Canadians are getting ripped off by some air duct cleaning services sold by offshore telemarketers, a CBC investigation has revealed.

In a recent report, Marketplace found many Canadians are facing harassment from foreign telemarketing firms who defy Canada’s do-not-call list. One of the most common services for sale is cheap air duct cleaning.

But Marketplace found that, even with the low price, Canadians aren’t getting what they’re paying for.

"Customers have to be really very careful," said air duct cleaning expert Gino Merenda. "Many, many people are getting ripped off in this business in so many different ways."

Marketplace hired multiple Toronto duct cleaning companies from telemarketers using fake, or "spoofed" phone numbers to conceal their locations. It’s a common trick used by some call centres to avoid CRTC penalties for calling numbers on the do-not-call list.

Merenda took part in the Marketplace investigation, which used hidden cameras to monitor the techniques and the overall effectiveness of the discount duct cleaners.

In one house, Merenda estimated one duct contained almost four centimetres of dust before the cleaners arrived, with very little change afterward.

"[The dust] hasn’t been moved at all," Merenda said inspecting the same duct after cleaning. "It hasn’t moved, not even an inch."

Work raises red flags

In two cases, the cleaners completed the job in approximately one hour and charged $115 and $99 before tax, respectively.

Air duct cleaner Kassim said he too receives telemarketing calls selling cheap duct cleaning services. (CBC)

That’s a red flag, Merenda says, since a proper job should take at least two hours and cost more than $300, depending on the size of the home.

One cleaner said they don’t have time to do a proper job, because the telemarketers schedule too many jobs — as many as eight in one day.

"My boss gives me hour [for every] job," said one employee who gave his name as Kassim. "He wanted me to run to one job and then go to other job."

Merenda agreed that’s not enough time.

"It’s really ridiculous," he said. "There’s no way you can do a proper job, [cleaning] eight to nine houses a day."

Despite the tight time constraint, the cleaners don’t spend all their time working. Hidden cameras captured one man blowing compressed air onto his pants while his co-worker sat and watched. Merenda guessed the worker was "just making noise" to appear busy.

Watch Marketplace's episode, When The Repairman Knocks, Friday at 8 p.m. (8:30 p.m. in Newfoundland and Labrador).

Kassim also admitted that some homeowners don’t want their ducts cleaned: they hire the cleaners in order to confront them face-to-face about the telemarketing calls.

It’s a problem Kassim understands, since he also gets calls selling duct cleaning services.

Upselling a common scam

Debate exists about the necessity of cleaning home air ducts. A November 2012 study by the U.S. Environmental Protection Agency said that dusty ducts aren’t a health hazard and don’t conclusively increase dust levels in a home.

The National Air Duct Cleaners Association argues that dirty air ducts can recirculate contaminants and create or aggravate respiratory problems, allergies and other illnesses.

The EPA and NADCA both suggest that clean ducts can increase household energy efficiency. Both also agree that if homeowners choose to get the work done, they should hire qualified companies.

Merenda and NADCA also warned that aside from just doing a poor job, the dodgier duct cleaners sometimes pull another scam: upselling.

Homeowners in Canada and the U.S. have reported duct cleaners who do the work, then claim to discover problems that will cost more money to fix — including full furnace replacement.

Merenda says homeowners can avoid the ripoffs just by doing their homework.

"You need to research," he warns. "Ask [the company] how long they’ve been in business, ask them if they’re a member of the National Air Duct Cleaners Association, and most importantly, have the companies show you before and after the cleaning is performed, so [you] know that [you] had a good job done."

Watch Marketplace's episode, When The Repairman Knocks, Friday at 8 p.m. ET (8:30 p.m. in Newfoundland and Labrador).