Return - How Large

[aaCharley]

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?

[dan sw fl]

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".

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.

... Use of 18" x 24" x 5"
MERV 8 Pleated filter.

[beenthere]

Easier to give answer if you can post a pic of the box.

[aaCharley]

The old furnace used two 20" X 20" filters mounted on an angled frame. I had been using two of the 1" MERV 10 filters and changing monthly. Seemed pretty inexpensive and seemed to get good results. I would rather get the box designed and fabricated to hold two 20" X 20" filters 4" thick if anyone would want to use them. I plan to continue with the 20" X 20" though.

However, my real question is about the size of the hole connecting the return into the bottom box. It would seem that larger should be better from an airflow standpoint. But, I don't know much about how these Variable Speed fans are balanced and what brain system they use to decide when to speed up or down.

[aaCharley]

I'm not camera capable enough to post a picture.

The flat metal box is 10" high X 28" deep X 38" wide. It is used as a base for the furnace to sit upon.

Furnace sits on top of the box on left side. Hole cut in the top of the box (beneath furnace) to match the bottom of the updraft furnace.

Return will join into the right side through the top, same manner as the furnace. Hole was made for the return duct inlet to be 10" X 24". It could be made larger without any problem.

[beenthere]

I thought thats what you had. Making the hole bigger will help, those are not the best return boxes.

[aaCharley]

Can you give me an idea of the design or style of an alternative for that floor box?

Nothing is really plumbed up yet, other than the A/C connections and those were made up to have quite a bit of flexibility to them. Nothing for the furnace exhaust, inlet air, or condensation - now just connected to the old line for the water drain.

[beenthere]

I cut out the side of the furnace for the return. It has stampings where to cut it. Its a much larger opening.

There is another style of return box you can use. Its 18" high though. And it will be teh width and depth of the furnace. Use them for furnaces with 5 ton A/C's. Around here its called a horizontal return box.