from September 16 to May 14, which is close to Toronto's October 1 to May 15 but starts a fortnight earlier. "Both dates inclusive" is doing real work — the exemption covers May 15 itself, so the duty resumes on September 16 rather than the 15th.
The instrument is titled the Adequate Heat By-law, not a vital services by-law, which is worth getting right in any citation.
The appliance is part of the standard
Hamilton's definition does not stop at a number. Adequate and suitable heat means "the maintenance of an air temperature of at least twenty degrees Celsius (20°C) in all habitable spaces, by a safe, operable and permanent heating appliance capable of maintaining that temperature".
"Permanent" rules out achieving compliance with space heaters as directly as Whitby's and Ajax's express prohibitions do, and "safe" and "operable" fold the condition of the equipment into the temperature standard itself.
So in Hamilton a unit at 20 degrees heated by borrowed portable units is not compliant, and neither is one at 20 degrees heated by a permanent appliance in poor condition.
The non-payment exemption
Section 4(b) also disapplies the duty for "a lack of heat arising from the failure of a Tenant to pay for a required service which is the cause of the lack of heat, where the Tenant is obligated by a tenancy agreement to pay for the required service".
That is narrower than it first looks. It requires the tenancy agreement to place the obligation on the tenant, and it requires the non-payment to be the actual cause of the lack of heat.
It does not touch the landlord's separate obligations under sections 20 and 21 of the Residential Tenancies Act, and section 21(2) of that Act deems a landlord to have withheld a vital service where the landlord is the one who failed to pay. The two instruments allocate the risk to whoever holds the account.