The filter size is determined by the amount of air flow which must pass through it. You have to determine when the air flow is the highest between the furnace and AC. The formula I use is for a filter rated for a velocity of 300FPM:

air flow (CFM) / 2.08 CFM/sq. in. = filter area (sq. inches)

Let's assume you have a 2 ton AC which would normally be using 800CFM of air. The filter area calculation would be:

800CFM /2.08 CFM/sq. in.= 385 sq. in.

A 16 X 20 filter (320 sq. in.) would be too small. You would need a 16 X 25 filter.

The 3M merv 11 filter is very restrictive. This may be part of your problem. Check the air flow with the filter removed.

What is your location? A 45% indoor humidity level when it is 10 degrees outside seems high. Does the basement have a moisture issue?

Thank you for the reply.

I can see on the furnace body where it would accept a larger duct size than being used. Metal was cut and installed and sealed with silicone, to fit up to the current duct work.

I am located in southwest, central, Indiana, humidity is an unwelcome neighbor here. As far as the basement, I do not have water laying anywhere, but I have an outside parameter drain that comes into the basement (built into the floor) and dumps to a sump pump for removal. (The entire basement is unpainted, bare block walls, no cracks.) Water there is dependent on rainfall. Humidity down there is always high 60s to low 70% range. And I know that's not good news. I am wondering if that's wood rot starting or something that I am seeing floating in my air all the time. I do have a 70 pint dehumidifier down there. I have those small weather stations upstairs and down to watch the humidity.

On my old fuel oil furnace, a different installer put an a/c unit on top of it. we ran it 10 years. It would pull the humidity down to 50% all day long when the air was running.

Is the fan speed relevant to any of this? Where I mentioned it was slowed down.

I was told this unit is supposed to have a variable speed fan, that depending on demand for heat and maybe A/C when to use a higher speed. I can try and dig up, my paper work for the sizes of the a-coil, condenser, and BTU rating of the furnace. Or look for model numbers. I can remember the a-coil and condenser were not of the same tonnage, he said it would make something better, maybe dehumidify, if so, that didn't work.

So for now it looks like I do have too small of filter. I will come back hopefully with some numbers later this evening. I will also take the filter all the way out and smoke check the cold air return again.

Thanks