9NEWS viewers alerted us to the dangerous mistake already discovered on dozens of homes that had new roofs installed. After a summer of severe weather, it's an alert thousands of Coloradans who got new roofs should know about.

"It just wouldn't go," said Tonya Genazzi as she explained how she discovered the problem with her furnace system.

She called a heating and cooling company to figure out why her furnace wouldn't start. The technician immediately discovered the problem was on the roof. A new cap on the home's furnace vent pipe was pushed down too far. When the furnace gases couldn't escape, a safety features on the furnace shut it down. But not all furnaces have that safety feature.

"We have hundreds of people getting new roofs in Parker from the storms this summer and if contractors are installing these wrong, this could be dangerous," Genazzi said.

It was indeed dangerous for the Felche family in Parker. Roofers also pushed the cap down too far on their vent pipe. The furnace seemed to be working just fine, but they were having trouble with their hot water heater.

"Our pilot light just started to turn off. I tried to light it again. It wouldn't stay on. Wake up the next morning, have cold showers," Brock Felche said.

When he described the problem to Rick York of Parker Heating & Air Incorporated, York immediately knew it was another vent cap issue. He had already received a number of these calls.

"Instead of going through the roof, the gas is blocked," said York as he showed Felche the problem.

He demonstrated where that gas was going instead. It was traveling down into a pipe connected to the hot water heater. There, it was blowing out the pilot light and carbon monoxide was seeping into the home.

"I'm very concerned," Felche said.

Not just for his family, he said, but also for the many other Coloradans who got new roofs and may not discover the vent cap problem as soon as he did.

York and homeowners reported the problems to their town's building department. All roof inspectors in Parker have been notified to look out for the ill-fitting vent caps before signing off on a new roof.

York says it won't be a problem in homes with new, high-efficiency furnaces. He's especially concerned about homeowners with older furnaces that don't have a safety feature that will shut the furnace off if gasses get trapped. He says it's one of many reasons homeowners should always have a carbon monoxide detector in their house.