Gloucester Towing owner Bill Mackay calls minus 20 C "the magic number" because when the mercury drops below that point, like it did Sunday night, Mackay gets a lot of work.

"If it's minus 20 or colder we're very busy. If it's minus 19 or warmer, we don't see more than a regular day," Mackay said between jump-starting stalled vehicles in Ottawa Monday morning.

Among Mackay's customers on this chilly day is the Ottawa Food Bank where three trucks won't start. It was Mackay's 20th call in six hours.

For the capital's homeless population, most who depend on the food bank to eat and on shelters to sleep, these cold temperatures — Monday's wind chill was -24 C — mean anything but a regular day.

There are nearly 1000 permanent beds spread throughout eight shelters in the capital. When the temperature plummets, shelters can add upwards of 200 overflow spaces, but in spite of efforts to accommodate everyone, there are some who refuse to take refuge, preferring instead to sleep rough on the streets.

"We can't force them (to go to the shelter)," said Salvation Army outreach worker Cindy Steward. "We can try to talk to them and try to deal with the issues that are preventing them accessing those services, but ... it's free will, they want to stay on the street so we just try to support them as best we can."

Stewart and her colleague Richard Charles roam the cold streets and alleys, distributing sandwiches, blankets and sleeping bags to help the homeless who won't take shelter get through the night.

"There is a certain segment of the homeless population that you may not see, they may live underground in a garage or something, they're pretty transient," said Charles who makes regular rounds checking on people and looking for others.

"We look for signs of life where they may be staying, such as, it's unfortunate to say, but discarded beer bottles or perhaps a piece of clothing," Charles said. "We do engage with them and of course, ultimately try and see that they get into housing."

Mental health issues are among the reasons why some homeless will forego the warmth of a shelter, even on the coldest days, said Steward.

Rick Roberts hunkers down on a blanket and asks for change at Bank and Albert Streets. While the deep cold is painful on the metal pins and plates holding his broken leg together, spending the night on the street is still preferable to the shelter.

"The bed bugs are chewing you up," said Roberts of his experiences at shelters. "People are always trying to pick fights. I'm not into that."

So far Ottawa's paramedics said they have not received any calls to deal with those suffering from exposure, but that could change when the temperature is expected to drop even further on Tuesday.