An Older House
The ductwork in a pre-war Toronto house is not the ductwork the equipment was designed for, and that affects both what is in it and what cleaning achieves.
Short answer
Older Toronto houses frequently have oversized gravity-era trunks adapted for a forced-air furnace, joist-space and stud-space returns using the building cavity as duct, and asbestos-containing tape or insulation on the trunk. All three change the job, and the third one changes who should be touching it.
What is different
Oversized trunks from a gravity hot-air system, converted to forced air. The air moves slowly in them and debris settles rather than being carried along.
Joist-space and stud-space returns, where the building cavity itself is used as the duct. Those are dusty, leaky, and cannot be cleaned the way sheet metal can.
Returns pulling from a basement or a crawlspace through an unsealed cavity, which is both an air quality and an energy problem.
Panned joist returns with the floor above as one surface, which collect everything that falls through the floor.
Old registers and boots with layers of paint and debris.
And modifications over a century, including branches abandoned in a wall and still open.
Asbestos on the trunk
Tape on the joints and insulation on the trunk in a Toronto house of a certain age may contain asbestos. It is stable undisturbed and it is a regulated material when disturbed. A duct cleaning crew brushing against it is disturbing it. Have it tested before any work on an older system, which is inexpensive and settles the question.
What cleaning can and cannot do
Sheet metal trunks and branches clean well, including the oversized ones.
Registers, boots and grilles clean well.
The furnace, blower and coil clean well and matter most.
Joist-space and stud-space returns can be vacuumed and cannot be properly cleaned, because they are a building cavity rather than a duct.
A panned joist return full of a century of debris is frequently better sealed and replaced with proper duct than cleaned.
And an abandoned open branch should be found and sealed, which is a better outcome than cleaning it.
The improvements worth making
Seal the filter slot and the cabinet, which on an older installation is frequently wide open.
Convert cavity returns to proper sheet metal duct where the access allows, which improves the air quality and the system performance together.
Seal the joints on the trunk and branches, which on an old system are leaking substantially.
Insulate duct running through unheated space.
Find and seal abandoned branches.
And have the asbestos question answered first, because it determines who does all of the above.
Common questions
Can joist-space returns be cleaned?
Vacuumed, not properly cleaned. They are a building cavity rather than a duct.
Should I worry about asbestos?
Have the tape and trunk insulation tested before any work on an older system.
What is the best improvement?
Converting cavity returns to proper duct, and sealing the filter slot and the joints.