HVAC Vents Needed or Not?

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rubenhak
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HVAC Vents Needed or Not?

Post by rubenhak » Fri Oct 05, 2018 5:49 pm

Hi Everybody,

I'm in the middle of a repair of HVAC closet after a water leak. The unit is out and I noticed that there are two vents that go to unconditioned attic space, one to the top, and another to the side wall. My questions is if this is needed or The unit can be sealed?

My understanding was that the furnace takes the outside air to burn and it does not need the inside air. I'm hoping I can seal the unit so that the unconditioned attic air does not get into the house. Here is a picture.

I've got a modern furnace and A/C - Rheem 96MDV. I think the unit uses separate inflow and outflow pipes.

Thanks,
Ruben
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Shannon
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Re: HVAC Vents Needed or Not?

Post by Shannon » Fri Oct 05, 2018 6:22 pm

Your furnace will need combustion air from outside the home if you have a high or mid. efficient unit. You will also have a cold air return of interior air from the house.
From these pictures and your description I can not say for sure if these ones you are describing are needed or not.
Are these pictures both upside down??
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A. Spruce
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Re: HVAC Vents Needed or Not?

Post by A. Spruce » Fri Oct 05, 2018 6:23 pm

I do not know the specific needs of your unit, nor am I an HVAC guy. What I can tell you is that a gas unit does require combustion air, and the crawl space/attic venting is to allow for that, if you seal them up, you not only draw from the house air, you create combustion issues that can cause invisible, odorless, and lethal carbon monoxide build up.

With those vents, your closet door should have a seal on it, just like your front door, this keeps the unconditioned closet from leaking into the conditioned space of the house. So long as the closet door has this seal, you shouldn't have any issues. If the door does not have a seal, I'd install one or replace the door with one that does have a seal.
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rubenhak
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Re: HVAC Vents Needed or Not?

Post by rubenhak » Fri Oct 05, 2018 6:31 pm

Shannon wrote:
Fri Oct 05, 2018 6:22 pm
Your furnace will need combustion air from outside the home if you have a high or mid. efficient unit. You will also have a cold air return of interior air from the house.
From these pictures and your description I can not say for sure if these ones you are describing are needed or not.
Are these pictures both upside down??
Picture 1 is lookup up where the box where ducts attach. My unit is removed from the closet.

Picture 2 is should be rotate 90 degrees clockwise. The framing you see is where the unit is supposed to stand. The plywood is removed. This is where my air return is.

The unit is high-efficient 96%.

My question was if I can completely seal the unit closet from attic air or the unit still needs air from the attic for some reason?

rubenhak
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Re: HVAC Vents Needed or Not?

Post by rubenhak » Fri Oct 05, 2018 6:33 pm

A. Spruce wrote:
Fri Oct 05, 2018 6:23 pm
I am do not know the specific needs of your unit, nor am I an HVAC guy. What I can tell you is that a gas unit does require combustion air, and the crawl space/attic venting is to allow for that, if you seal them up, you not only draw from the house air, you create combustion issues that can cause invisible, odorless, and lethal carbon monoxide build up.

With those vents, your closet door should have a seal on it, just like your front door, this keeps the unconditioned closet from leaking into the conditioned space of the house. So long as the closet door has this seal, you shouldn't have any issues. If the door does not have a seal, I'd install one or replace the door with one that does have a seal.
I understand it and this was my concern as well. But I was hoping that the unit would draw the outside air to burn and not unconditioned air. The unit has separate inflow and outflow pipes.

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A. Spruce
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Re: HVAC Vents Needed or Not?

Post by A. Spruce » Fri Oct 05, 2018 6:50 pm

rubenhak wrote:
Fri Oct 05, 2018 6:33 pm

I understand it and this was my concern as well. But I was hoping that the unit would draw the outside air to burn and not unconditioned air. The unit has separate inflow and outflow pipes.
Not sure what you're referring to as "outflow/inflow" pipes. The air handler portion of the furnace will be connected to the ducting in the house, one is the "supply", the side that feeds the house, and the other is the "return", the side that returns from the house to the furnace.

As for ducted combustion air, I've never seen this on any furnace before, not that it may not exist, I've just never seen it.
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Re: HVAC Vents Needed or Not?

Post by rubenhak » Fri Oct 05, 2018 6:56 pm

A. Spruce wrote:
Fri Oct 05, 2018 6:50 pm
rubenhak wrote:
Fri Oct 05, 2018 6:33 pm

I understand it and this was my concern as well. But I was hoping that the unit would draw the outside air to burn and not unconditioned air. The unit has separate inflow and outflow pipes.
Not sure what you're referring to as "outflow/inflow" pipes. The air handler portion of the furnace will be connected to the ducting in the house, one is the "supply", the side that feeds the house, and the other is the "return", the side that returns from the house to the furnace.

As for ducted combustion air, I've never seen this on any furnace before, not that it may not exist, I've just never seen it.
I'm referring to direct venting where the combustion air is supplied by outside and exhaust also goes out.

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Shannon
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Re: HVAC Vents Needed or Not?

Post by Shannon » Fri Oct 05, 2018 11:03 pm

Yes I understand your question now. I thought you wanted to close off the ducts themselves. Yes you should be able to seal off the closet to a point. In most cases you can seal everything since you have incoming combustion air supply and out going exhaust. My fear is if you make it completely air tight you could cause issues with back drafting possibly if the vents are not sized right. If you seal it all up I would keep a close eye on it and be darn sure you have a working CO detector close by. I think you may find you will need a vent in the door or at least a good space under the door . I would consult a Hvac pro to be sure.
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rubenhak
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Re: HVAC Vents Needed or Not?

Post by rubenhak » Fri Oct 05, 2018 11:25 pm

Shannon wrote:
Fri Oct 05, 2018 11:03 pm
Yes I understand your question now. I thought you wanted to close off the ducts themselves. Yes you should be able to seal off the closet to a point. In most cases you can seal everything since you have incoming combustion air supply and out going exhaust. My fear is if you make it completely air tight you could cause issues with back drafting possibly if the vents are not sized right. If you seal it all up I would keep a close eye on it and be darn sure you have a working CO detector close by. I think you may find you will need a vent in the door or at least a good space under the door . I would consult a Hvac pro to be sure.
Thanks for the comments. Do you think the manufacturer can comment on this?

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Shannon
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Re: HVAC Vents Needed or Not?

Post by Shannon » Sat Oct 06, 2018 4:36 pm

They should be able to
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