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Combustion Air From Return Plenum

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Old 02-21-2018, 12:04 PM   #1
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Combustion Air From Return Plenum

There is a water heater and 90% 2 vent (direct vented) furnace in a mechanical room.

But there is a vertical pipe that vents from supply plenum to 1' off the floor. The previous guy who did this says this was for combustion air. But the furnace is sealed and already has the direct vented combustion air.

I don't if this room needs exterior cold air return as well. Or why that pipe is cut from the return. May be for the water heater's combustion air.

Water heater exhaust is sealed to exterior, so there is no CO leakage in to the house from that. I am concerned if there is a gas leak in this room, then it can come in to house through this return and then the supply.

Last edited by viks; 02-21-2018 at 02:59 PM.
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Old 02-21-2018, 04:27 PM   #2
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Re: Combustion Air From Return Plenum

Do you have or can you get the install instructions for the furnace and water heater?

The code says (in part) "Direct-vent gas appliances, gas appliances of other than natural draft design, vented gas appliances not designated as Category I and appliances equipped with power burners, shall be provided with combustion, ventilation, and dilution air in accordance with the appliance manufacturers' instructions" (M1701.1 & G2407.1).


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Old 02-22-2018, 05:49 AM   #3
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Re: Combustion Air From Return Plenum

No manufacturer provides/allows for combustion air to come from the homes central duct system.
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Old 02-23-2018, 11:37 PM   #4
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Re: Combustion Air From Return Plenum

Is the cold air pipe in a separate plenum isolated from anything else but the outside/exterior air?, NOTHING to do with the furnace heating and cooling plenums?

fan forced exhaust on Hot water heater or just chimney?

Moderator: Any competent HVAC person would KNOW the answer to this Question.

Please stop working with out a journeyman level adviser till you have at least a journeymen's level license/training to work on HVAC....

Do you want someone dead or injured due to your greed and false confidence?

If you have worked on your own home's HVAC, you could be suffering the effects of CO or CO2 poisoning, making you "stupid" and untrainable.
I might be an Idiot, but I know things that You don't: Please wait till after you get my know-how to insult me....

Last edited by Fouthgeneration; 02-23-2018 at 11:45 PM.
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Old 02-24-2018, 10:39 AM   #5
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Re: Combustion Air From Return Plenum

You mentioned the pipe is coming off the supply plenum. I am wondering if it's possible that somebody wanted to "condition" the mechanical room during the summer ?

The combustion air thing seems like a stretch, seeing that one could simply shave the bottom of the door, or, install a transfer grill. on the walls.
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Old 02-24-2018, 04:57 PM   #6
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Re: Combustion Air From Return Plenum

Code requires unvented combusters of air to have at least ?1 square inch of free vent per KBTU of fuel use.... preferably Two grills one near the ceiling and one near the floor and the volume of the space = or > the BTUs/ over 10, 50,000 BTU hot water heater need 500 cubic feet +.

Super tight houses will need outside air plumbed to the mech room to prevent partial vacumns created in the house which could reverse the flow of toxic waste gases and suck sewer gases back through traps....

The solution is to insulate/fire proof the mech room and run an air supply pipe of the right size to the out of doors with a rain and rat barrier installed, and tighten the mech room door to exterior door standards.

you still might have issues with Bath room vent fans, range hoods, and Clothes dryers creating low pressure conditions during operation in a tight house.


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