Thermo Pride Gas/Propane Furnace Induced Draft Highboy/Counterflow/Horizontal MHA1 - Manual (Page 14)

All installations and services must be performed by qualified service personnel.
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Table 2: Minimum Filter Area Required (Sq. in.)
Furnace Model
Filter Type
Maximum
Rate of
Velocity
(FPM)
MHA1-50
MDA1-50 MHA1-75
MDA1-75 MHA1-100
MDA1-100 MHA1-125
MDA1-125
*THERMO
PRODUCTS
SUPPLIED
PERMANENT
FILTER
600 172 254 328 402
STANDARD
PERMANENT
FILTER
500 208 304 394 484
DISPOSABLE
TYPE
FILTER
300 344 506 656 804
* The Thermo Products supplied filter can be cut to size to fit other filter retention
systems as long as the minimum size requirement is met.
NOTICE: The filter areas in Table 2 are the minimum areas required based on
the flowrate generated by the furnace for standard heating speeds only. The
following formula can be used to determine the minimum filter area required for
cooling, if the unit is equipped with cooling. This value should then be compared
to the value shown in table 2 and the larger of the two should then be used for
determining the minimum filter area required for that installation.
FORMULA:
(tons of cooling)(400 CFM per ton)(144 sq. in. per foot) = filter area sq. inches
(max. air velocity of filter from table 5 for the filter type)
EXAMPLE: If you had an MHA1/MDA1-100 furnace and 4 tons of cooling and a
standard reusable filter.
4 tons x 400 CFM x 144 = 460 square inches for cooling
500
For heating an MHA/MDA-100 needs 328 square inches of standard reusable
filter. The filter system must be designed for the larger flow requirement
determined for cooling of 460 square inches. A filter would have to be sized so
that the area (length x width) was at least 460 sq. in.