In Utah, these homes are a cheap alternative to what residents in this economically struggling region call a "stick-built" house. At least 100 dot Sanpete County.
"Our economy is about the worst in the state," said Todd Mortensen, who owns Trade Winds Heating in Manti, which has a population of about 5,000. "This is how people live when you can't afford to have a house built."
A manufactured house can cost as little as $68,000; the property might cost $25,000, according to Mortensen.
Mortensen recommends a roof jack extension to raise the vents, "but you can't eliminate the problem 100 percent," he said.
In normal "thermal buoyancy" hot fuel gases rise above the roof and float into the air. But during the winter, dense, cold air can cause exhaust fumes to hover over the roof. And with certain wind currents and snow blocking vents, "it's like a perfect storm for it to happen," he told ABCNews.com.
Daniel Conrad was born at home with the help of a midwife in August 2007 with the home's air conditioning and hot water heater at full force. Looking back, his father says the newborn was listless and his mother was nauseated.
"He looked awful," said David Conrad, 47, and a self-employed day trader. But throughout the early fall, the baby thrived until the family turned on the heating system in October. By winter, the child was spitting up more milk than he was ingesting.
The Conrads' other kids were fine, though he said the family experienced some low-level nausea. Church members started to talk and reported the family to authorities.
"We had just moved here and they see we are giving a home birth and they find out we are vegan," said Conrad. "This wouldn't have happened in our previous place, where people knew us well and respected us. They were well-meaning but they didn't have the confidence we had. They thought we were harming our baby and were irresponsible."
Daniel was put under court-ordered doctor supervision, but wasn't tested for carbon monoxide poisoning. Rodgers, who founded the nonprofit LifeSave Biological Research, suggested carbon monoxide poisoning might be a possibility.
Together, he and Conrad blocked the intake vent with a plastic top and duct tape and opened a window to draw in fresh air, a quick solution they have shared with neighbors.
Since then, Conrad said Daniel has been "happy, healthy" and thriving physically. The state's case against the Conrads was dismissed in October.
Clark Watkin, an Arizona lawyer who handles cases of carbon monoxide poisoning, confirmed that "proximity of the intake and the exhaust is a factor -- whether it's a building structure, a boat, wherever there is a combustion engine."
"This is actually quite common," Watkin, who has no involvement in the Conrad case, told ABCNews.com. "Depending upon the force of the exhaust and whether there is any breeze directing the exhaust fumes away from the intake vent, you can have a dangerous situation."
Retired chemical engineer Jay Bishop of Bountiful, Utah, has joined Conrad's crusade to alert residents in Utah's 80,000 manufactured homes, many poor and some on American Indian reservations.
"We need to absolutely require 12 feet [between furnace vents]," he told ABCNews.com. "We are openly contesting this, informing people and hoping HUD will eventually change the regulations."