Many safety authorities in many jurisdictions have archived accident reports for fatality and near-fatality incidents on-line these days. It sometimes takes a bit of effort to find them – but they make for grim reflection. These resources should be mandatory reading for engineers, inspection persons, suppliers and tradespeople – anyone who works with combustion appliances.

Albert, I don’t know. That pipe on the wall behind the water may be it.

Roy, I totally agree that unvented gas appliances are lunacy.

Bob, the only good thing about that scenario is that with the dryer running and backdrafting the water heater, most of the exhaust gases and carbon monoxide would get exhausted through the dryer. But as soon as the dryer goes off, exhaust gases will collect in the room if the water heater were still operating. What could make it worse is if you had leaky return ducts in there to pull in the CO and distribute it to the bedrooms.

Bill, excellent point. Incomplete combustion occurs when there’s not enough oxygen and that requires enough air movement through the burner.

Lee, the closer you look at the photos, the more problems you’ll find. On the amount of rise required, though, the code (NFPA 54) says all that’s needed is 1/4″ per foot, not 45 degrees.

Mike, my guess is that the answer to your question is no.

Yeah, I’ve seen some bad installations of combustion appliance venting myself and even more photos. It’s definitely a problem.

Paul, yes, it’s similar. A recirculating hood can remove some of the stuff from the exhaust, but it depends on the type of filter and how recently it’s been replaced. Ventless gas fireplaces are worse, though, because they dump all of their exhaust gases into the home.