Why is a sealed draft hood and air gap bad?

Earlier today myself and another inspector were at an inspection and saw that the exhaust flue above a water heater did not have an air gap or a draft hood for the house air to be drawn into the flue with the exhaust. We knew that this was an incorrect way to set up the exhaust, but we couldn’t figure out WHY it’s wrong. Our assumption was that without that air gap, the combustion air rising through the flue would create too high negative pressure in the combustion chamber. This would lead to combustion air being sucked into the combustion chamber much faster than the burners could handle, leading to inefficient combustion (I.e. more energy lost than necessary. think of a candle flickering in the wind) and higher levels of carbon monoxide (because the flames aren’t getting all the oxygen they need). Is this correct?

Your help is greatly appreciated!

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Pictures are most helpful for answers. Can you share?

Think about that for a minute.

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Slava, this may help you understand draft hoods (from A.O. Smith): https://www.hotwater.com/lit/bulletin/187010-000.pdf


Hello Larry, thank you for this information. However, it still doesn’t answer my question - WHY is it bad when the air gap is closed off? What’s the physics behind it?

Does that make sense?

I’ve been thinking about it for a few hours :stuck_out_tongue:

Certainly, please see the attached photo

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The second paragraph from my link: https://www.hotwater.com/lit/bulletin/187010-000.pdf



"A draft hood is critical to the operation of a gas water heater. (It is attached between the heater’s flue outlet and the vent pipe.) When properly installed, the draft hood cools the combustion products and allows them to flow safely from the building. It also protects heater operation during updrafts and downdrafts that are caused by indoor/outdoor air pressure differences."

The above does not happen if there is no draft hood.


Ahhh I somehow overlooked that paragraph, my apologies. Thank you for your help!

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You’re welcome, Slava… :smile:

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