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I have a furnace located in my basement in a small enclosed room. I also have a large vent upstairs above this room in my kitchen. I always thought that the furnace takes the air from the kitchen upstairs, heats it up and circulates it back. However, when I was changing the air filter today, my wife asked about the vent located downstairs near the furnace.

It seems to me that the air is bring pulled from 2 locations. First, upstairs by the kitchen and second also directly in the room where the furnace is located (the second location is not ideal. In an enclosed room with hot water heater and sewer pump)

I searched around online and I couldn't find anyone with a furnace design similar to mine that had another intake in the air return. I closed the second intake off by wrapping the vent in a plastic bag to prevent any additional airflow. Once the furnace started again, we noticed a much stronger pull from the first intake upstairs.

Does anyone know why there would be two intakes and will blocking off the second intake in the enclosed room be a problem?

Edit: The main problem is there is a slight smell that gets circulated from that room. Once I closed it off, the air got quite a bit fresher. My worry is that there was some good reason it was there in the first place.

Enclosed Room in Basement Furnace with second vent First vent in kitchen

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  • Was there a problem you thought you were solving by making this change? As in you were having a problem befroe you noticed the vent, and it's better now, rather than "you noticed a vent and decided to close it?" One obvious effect will likely be poor heating of the basement as any hot air delivered to the basement will just run across the ceiling on its way to the kitchen upstairs.
    – Ecnerwal
    Commented May 17, 2020 at 3:31
  • Thanks for the reply. The main problem is there is a slight smell in that area and the air gets circulated and isn't fresh.
    – Leon
    Commented May 17, 2020 at 4:25
  • Closing off the vent will affect the system as it was designed for multiple intakes. As for affecting the smell stagnant air in that area it will usually get worse down there an activated carbon filter will do the best to eliminate odors in the air but the root cause should be addressed , the sewer pump /pit should be sealed this is the reason every sink tub and or toilet has a water trap to keep the smell and possible hazardous gasses out. The edit made the much different that the original.
    – Ed Beal
    Commented May 17, 2020 at 16:00
  • Terminology point of order: Your furnace (oil,gas) burns fuel and needs air to do that. Old school furnaces just pulled that out of the ambient air around the unit. Newer tech uses a separate "intake" vent. The exhaust from this process is vented to the outside. The ductwork that pulls air to be heated/cooled from your house is called the "return". As the answers show, practice has changed over time. It should come from conditioned space. Blocking the basement return is a good idea, but I've opened mine on occasion in the summer as a source of cool air.
    – DaveM
    Commented May 17, 2020 at 16:14

3 Answers 3

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Actually it is quite common to have multiple air return ducts, with both heating and air conditioning it can save 20-30% by making slight adjustments and changing the flows. I usually suggest dampers on both the supply and return trunk lines for each level for the maximum savings, summer with ac more flow to the upper floor(s) in the winter more flow to the lower floors when heating. This balancing if done well with a air handler appropriately sized can save up to 30% so it may be uncommon in your area but higher end systems do have multiple intakes.

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Having single returns on each floor is now the new (cheaper) normal.When I was still doing some residential heat & a/c it was imperative to have a return and a supply in each room. Today, in order to save money on the installation, most if not all of the of the multi-unit housing plans are opting for the single returns. This single return concept lowers the overall efficiency of the system to control the home's temperature while also making noise transmission a problem since the bottom of all doors need to be cut to allow for air flow out of that room. If smells are a problem with your home try to find a charcoal filter that fits your furnace. It was a problem with mine since my wife can smell a "rabbit fart" at 2 miles. I have an electronic air cleaner in which I removed the pre-filter and in it's place I put a cut to size charcoal filter called "the absorber" which I purchase from the orange or blue store. This is a pain in the a*s but it keeps my wife happy (you get the picture). If you look around your area at the newer patio homes and duplexes you will see this single or double return concept. my 2 cents

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  • I don’t know how you say it is cheaper as an additional intake grill is required . I still have installed electrostatic filters prior to the furnace and use cheap fiberglass at each intake to keep dust from building up in the duct work.
    – Ed Beal
    Commented May 17, 2020 at 15:45
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The contractor likely put in the minimum air recycle inlets required for that furnace By choking-off one off you may cause the furnace heat exchanger to over-heat. I would remove the blockage as soon as possible. The more recycle air supply , the more efficient your furnace. My house was built with a single ( cheap) air return. I have added two more ; now a total of 20 X 30 + 16 X 20 + 12 X 12. , works fine. When I lived in IL every room had a return ( built in the '60's). I believe the lower air velocity through my 3 filters helps the filters work more efficiently also. My house is one level so no summer/winter air flow adjustments are needed.

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