Oh yeah, sorry about that. I keep saying furnaces in my posts but you are dealing with a boiler/water heater, lol. But in any case, the vent terminology and requirements are mostly the same. The water heater may have a lower BTU output than typical furnaces so that can change things when it comes to clearances and comparing a furnace to a water heater.
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I’ve seen many Navien tankless water heaters pull combustible air from the interior according to manufacturers recommendations.
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I agree with him for the most part. And I don’t call it out if the PVC pipe isn’t installed. I usually just tell the client to make sure not to set anything on top of the furnace that could block the intake. A pipe is nice for preventing that.
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I meant that as in "…in order to be considered “direct-vent” "
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Totally agree. After the install and when the system was fired up, I placed my hand over the intake and was surprised at what little air it was pulling, but keeping it clear is #1.
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Does anyone know why this clearance is needed?
I bet there is a lot of CO concerns out there.
My first thought would be moisture intake from the exhaust during certain times of the years, which could possibly lead to internal corrosion. Maybe? 
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wouldn’t it create some extent of energy ineffeciency if some amount of exhaust was sucked up through the combustion air intake? the only time combustion air is being sucked in is when exhaust is being expelled. there would theoretically be less oxygen supply than optimal for the gas to burn?
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Air is only 21% oxygen. That’s what the furnaces and water heaters are designed to burn (10:1 air to natural gas). If you dilute the air supply with non combustibles You screw up the burn efficiency and create more noxious exhaust and less heat (rich mix).
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