Furnace Air Return in Forced Air Systems
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.
One more furnace air return condition – this 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.
Another 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!
Search existing Q&A in Heating and AC Questions