Direct vent fireplace are the ones that have the glass front that doesn't open. They are supposed to be a sealed chamber that shouldn't allow flue gases to enter the home. The direct vent flue pipe has an inner pipe that exhausts the flue gases to the exterior and an outer pipe that draws combustion air into the firebox for the flames to have oxygen.
I had the pleasure of inspecting this large house built in '96 in Wahoo, Nebraska. With it came 2 fireplaces. When I was testing the propane direct vent fireplaces, they both appeared to operate normally, but I started smelling fumes. This led me to take off the front face of both of them. I found that the paint had been burnt right at the top where the seal was bad. My carbon monoxide detector confirmed what I was smelling. Both were leaking around 50 ppm of CO gas directly into the house.
Had the homeowner turned either of these on for only a few hours, the lower levels of carbon monoxide would have been sucked into the cold air returns of the furnace and blown pretty consistently throughout the house. The levels were under the 70 ppm that store bought CO detectors are designed to alarm at, so even if they had regular CO detectors, they would not have been warned of the carbon monoxide poisoning.
Lower levels of CO like this over long-term exposure can affect brain development in all ages of children. I've personally inspected a large 4 year old ranch house on an acreage that was leaking low levels of carbon monoxide into the basement bedroom where the homeowner's daughter slept. I was the one that found the carbon monoxide leaking. The CO leak was coming from a bad glue joint in the exhaust line of the hi-efficiency furnace. They figured she was exposed to the low levels of CO gas for at least 3 of those 4 years they had lived in the house. Seeing the long-term effects it's had on her is heartbreaking.
To best protect your family, you need to install low level carbon monoxide detectors. In both situations described above, these low level detectors would have alarmed and alerted the families of the carbon monoxide poisoning. To read more about low level carbon monoxide detectors, go to this article on our website: http://www.omaha-home-inspection.com/hometips-low-level-carbon-monoxide-detectors-11-3-13.pdf.
You can't buy low level CO detectors at any stores as they are not Underwrited Laboratory approved. They aren't approved because UL doesn't test for CO under 70 ppm. I was able to find one for purchase at www.truetechtools.com. Just search for "low level CO detector" and their Defender LL6070 will pop up. It's available to purchase around $150. Some HVAC Contractors are also selling different models so you can ask them also.