I'm switching out the old gas furnace and air conditioner.
Replacing with a Rheem 3 ton Heat Pump and a Rheem 105,000 BTU 90% Mod furnace.
I've got a partial install at the moment with the HP and furnace fan operating for A/C.
The existing metal return duct is 14" X 24". This has not really been tied into the furnace yet - just using the furnace box to join them together.
The furnace is an upflow version of the Rheem Mod. It is set on a large metal box that the contractor cut a hole in to match the bottom of the furnace. There is another hole cut into the box for the return duct to join. That hole is only 10" X 24". It came that way from his supply house.
Would it be an improvement to increase the size of the hole to the full size of the return duct? It seems as if there would be less restriction in the airflow that way. I want to mount some filters into that transition section so that box can be about any form. I do not have any space constraints as the furnace unit is sitting in the unfinished basement with lots of room to spare. The installer checked with the Rheem Distributor / supply house and was told that the 10 X 24 could actually handle up to 1700 CFM airflow. That seems like it would be moving pretty fast. And it is probably more than I need for my situation, but maybe not.
It would not be a problem now to make the hole a little larger if there would be a benefit in lowering pressure on the inlet side of the blower. However, is there any benefit to be gained?