Bernoulli's Principle

[Neal Lewis]

I've had on occasion poked a sizable hole in a gas water heater or Cat I gas heater rusty exhaust/vent connector (aka flue pipe). Usually someone will gasp how carbon monoxide will be spewing outta the flue (I carry aluminum tape for repairs). I try to explain to them that Bernoulli's Principle says that air will be drawn into the flue because of the pressure differential, which usually results in a blank stare. I might try to give a real world example if someone seems interested.

So here's the question. At what point do the products of combustion indeed spew forth from the damaged flue. Does the hole have to be a certain size or the flow rate increase to say, a Cat IV appliance? Where's the threshold?

[kurt]

It's about pressure differentials, which are highly variable.

All things in balance, hardly any combustion gas escapes the flue. My experience tells me that nothing is in balance, because when I put my hand adjacent to such holes, I can always feel combustion gas coming out.

I pretty much stick with the basics. It's a flue connector; there's a hole in it because of XYZ. Replace it.

[Michael Carson]

I have seen natural draft water heaters back draft with only the bath fans turned on.

[Terence McCann]

The water heater has a flue pipe for a reason. If it has holes in it replace it. If the flue was meant to have holes in it then it would have been shipped that way. There is no need to impress your clients with home inspector techno babble. Just adhere to the KISS principal.

90% of the people won't understand and the other 10% will want to argue about the theory.

[kurt]

I have seen natural draft water heaters back draft with only the bath fans turned on.

That would tend to indicate there's no makeup air supply. Tight houses have to have makeup air, or lots of bad things can happen.

[Jim Katen]

I've had on occasion poked a sizable hole in a gas water heater or Cat I gas heater rusty exhaust/vent connector (aka flue pipe). Usually someone will gasp how carbon monoxide will be spewing outta the flue (I carry aluminum tape for repairs). I try to explain to them that Bernoulli's Principle says that air will be drawn into the flue because of the pressure differential, which usually results in a blank stare. I might try to give a real world example if someone seems interested.

Don't mention the dead guy's name. Just say that a moving column of air has lower pressure than the non-moving air. Illustrate the point as you speak by holding a piece of paper near the hole and watch it get sucked against the pipe.

So here's the question. At what point do the products of combustion indeed spew forth from the damaged flue. Does the hole have to be a certain size or the flow rate increase to say, a Cat IV appliance? Where's the threshold?

The threshold is when the pressure inside the pipe is greater than the pressure outside the pipe.

With a Cat IV appliance, the pressure inside the vent is greater than the pressure outside whenever the appliance is running. With a natural draft appliance, the pressure inside the vent is lower than the pressure outside (if the vent is working properly). The amount of pressure that drives a natural draft vent is really really small. I've heard is expressed as equal to the pressure of 1-1/2 gnat farts. It's really easy to overcome a natural draft vent and cause it to backdraft. But that fact is completely unrelated to the presence of holes the pipe.

Now, if there's a very large hole in a natural draft vent pipe, it can cause a region of turbulence that allows leakage. There's no way to predict turbulence without some kind of visual marker such as smoke.

Also, if the appliance has a draft inducer fan, the vent will be under positive pressure during the purge cycles before and after the burners fire.

A small hole in a natural draft vent is not a big problem. If there's positive pressure inside the vent, it's going to backdraft hundreds of times more gas from the vent hood than it every will through the hole.

[Marc]

Perhaps a simpler way to explain spillage via a hole in the flue in a natural draft appliance is that the hole results in a reduction of negative pressure at that point as well as at all points from there down to the draft hood. The flow of air into a flue implies some relief of the negative pressure that caused it in the first place. A large enough hole may result in sufficient loss of negative pressure to cause spillage, not only at the site of the hole but at the draft hood also.

Holes may also delay the generation of negative pressures at the initial start up of the appliance, increasing the amount of draft hood spillage that occurs at each time.

Marc