Confusion on cutting top plates for gas wall furnaces. Fire spread

I’m getting really confused

I can tell I’m about to get called out on an inconsistency. I’ve been calling for fire blocking to prevent fire spread from gas wall furnaces to the attic, but my sources are super weak. I have yet to be questioned, and I want to get solid before continuing.

I’m seeking any resources to help resolve this from other inspectors.

A gas wall furnace looks like this:

And are installed by cutting open a stud bay:

And cutting through the double top plate of the wall, regardless if it’s load bearing or not:


So a vent can be installed:

My issue issue is fireblocking.

Viewpoint one: The furnace people in my area insist that the double top plate remain open, else “too much heat will build up in the wall”. They want the heat go to the attic.

Viewpoint two: Some inspectors in my area insist on an inspection vent in the wall above the furnace. The only purpose of the vent is to observe the fire blocking. In this case the vent provides fire access to the inside of the wall as well. One local furnace installer told me of arguments with the City inspector that ended up in a police call, over this exact issue.

Viewpoint three: The main wall furnace vendor, Williams, has conflicting instructions. In all cases for multistory buildings they want a firestop between levels. But only on multistory buildings:

For a single story building they strongly imply the double top plate should remain open to the attic.

Pre-made fire stops are sold separately and look like this DuraVent 4GWFS. They sort of cradle crudely around the oval pipe.

These various sources are somewhere between incompatible and contradictory. All of the approaches leave a place for a fire in a unit to enter an unprotected wall cavity with no gypsum. Most leave a way for the fire to spread to the attic.

What’s the right answer here? Don’t install this type of heater is likely the correct answer, but not practical.

I think if I was to recommend it, I’d stop right there AND defer it to a licensed HVAC contractor

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Same here, Dan.

do you really think that if the furnace caught fire a top plate would stop it? I think not. You are way over thinking this. Furnace pipes go through walls and ceiling all the time.