"Any idiot can see [the ventilation] is foolhardy and dangerous," said David Conrad. "Calling it safe violates any code in the whole country and I say it's insane."
In Conrad's case, the stubby vents were located on the north-facing, shady side of the roof where snow is protected from the sun and "inclined to pile up."
Blocking a combustion air supply return diminishes oxygen to the combustion chamber of the furnace and can cause carbon monoxide levels to rise to dangerous levels.
Those exhaust fumes, say Conrad, had "easy access for reentry into the living quarters" through the fresh air vent.
"I would prefer that there be a better and bigger separation than that," said Terry Brennan, a 28-year building scientist who owns Camroden and Associates in Westmoreland, N.Y., referring to the three-foot HUD code.
"The amount of percentage of intake air goes down exponentially as you go away with distance," he said. "Ten feet away isn't three times less, it's 20 times less -- a big difference. It's hard to measure, but it's likely we have a lot of similar cases to this."
The CDC reports that more than 600 Americans accidentally die each year of acute carbon monoxide poisoning. The gas is odorless and colorless.
"People can be overcome in a minute or so," said Dr. Philip Landrigan, a pediatrician at Mount Sinai School of Medicine in New York City. "All it takes is a few breaths."
It is equally dangerous in infants and adults. "Both go down pretty fast," he told ABCNews.com.
An estimated 11,000 cases of low-level carbon monoxide poisoning go undetected each year, according to a yearlong study at Rhode Island Hospital that was published in a 2008 issue of the Journal of Emergency Medicine.
That's because its symptoms -- headache, dizziness, chest pain, nausea and vomiting -- are often diagnosed as the flu.
Daniel's failure to feed properly was a "classic symptom," according to expert Greiner, who was not part of the Conrad investigation.
Carbon monoxide is a byproduct of combustion heating and air conditioning and must be properly vented.
Under international building codes, vents in a conventional home must be placed a minimum of 10 feet apart. But critics say local officials often ignore manufactured housing because it carries HUD's red seal of approval.
"You cannot expect consumers to be building scientists," said Rebecca Morely, executive director of the National Center for Healthy Housing. "Most people are not skilled enough to see if it's flawed."
"The larger issue is, is HUD the best regulator of the units?" she said. "There should be more oversight. Who is responsible for quality control for both health and safety?"
HUD spokesman Lemar Wooley told ABCNews.com that the Conrad complaint was the first the department had received about carbon monoxide dangers in manufactured homes.
"Because these indoor air quality concerns have been raised, HUD will be looking into the matter and may refer this issue to its Manufactured Housing Consensus Committee to obtain their recommendations on whether the current requirements should be changed," he said in a statement.
About 8 million Americans live in manufactured homes -- once called mobile homes. Not all are installed with combustible gas furnaces, according to HUD.