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New house–Air intake vent is about 4-ft below high efficiency furnace & water heater exhaust vents. Dangerous?

aunsafe2015 | Posted in General Questions on

Recently bought a house that has natural gas furnace w/ 95% AFUE, a natural gas Rinnai tankless water heater w/ UEF 0.92, and a Fantech FG8 air intake (which I believe may operate at around 300-400 cfm).

Attached pics show the intake of the Fantech (white hood near ground) is about 3.5-feet beneath the exhaust of the concentric Rinnai vent (black), and about 4-feet diagonally beneath the concentric furnace vent (white).

My understanding is that ASHRAE recommends 15 feet of distance from an air intake to a combustion appliance vent.

Is this 3.5 to 4 feet of distance at my house dangerous such that I should disable the Fantech until I’ve moved its intake further away from the furnace and Rinnai?

Or is this setup adequately safe that I could allow the Fantech to run 15 minutes per hour for a few weeks until I have time to move its intake further from the furnace and Rinnai?

Thanks for any input!

P.S. I think ASHRAE also recommends that intake vents be 2 feet above grade.  Mine is more like 6-8″.  When I move it, I’ll plan to fix that problem as well.

P.P.S.  For what it’s worth, this was permitted and inspected (apparently passed) by the local AHJ, and installed by a fairly reputable HVAC company in the area.

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  1. user-5946022 | | #1

    It may be just within tolerance.
    Look up the installation instructions for the model furnace and water heater you have...
    I suspect you will find that a mechanical supply inlet (which is what your FG8 functions as) that is within 10' of your water heater concentric vent, must be at least 3' below such vent. Furnace will have similar requirements.
    If your install does not meet code, I would contact this reputable HVAC company to have it relocated. (they may however tell you another trade made the penetrations; I'd tell them it was up to them to tell the contractor something was not to code and needed to be relocated)

    I've attached a clip from the Rinnai install guidelines.

    1. aunsafe2015 | | #3

      You are exactly right, thanks so much. The Rinnai manual is exactly like the one you posted and shows that if it is within 10 horizontal feet, the Rinnai exhaust needs to be at least 3 ft above the intake. The furnace manual just requires 3 ft of clearance and does not seem to care whether it is horizontal or vertical.

      So I suppose the 3-4 ft of clearance that I have is probably "minimally" safe in the sense that it complies with the manuals, but I'm pretty sure I saw ASHRAE recommendations for much greater distances. Are those recommendations perhaps with reference to say, an 80% AFUE furnace rather than a 95% AFUE?

      In other words, am I just barely squeaking by here, and I'd be better off relocating the Fantech intake even though it currently complies with manuals? Or is compliance with the manuals 100% adequate such that relocating the Fantech intake would be a complete waste of time?

  2. Expert Member
    Deleted | | #2


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