A couple weeks ago I inspected a home in Beloit that had a few significant defects. The most important one was in relation to the water heater. It appeared to me that the homeowner wanted to take the chimney down at the time they last replaced the roof, the problem was that chimney was still being used to vent the water heater. Seems simple right?, not so much.
First off, I never recommend that homeowners do repairs to gas appliances unless they are trained professionals themselves. There are too many risks involved when it comes to gas fired appliances. In this particular case, it could have caused death. Instead of 
hiring a plumber or HVAC technician to evaluate and install the correct vent, they simply used the single wall vent connector and ran it out the wall like a power vent PVC flue. When there is no draft inducing fan (power vent) and no, (or very little), rise in the flue piping, there would not be sufficient draft to draw the exhaust gasses to the exterior resulting in likely carbon monoxide leakage into the living space. When I tested the base of the flue with an ambient air carbon monoxide meter I found 24 ppm of carbon monoxide leaking into the basement. This would cause the buildup of CO into the living space and could have caused the occupants to become ill or worse. And to make matters worse this was a 40 gallon tank water heater so this would fire at least a few times a day to keep the water to temperature.
So, I had to turn the gas off to the water heater and inform both parties that the water heater should not be used until the flue pipe was repaired. As an added bonus, when the gas was turned off to the water heater, the gas was also turned off to the furnace. Gas supply should never be installed in this manner. Each appliance should be able to be operated independently. So to add insult to injury, not only were they without hot water but they were without heat as well.
So this little situation led me to think that a post solely about water heater flue configurations was in order since this is a situation that comes up somewhat regularly and can cause carbon monoxide poisoning.
When I inspect the flue I first determine if it is an induced draft appliance or a natural draft. Many newer water heaters will have an induced draft (power vent) and PVC flue. This is perfectly acceptable if it is installed on an appliance designed for this application. A natural draft water heater is easy to spot, usually they will be a metal flue connector that runs to a masonry chimney or a class B bent that penetrates the roof.
When a class B vent is in use minimum distances to combustible materials must be 
adhered to. These dimensions are usually stamped into the pipe wall. Generally speaking 2 inches to any combustible material is required, although, some only require 1 inch. The minimum distance to combustibles for a single wall vent connector is 6 inches, so if your vent connector travels through a wall prior to entering the chimney you may need to protect that penetration.
Often times when we encounter a water heater that vents into a chimney, and the chimney flue is not re-sized. When a furnace is upgraded and vented through the wall the water heater is often abandoned to the original flue. This sometimes leaves too large of a flue for the size of the water heater and usually results in corrosion on the flue connector from the exhaust gasses condensing in the chimney. Generally a stainless steel liner can be installed in the chimney to rectify the situation. But, under no circumstances is it OK to use a natural draft flue connector through the wall without a significant rise to create draft. That is what caused the carbon monoxide problem in the story above.
The moral of this story is that there are many things to consider when changing the venting on any appliance, and if you are not well versed in these applications and their effects on the actual venting of the appliance, you really need to call in a professional..
As always, if you have any questions with your home don’t hesitate to send them to: Aaron@zuehlkeinspections.com
I hope this was helpful and informative.
-AZ
Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-
service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection and Residential Thermal Imaging. He can be reached by email at Aaron@Zuehlkeinspections.com or by calling the office at 608-931-7485.