Furnace Air Return in Forced Air Heating System

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Poorly designed or older forced air heating, and air-conditioning systems often have only a single air return compartment.

With a single story home, such furnace air return installation is still popular, simply because it saves money and material.

However, single furnace air return configuration might not be as efficient as the one with individual air return ports installed in every room. You should also remember to have doors in all rooms undercut approximately 3/4″ above the finished floor surface to provide proper air circulation in home.

Sometimes, carpet installation over the hardwood floor completely eliminates this necessary gap under the door bottom edge. Single furnace air return systems are also noisier (noise from air returning to the furnace through only one opening) than the ones with returns distributed evenly through the house.

Single furnace air return in two-story residences

Usually installed on first floor becomes little more problematic. Such designs are very common in older houses, built when air conditioning wasn’t yet popular or available. For heating purposes it almost works, warm air naturally raises up, so upper floor should be warmer…, and as long as your house is not too “air tight” this is true.

However, if you notice that temperature on the second floor appears to be dropping, slightly open the window to release air pressure, and you’ll be OK. Over the years, air conditioning units were added to those air forced heating only systems.

Most of the time without any concern about proper sizing of ducts, and installation of an additional furnace air return on the second floor. That extra furnace air return would significantly help to optimize performance of the entire system.

Forced air systems AC/heating duct work is designed primarily for cold air distribution, because more cold air is necessary to lower room temp., than hot air to raise it.

Therefore, summer time on the second floor, and without the furnace air return duct, might be painful (you can always open the window to release pressure). The easiest way to improve this condition is to use part of the first floor closet, or any wall corner space, to install return duct between the basement furnace and the second floor area.

fFurnace air return should not be placed closer than 10' from any area of combustion, or should be located in a separate roomOne more furnace air return conditionthis time a safety hazard, is an installation of air return in close proximity to any source of combustion (process of burning fuel). Because each floor (including basement), should have at least one furnace air return port for proper circulation, such ports are often installed next to the furnace enclosure, on the furnace air return duct.

By code, and for safety reasons, furnace air return (in forced air systems) should not be placed closer than 10′ from any area of combustion, or should be located in a separate room. This condition doesn’t apply to high efficiency systems, and regular efficiency furnaces with sealed combustion chamber which often (not always) use combustion air from exterior.

Furnace air return compartment with a side duct and improperly open bottomFurnace air return compartment with a side duct and improperly open bottom - missing metal plateFurnace air return compartment warning information tagAnother furnace air return compartment safety issue – an item often forgotten by the furnace installers, and at the same time required by the furnace manufacturers is a bottom closure panel. Furnace enclosures are sometimes shipped with an open bottom for return duct attachment.

However, installation location might require return duct to be attached on the furnace side or rear, in which case special metal plate (bottom closure panel) must be installed – same hazardous condition as in previous paragraph – might cause bi-products of combustion, Carbon Monoxide to mix with and contaminate the circulating-air.

Go and check your furnace air return right know!

4 Comments
  1. Don Dineen says

    My daughter is in the process of remodeling a house. It is two story, about 2,600sf. Her husband had to do a total gut before starting. They put in a large single air return in the basement, about 30″x30″. There is no return on the top floor, just an open staircase to the basement (living spaces).
    Since the “other” grand dad was in heating and air conditioning they followed his ideas. I suggested that each room should have had its own ceiling air return so the warm air would be used more efficiently, and it should stop the drafts associated with all the air in the house funneling through the house to the floor height return.
    I get cussed out suggesting that I think it would be better. The other grandpa just keeps repeating hot air always rises. Well it also conducts and radiates, and this reheating of the cold air causes the furnace to work harder.
    I am persona non grata when it come to furnaces.

    Is it better to use individual ceiling hieght returns in each room? Our 1950’s era home needs a new furnace and I was thinking of routing returns high on the walls or in the ceiling. We only have 800sf to deal with, but it is heated very un-uniform during our 5 months of ice. There are two floor mounted returns in the living room, none in the other rooms.

    We did insulate the house. After we bought it we found out that none of the walls were insulated. At that time the furnace ran 24/7. When I found the crawl space heated to 65 F I added insulation under the house. That is a big improvement.

    1. CTH Experts says

      Hi Don. I apologize for a delay but I was away for a long weekend + a little more. You are absolutely correct with the air returns in each of the rooms. This would dramatically improve air circulation and help to equalize temperature through the whole house.

      They should at least install a single air return port on the main floor so the air has a way of being recirculated. Single returns used to work in older, poorly insulated homes with a lot of air leaks. All those gaps and holes allowed the heated air to escape and make room for more heated air (it’s called stack effect). Newer (or newly remodeled) homes are much tighter, thus keeping all the air inside, which requires circulation.
      Even one properly sized air return on that main floor will help them stay comfortable. Ideally (if using heating and cooling), double air returns located in the upper(or ceiling) and lower sections of the wall perform best.

  2. Rindie says

    We are in negotiations on a house where the inspector showed us an open return by the furnace in a 1.5 story house. No ductwork. It looks to my uneducated in HVAC eye like an open metal trough that sucks air into it. The inspector said it may create a carbon monoxide hazard. I would like to have the company who installed and services it come in and explain why it’s right or wrong. Also considering an opinion from an uninvolved company. Any thoughts or suggestions?

    1. admin says

      Hi Rindie,
      An open return by the furnace issue is always associated with the setup of the area this furnace and maybe some other gas burning appliances are located in. It has also to do with the combustion air required for the gas (and other fossil fuels) burning process. Forced air furnace air return must be a min of 10’ from the firebox (furnace burner chamber, water heater burner chamber, fireplace firebox, etc.) and from the draft hood (water heater, some boilers, and furnaces), or located in a separate room (not in a bathroom, closet, bedroom, kitchen, garage, mechanical room).
      This requirement applies in situations where fuel burning appliances rely entirely on the combustion air from within the room they are located in. For the appliances using house exterior combustion air only and equipped with sealed combustion chambers (high efficiency furnaces with two PVC pipes, high efficiency water heaters with exhaust and air intake pipes, etc.) this wouldn’t be necessary.
      The 10’ forced air furnace air return opening distance is required to minimize the possibility of creating a negative pressure within the area surrounding the appliance. This negative pressure could create a backdraft through the draft hoods and combustion chambers of the appliances. In an air tight / well weather sealed home this could result in exhaust gases (might contain Carbon Monoxide) being sucked back in to the house from the vent pipes / chimney, and distributed through the house by the air circulating system.
      Another problem is the combustion air requirement – without the proper amount of oxygen / air being removed / diluted by the return port from the area surrounding the appliances, Carbon Monoxide might develop during the burning process. With the air return being so close, CO could be distributed through the house.
      So, besides the air return port being too close to the fire box / draft hood, you have to make sure that all of the fuel burning appliances are provided with an adequate amount of combustion air – read that post!
      Let me know if this fully answers your questions and if there’s anything else I can help you with…

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