It's located above the evaporator, with the duct from its side connecting it to the return duct. Flow presently is from the supply plenum to the return duct.
I was measuring the temperature rise across the furnace and was surprised to see the return air right into the furnace at 85F (with the damper in the bypass duct in the summer position), and 101F in the winter position. (No water flowing in either case.) These temperatures are well above ambient of 70F. (Furnace output temperatures at the side of the evaporator were 134F and 155F, giving rises of 49F and 54F, within the label spec of 45-75F.)
Strange thing: I also measured return air temperatures almost two feet ahead of the bypass duct. In both cases, the reading was 77F, still surprisingly above 70F ambient.
With the damper in the summer position, it takes 54 minutes to raise the temperature at the thermostat 4 degrees, from 65 to 69 degrees. In the winter position, again with no water flow, it takes 80 minutes, even though the air temperature out of the furnace is 21 degrees higher.
Measurements were made at 8:00 am. Summer measurements were made with attic ducts in an ambient of 45F. Winter measurements were made same time next day with attic temperature 40F. This accounts somewhat for the longer response time. Attic ducts are 1" duct board.
Sorry for the long post, and drifting somewhat off the Subject.
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Ray