American Standard Condensing Gas Furnaces Fan Assisted Combustion System Upflow Horizontal and Downflow Horizontal User's Information Manual page 2

Filter maintenance reduces energy use.
A clean filter saves money.
When the furnace circulates and filters the air in your home, dust
and dirt particles build up on the filter. Excessive accumulation
can block the airflow, forcing the unit to work harder to maintain
desired temperatures.
And the harder your unit has to work, the more energy it uses.
So you pay more any time your system is running with a dirty
filter.
CAUTION: Never operate your unit for either heating or
cooling with filters removed.
Help ensure top efficiency by cleaning the filter once a month.
Clean it twice a month during seasons when the unit runs more
often.
You can clean the filter with a vacuum, OR you can wash it with
a household detergent.
Both methods are quick and easy, and guaranteed to improve the
performance of your system.
Your filter may or may not be framed.
Replacing your filter.
When replacing your furnace filters, always use high velocity
type which are the same size as originally supplied. Filters are
available from your dealer.
Where disposable filters are used, they must be a high velocity
type which are the same size as originally supplied.
How to remove your filter.
WARNING: Disconnect power to unit before removing
blower door.
Upflow furnaces use a high velocity type air filter which may be
located within the furnace blower compartment in either a
BOTTOM or SIDE (left or right) return air inlet. The furnace
may be secured with filter retaining brackets (as shown) or a
filter retainer wire.
To replace filters, remove blower access door, push back to flex
the filter and clear the filter retaining bracket at the front of the
unit. After cleaning, replace the filter in the same manner
making sure that the filter wire is secured in place in both front
and back filter retaining brackets. Replace blower access door.
A bottom return air inlet as shown features a 17" x 25" x 1" filter
in the 17-1/2" wide furnace cabinets; a 20" x 25" x 1" filter in the
21" wide models; and a 24" x 25" x 1" filter in the 24-1/2" wide
cabinet.
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UPFLOW BOTTOM RETURN
For vertical UPFLOW ONLY, a left or right return air inlet as
above (left side shown) requires trimming of the factory supplied
filter to 17" x 25" x 1" for both the 21" and 24-1/2" wide furnaces.
UPFLOW SIDE RETURN
Air filters may also be located outside of the furnace using a
SIDE FILTER FRAME.
UPFLOW ONLY
Pub. No. 32-5022-10