Direct vent model installed on B vent

Saw a direct vent model water heater (not a power vent) installed on a standard vertical B vent flue with a furnace. Manufacturer documentation has no provision for this type of installation. Is this inherently dangerous?

Besides not being installed according to mfg spec, what are the potential issues?

If the configuration is not supported by the installation standards for a listed device, then it’s improper.

The unit is designed to use a concentric vent and pulls the combustion air from the outer vent, which is why you see an open collar on the top of the water heater. The only place it can get combustion air as it’s configured is to obtain it from the room in which it is located. Do you believe that this unit / installation is supposed to obtain combustion air from the interior?

I get that it pulls combustion air from the top and that it is not installed according to any mfg spec. That I am writing up as a defect, possibly a safety issue. My question is more oriented to what-if scenarios as my clients are curious as to the possible ramifications of such a set-up.

That depends on other factors. I don’t suggest you help justify an improper installation of a listed appliance based on whatever you and any other group of inspectors, including myself could come up with.

I can’t assume that that the “typical” 50 cubic feet of space per 1,000 BTUs is adequate for that device, because it was not designed to be installed that way, or the installation instructions would have said so.

My answer would be “I don’t know what all of the potential implications are, but the unit was not designed to be installed in that manner. You would have to consult with the manufacturer.”

The purpose of the missing draft hood is to control the draft pull that may be generated from the rushing wind at the end of the flu pipe, has nothing to do with combustion air unless the flue is clogged. what you may end up with is a flame out on the burner or a pilot light if it has one, or nether. must have the cap required by code.