Does It Work
Duct cleaning is worth doing in specific circumstances and is oversold as routine maintenance, and both of those things are true at once.
Short answer
The circumstances that justify it: visible mould on the duct interior, evidence of vermin, or enough debris that it is being released into the living space. Also after a renovation, after a fire, and when moving into a house with a history you do not know. As an annual service on an otherwise clean system, it is not necessary.
When it is genuinely worth it
Visible mould on the duct interior or on the coil, which is a remediation rather than a cleaning and needs the moisture source found as well.
Evidence of vermin — droppings, nesting material, a smell — which is a health matter.
Substantial debris: construction dust after a renovation, drywall dust, sawdust, insulation fibres.
After a fire or a smoke event anywhere in the house, because the system distributed it.
Moving into a house where you do not know the history, particularly after renovation work or after a smoker.
And a system that has visibly never been cleaned, where pulling a floor register and looking down with a light settles it in ten seconds.
Pull a register and look
The single best piece of advice: take a floor register off and look down the boot with a flashlight. If you can see a layer of debris, loose insulation, pet hair, or something built up, the cleaning will do something. If it looks like sheet metal with a film of dust, it will not. That costs nothing and it is better information than any quote.
When it is not
As annual or biennial routine maintenance on a system with no identified problem.
For allergy relief as a general proposition, which the evidence does not support in the way it is marketed — filtration and source control do more.
In a condo suite with two metres of duct, where the fan coil is the real job.
Before a renovation finishes, because the system reloads.
As a substitute for changing the filter, which is the actual routine maintenance.
And on the basis of a photograph shown at the door that may or may not be of your ductwork.
What a proper job includes
Supply and return runs, and the trunk, with mechanical agitation rather than only vacuum.
The furnace or air handler cabinet, the blower wheel and the evaporator coil, which hold more than the ducts.
The return plenum and the filter slot, including sealing the slot afterwards.
All registers, grilles and boots.
A new filter at the end.
And negative pressure on the system while working, so what is dislodged leaves the house rather than being redistributed.
Common questions
Does duct cleaning help allergies?
Not as marketed. Filtration and source control do more. It helps where there is real contamination.
How do I know if I need it?
Pull a floor register and look down the boot with a flashlight.
Should it include the furnace?
Yes. The blower wheel, the cabinet and the coil hold more than the ducts.