Combustion air intake near exhaust

Little help, gentlemen and ladies?

Tankless combustion air intake in close proximity to the exhaust.

Field research and common sense tell me probably not good. The exhaust should be 3’ above the intake, correct?


What does the manufacturer’s instructions say? You take a closeup of the nameplate and serial number. Then when there is a question about installation you look up the install instructions. Even the IRC defers to install instructions.

NPE_Installation and Operation Manual_200110_Ver5.4_EN.pdf (31.9 MB)

1 Like

Based on your second and third pictures, it appears that the exhaust (exterior) is on the left and is higher than the intake.


1 Like

What a guy, thank you Bob.

Is this what I’m looking for? It’s the standard I thought I found when googling around

High efficiency water heaters’ intakes are mechanical intakes, right?

Looks like a Navien. 36”.


thank you!

1 Like

Looks fine to me. The intake is on the right side with a pipe that goes down. The intake always has the screen installed on it.

1 Like

that’s not 36" vertically, though. shouldn’t the intake be extended?

Looks good to me. What do you think of concentric vent/exhaust?
Not a problem.


Keep in mind the intake also about 36 inches away from the exhaust. It’s your call you’re the inspector, me I’d leave it alone.


As Martin noted the key words are Away From.

1 Like

They can be much closer than 36." Look at pages 41 and 42 of the manual Bob posted above. There are several different options for venting in a direct vent situation. In your case, the following applies…



stumped. good point, thanks Bert!

hm, yea that looks like what I have there, doesn’t it. thank you!

so I assume that “3’ above mechanical air inlet within 10’ horizontally” clearance I was referring to above is in reference to a fresh air intake for HVAC?


Yes, a mechanical air inlet is different than a combustion air inlet. Combustion air, for the most part, is contained to the appliance, and not distributed around the home. Think of the combustion air and exhaust system of a direct vent appliance as a “closed circuit,” if you will.

1 Like

yea, it makes a lot of sense now.

I think I was confusing/conflating “mechanical air supply” with “forced air/direct vent”.

Also make sure to look at the manual and see the differences in clearance for “direct vent” vs “non-direct vent.” You have a direct vent arrangement in your situation. But something I see a lot of people get confused by is that just because the furnace is high efficiency and has a PVC exhaust out the side terminating horizontally, that does not make it a direct vent. In order to be direct vent, the appliance must pull combustion air from the outdoors.

Probably 1 in 5 of the high efficiency gas appliances I encounter are non-direct vent, meaning they have a PVC exhaust terminating horizontally, but they pull combustion air from the indoors. This changes the exhaust clearances compared to a direct vent arrangement.

1 Like

yep, ive read a blog post before about that common misconception. thanks for the reminder.

the unit has to directly pull the air from the outdoors, correct?

if it were the same setup in a garage, but instead there were combustion air tubes/openings in the wall or ceiling, that is non-direct vent?

1 Like

In order to be considered direct vent, yes. But keep in mind that the manufacturers allow non-direct vent (combustion air pulled from indoors). It is up to the installer to determine the best arrangement for each particular home.

Usually non-direct vent setups in my area look like the picture below. Sometimes they don’t even put a PVC pipe on the intake, it just pulls through the hole in the furnace cabinet.

(Picture from Google)


yea, don’t see too many HE furnaces down in TX. Once in a while.

When I see tankless water heaters in garages it’s usually with a concentric exhaust or two ~6" combustion air tubes in the top & bottom of the exterior wall, leading to a screened hood on the exterior. I think this is the first time i’ve seen a two pipe direct vent system on a tankless.

1 Like

Mine is set up without the PVC on the intake. When it was installed last year, I ask the HVAC tech if it was needed and he said no.