Installation intructions vs InterNACHI standards for flue pipes

When reading installation instruction manuals for various furnaces, I see language that specifies that the vent or flue should terminate a certain distance above the “average snow level,” or “above the minimum snow level” on the roof. These instructions for how high to install the pipe above the roof often conflict with the InterNACHI standard of 3 feet. The point being that the installation instructions for furnace flues/vents have different standards than InterNACHI, and rarely say 3 feet.

Do installation instructions trump InterNACHI standards when doing a home inspection where the inspector has a contract saying that they follow InterNACHI Standards of Practice?

How do I measure the average snow accumulation on a roof? Is it the most snow that will accumulate on any given day?

Yes, IMHO. Some AHJs don’t think about that or care.

You use an average depth of snowfall in the usual storm, IMHO.

I had to dig mine out after one storm (Though wall). You might want to alert your clients to that potential.

Thanks for sharing your perspective, Larry.

When Manufacturers instructions direct you to be 12" above ave. snow accumulation, you have to refer to the 2015 IRC

  • Figure R301.2(5) Ground Snow Load

Manufacturers’ recommendations usually take precedence over local building codes, but you won’t know for sure which takes precedence or what those manufacturers’ recommendations are. So, if you see a vent that doesn’t meet the 2-10 Rule, you should recommend evaluation by a qualified HVAC contractor.

And, of course

  • G2427.6.1 Installation, general. Gas vents shall be installed in accordance with the manufacturer’s instructions.
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Different vents and chimneys have different installation rules. The 3 feet is for a typical masonry chimney used mostly by fireplaces and to vent oil-fired appliance. Naturally vented and HE gas appliance follow different rules. Your pic is of HE appliance venting. For this type of application you need to follow the manufacturer’s instructions. Contact your local AHJ and ask them how high they like to see the termination of such vents and or the anticipated accumulation snow level for your area. Believe it or not in NY, where I am, it’s surprisingly 0 inches.

Hello Marcel.
I looked at Figure R301.2(5) Ground Snow Load, and it does not provide any usable numerical measurement for snow anywhere. It only gives subjective terms for Ground Snow Loads in vast geographical areas using terms like “very heavy,” “moderate to heavy,” “slight to moderate,” or “none to slight.” The Figure does not appear to provide measurable snow loads for specified geographical areas.

The Fuel Gas section of the IRC that you provided was very helpful: G2427.6.1 Installation, general. Gas vents shall be installed in accordance with the manufacturer’s instructions.

Thanks, Marcel.

Hello Simon.
I will ask the AHJ what the anticipated accumulation snow level is for my area.

Simon, I called the AHJ and they don’t have an anticipated snow accumulation level for my area. They sound concerned about having a cricket on the high side if the vent is near the eave, to protect it from snow and ice.

There you go :slight_smile:

Who brought this up? a cricket is normally not used with standard HE PVC vents. Have you looked around your area to notice how PVC vents are installed, especially new construction (would have to be inspected & approved by AHJ)

There is anticipated snow accumulation for my area, and lots of it. While the AHJ does not track it, it is still here. I live in a mountainous area where sliding snow and ice could easily rip a pvc vent right off the roof without a hickup.
On newer construction, the builders are trying to go out the exterior walls with the pvc to avoid adding roof penetrations.

Currently, I am getting snow accumulation totals from the National Centers for Environmental Information, who help me obtain data for any given day from a nearby weather station. I use the highest snowfall from any day during the winter to determine the average snow level, but the installation instructions are vague. Therefore, the way the I obtain that data is, as well.

(In snowy climates, the vent terminal should be located one foot above the maximum snow level for that climate. )
I think that in most cases that elevation will be three feet above the ground, depending on where you are located. That can be established more accurately by contacting your National Weather Service for your area.

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That would have saved my furnace from shutting down when the intake of the concentric vent terminal clogged with snow.

Our InterNACHI SoP are association standards. Manufacturer specifications or local codes or if your state has home inspector licensing their SoP (whichever are more stringent) trump association SoP

As marcel stated, masonry chimneys have the 3’rule. That does not apply to gas fuel appliance terminations.
Neither does the 3-2-10 rule.

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To answer your question as to the way you asked, the 3’ rule is a minimum height set to allow gasses to flow properly with possibly reentering the home, manufacturer requirements always trump local codes and rules.

The 2/10 3’ min. rule is for fireplace chimneys.
For gas appliances (including furnaces), the manufacturer’s recommendations typically take precedence over any code. There may be exceptions. Ask your local building dept.

You guys are super helpful. Thanks for the replies.