gravity furnace vent thingy

[Kristi Silber]

Saw this today on an old monster of a furnace. Never saw anything like it before, and I'm wondering if/why it's necessary. It's a open-bottomed box on the vent, just before it joins with the water heater vent.

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[Eric Barker]

Draft hood.

[Kristi Silber]

Thanks, Eric! Feel like I should have known that. Geez, seems odd there somehow, but I know little about HVAC, especially these ancient furnaces.

Probably unrelated, and I can't say with absolute certainty it was coming from this house, but I noticed in the back yard quite strong combustion fumes, though it was vented through the chimney. Strange. Couldn't see where else it might have been coming from.

[John Arnold]

Looks like asbestos duct wrap, but you knew that...

[Kristi Silber]

Yup, but thanks anyway! Never seen a gravity furnace without asbestos.

[Bob Harper]

Unlisted horizontal draft hood. They were often added on when the heater was converted to gas. Of course, I have a bunch of boilers in my area still set up for mfd. gas, which burns totally different from NG or LPG. No approved way to convert them so these units really should be replaced.

The problem with draft hoods is they function exactly as designed: they disconnect the chimney from the appliance. They should be outlawed. Safer solution is to replace them with a bullhead tee to double acting barometric damper with spill switches. A lof of debate on whether this modification is approved or not but I can assure you it is one helluvalot safer and works.