Calculating the correct sizing for a return air on a electric furnace

[Adam/]

Here are some details on the electric furnace, its a Lennox Model # CB19-51-3p-TXV Serial # 5895F61762

Trying to figure out what size of return I need. Is there some more info I need to provide?

[handyandy]

To size a return air duct or grille for your Lennox CB19-51 electric furnace, you generally need about 200 square inches of free area per ton of system capacity, or roughly 2 square inches per CFM of airflow demand.

The "51" in your Lennox CB19-51 model number indicates it is a larger unit (typically a 4-to-5 ton blower capacity), which usually demands around 1,600 to 2,000 CFM depending on the specific external static pressure and blower speed set. This size chassis (like the CB19-51 and CB19-65) is actually designed to take dual side-filter kits or a combination bottom/side return setup from the factory to handle the high air volume.


To calculate the exact precise duct and grille dimensions using a Rectangular Duct Sizing Chart or a standard HVAC Duct Sizing Calculator, please provide:

The exact tonnage or kW size of the electric heat strips installed inside the furnace.

Whether you are sizing a central filter grille, a ducted return trunk, or connecting directly to the cabinet sides/bottom.

The total length and layout of the return duct run (including number of elbows or flex vs. sheet metal).

[Adam/]

The exact tonnage or kW size of the electric heat strips installed inside the furnace.
The furnace info says 68200 btu's @20kw. The sticker on the heating element says EBC19-20. 0 1P
Here is the best picture I can get of the return air as it is behind the unit and goes up the wall from the floor to the top of the furnace in a L shape.

This is in a closet and the air return grill is on the other side of the wall.

My furnace currently has two filters at the base of the furnace unit, one on each side. on one side it has a 20x25x1 and the other has a 16x25x1.
I want to remove them and put a single filter return air grille assemble which has a free air space of 24x24. I live in a very dusty area and the return air duct work gets so dirty every year and I am tired of having to spend several hours to clean it up. I have seen other systems that had the filter in the return grille instead of at the furnace.
So what I am trying to figure out how big of an air return hole I would need to see if I need to increase the size of the current return air inlet hole in the wall to do this.

Incase it helps here is a drawing I did of the current return air set up.