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Atypical open *tee* installed on furnace intake-air flue pipe


Jerry Simon
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5 hours ago, Jerry Simon said:

The gas-fired furnace and hot water heater are located in a garage closet.  Zero air-intake source from the garage.  Took me a while to figure out why the furnace's exterior air intake flue pipe had been *tee'd* with an open-end pipe stub.

Do you know why?

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To augment the exterior air intake? Its a garage closet so, what's the harm in taking air from the inside if its unconditioned and available?

Edited by Marc
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Hey Mark, no, not to augment anything.

And, you can't take intake air from the garage with the furnace closet located solely in the garage; if a car is warming up in the garage, automobile-generated CO could be drawn through the ductwork into the living areas

Edited by Jerry Simon
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54 minutes ago, Jerry Simon said:

Hey Mark, no, not to augment anything.

And, you can't take intake air from the garage with the furnace closet located solely in the garage; if a car is warming up in the garage, automobile-generated CO could be drawn through the ductwork into the living areas

Ok, I misunderstood.  I thought you were asking about the combustion air intake for the furnace.

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On 4/30/2020 at 6:51 AM, Jerry Simon said:

For the hot water heater's combustion air supply, you silly.

Smart-ass question: How does the furnace know to only pull the air from outside and not from the closet?

More serious question: Is this an AHJ accepted thing in your area?

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9 hours ago, ejager said:

Smart-ass question: How does the furnace know to only pull the air from outside and not from the closet?

More serious question: Is this an AHJ accepted thing in your area?

Not a smart-ass question.  The furnace closet doors have an air-tight perimeter weather-stripping/gasket and spring-loaded hinges to prevent CO from the attached garage from getting into the closet.  

AHJ acceptable?   Who the hell knows.  The thousands of IL cities/town all march to their own drums.  Not to mention the Country of Chicago codes.

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If it were a 60,000 btu/hr furnace and a 40,000 btu/hr water heater, that'd be a total of 100,000 btu/hr. 

The single-opening combustion air method requires an opening with a cross-sectional area of 1 square inch for every 3,000 btu/hr of all the appliances in the space. That would be  33.3 inches. 

Schedule 40 pvc pipe has an interior diameter of just a shade above 3". So (1.5 x 1.5)x(pi) is about 7". 

So if you had another 4 of those pipes, it'd be fine. 

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Jim, those are some interesting specifications for single pipe.  That pipe alone should be easily able to serve the furnace alone, as would another single pipe like that for a direct vent water heater. Do you know what the logic is in making a single source have to be so much larger?

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13 hours ago, ejager said:

Jim, those are some interesting specifications for single pipe.  That pipe alone should be easily able to serve the furnace alone, as would another single pipe like that for a direct vent water heater. Do you know what the logic is in making a single source have to be so much larger?

Not disputing your calculations, Jim, but every other furnace I've seen like that (sans water heater issue in my original post) has same size intake-air pipe.  They've all been wrong?

Edited by Jerry Simon
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The single-opening combustion air method does not apply to direct vent appliances. Those have their own dedicated combustion air pipes.  The two are not comparable. As soon as you make an second indoor opening in the pipe, as was done in Jerry's original picture, that appliance is no longer set up in a "direct vent" configuration. Now the room needs to have adequate combustion air - generally much more than you'd need if you direct-vented the appliances. 

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