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Old 09-16-2020, 08:50 PM   #1
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HVAC ok to use with bad outside air


My HVAC has an air intake in my basement goes to the bottom of the furnace and passes through an air filter. There is an air duct on the other side of the bottom of the furnace that goes to the ceiling, through the garage. I am not sure where it goes from there. I didn't notice a vent on the side of the house.

I was going to turn on my fan to try and clean the air in my house since there is smoke from fires far away but not sure if it will pull air from the outside and make the air quality worse inside. I would think there is minimal air from the outside since the furnace is used in the wintertime and outside air would defeat the purpose of trying to heat the air in the house.

The vent at the bottom of the furnace seems wide open and the air duct looks pretty big in the garage.

Any idea if outside air would come in if I turn on the fan only? I don't have an air conditioning unit for the house.

note: I attached a photo of the air quality. The fires are far away, but the winds are pushing the bad air up the coast all the way into Canada.

note: i'll send separate photos of the intake

Thanks
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HVAC ok to use with bad outside air-20181027_160345.jpg   HVAC ok to use with bad outside air-20200916_182830.jpg   HVAC ok to use with bad outside air-20181028_153421.jpg   HVAC ok to use with bad outside air-20200916_182806.jpg   HVAC ok to use with bad outside air-cur_aqi_usa.jpg  

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Old 09-16-2020, 08:59 PM   #2
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Re: HVAC ok to use with bad outside air


Here are a few photos of the air intake. The intake is in the basement. I took a photo under the furnace and you can see the air filter. Looking directly through the air intake in the basement, you can see the other side, where I assume air also comes in. That opening seems large. That duct goes up to the ceiling in the furnace closet and out to the garage, though i do not see a vent in the garage. I assume it might go to the roof area possibly or attic.
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HVAC ok to use with bad outside air-20200916_184216.jpg   HVAC ok to use with bad outside air-20200916_183518.jpg   HVAC ok to use with bad outside air-20200916_183528.jpg   HVAC ok to use with bad outside air-20200916_183533.jpg  
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Old 09-16-2020, 09:20 PM   #3
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Re: HVAC ok to use with bad outside air


Looking at the furnace I noticed the duct at the top of the furnace closet may connect to the top of the furnace. If so, that means hot air is being used on the intake, which might make sense. I attached a few photos.

Also, I noticed that I can set the fan speed %. I am not sure what speed I should keep it at for heating the house. It was at its lowest setting, something like 18%, so probably not too efficient.

Excuse the photo orientations. I can't figure out how to post photos in the right direction.
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HVAC ok to use with bad outside air-20200916_190522.jpg   HVAC ok to use with bad outside air-20200916_190507.jpg   HVAC ok to use with bad outside air-20200916_190449.jpg  
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Old 09-18-2020, 11:52 AM   #4
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Re: HVAC ok to use with bad outside air


On a 90%+ furnace like that, it will pull in fresh outside air from one the PVC pipes for proper combustion in heat mode. On fan only mode, it will just circulate whats in the house.
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Old 09-18-2020, 02:30 PM   #5
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Re: HVAC ok to use with bad outside air


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On a 90%+ furnace like that, it will pull in fresh outside air from one the PVC pipes for proper combustion in heat mode. On fan only mode, it will just circulate whats in the house.
Thanks. I'll run the fan since it has been smoky in the area and worried that the internal air has been affected.
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Old 09-19-2020, 08:48 PM   #6
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Re: HVAC ok to use with bad outside air


You're fine using your heating/cooling system. Combustion air supply and interior air are isolated from each other. Combustion air is pulled in from the outside, as stated above, burned, passed over the primary and secondary heat exchangers and then vented back outside. Interior air is in a totally separate conduit and does not mix with the combustion air from the outside. "Bad" air quality (from a breathing standpoint) will be fine for combustion air, so using your heating system will be fine.
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Old 09-19-2020, 10:53 PM   #7
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Re: HVAC ok to use with bad outside air


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Originally Posted by sktn77a View Post
You're fine using your heating/cooling system. Combustion air supply and interior air are isolated from each other. Combustion air is pulled in from the outside, as stated above, burned, passed over the primary and secondary heat exchangers and then vented back outside. Interior air is in a totally separate conduit and does not mix with the combustion air from the outside. "Bad" air quality (from a breathing standpoint) will be fine for combustion air, so using your heating system will be fine.
What is the other duct on the other side of the intake vent used for? Is it heated air from the furnace that is redirected to the vents in the house and also to the bottom of the furnace and reheated? In the photo, there is a bit opening at the bottom of the furnace but not sure where that air is coming from.
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Old 09-20-2020, 07:26 PM   #8
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Re: HVAC ok to use with bad outside air


Sorry, don't understand what you are referring to, it looks like you have a standard supply and return duct.
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Old 09-20-2020, 10:35 PM   #9
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Re: HVAC ok to use with bad outside air


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Sorry, don't understand what you are referring to, it looks like you have a standard supply and return duct.
I think I don't understand the return side of the furnace. There are two entries into the bottom of the furnace for intake. One intake is in the basement room. The other side of the intake goes through the furnace closet, into the garage and I am guessing it goes up into the house. I thought all the air vents in the rooms were pushing air into the room. Is the other air intake vent in the attic or the roof?

I attached a diagram but it only has one air intake into the furnace. I have two.
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Last edited by greentrees; 09-20-2020 at 10:36 PM. Reason: describe diagram
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Old 09-21-2020, 05:56 AM   #10
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Re: HVAC ok to use with bad outside air


Returns take air out of the room.
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Old 09-21-2020, 06:24 AM   #11
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Re: HVAC ok to use with bad outside air


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Originally Posted by greentrees View Post
What is the other duct on the other side of the intake vent used for?
"the other vent" is not the greatest description.

But on the outside of the house, there will (on a high efficiency furnace) typically be a roughly 2" or 3" white PVC pipe for combustion air in, and beside it, a similar white PVC pipe for products of combustion out.
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