Summer changes
 

It was getting too hot in the back hall and in the low 70s in the basement.

Summer mode:
I've unplugged the attic blower and used the X10 to power the old
attic exhaust fan (it vents to the outdoors).

That old exhaust fan has it's own thermostat (on@120F & off@100).

So it's enabled by the CAI program (via X10), but might not run.
(If it's not hot enough in the attic).
The basement fan will still run whenever it's commanded by the program.

Today isn't very hot, so only the basement blower is running.

Xringer

In ref to fan noise....
I run 6 inch duct fans in my solar heated hot air collectors
My fans are outside but are piped in thru the side of my house and run along under the floor joists about 4 to 5 ft to the registers.

When first installed I ran 6 inch metal tubing insulated on the outside all the way to where the registers were located. When the fans kicked in the registers sounded like a tornado, just nuts so I tried something different.

I removed about 4 to 5ft of metal tubing from the register back toward the collectors and installed flexible 6 inch insulated duct hose in it's place... man what a difference.

99% of the noise went away

Is there a way you could move your fan back into your attic and install the insulated duct hose leading to your register? It may help big time.

Something else I would do is change over to 8 inch fan and ductwork.
Just my opinion.

I don't think the little 4 inch fan is capable of moving enough volume to your basement to accomplish your mission.

Just some thoughts.. I'm not an expert but a larger fan would most likely work less and move a heck of a lot more air.

...at my house:
I recently installed a new Navian Combi hot water Boiler in the laundry room here at our house. The new unit required 1 1/4 inch copper pipe manifold below the boiler and a whole bunch more additional copper piping around the boiler which is radiating tons of heat so our small laundry is now heating up in there pretty good much warmer then the rest of the house.

My plan is to do what you are doing and pump that warm air down to our basement.
I'm going to use an 8 inch fan and controller so it turns on/ off on it's own once installed.
I would love to use a 12v powered fan powered by a single panel solar panel but not sure I have the patience to put all that together. :-)
I have a pool table down there and would really love to be able to shoot pool without having to wear gloves. :-)

Use one of the Gargantuan computer case fans. 15 bucks for an 8 inch, moves somewhere around 100 CFM with a little back pressure. Sooper quiet, ultra low power, but they only produce a millimeter or two of pressure.

The other thing I do a lot is recycle broken dryer motors. They're all rated for 300-350 CFM through a long dryer hose. GE are short, Whirlpool are taller. Straight out a short pipe, they can do over 500 CFM. Made for massive pressure drop on either side (inlet suction or outlet back pressure). I've actually got a GE one rigged to preheat and/or precool my garage. It literally sucks air from the space above the drop ceiling in the basement.

This thread is not as old as me, but it's old!
 

I rarely use the attic fan these days. The basement is nice in the summer.
(Still use the basement fan, if the Radon gets too high).
We keep it dehumidified and the slab is cool enough that's it's comfortable down there. The attic fan could be moved upwards a few feet. Not enough to cut the noise much. (My wife dislikes the woody smell of hot attic air).
As for the winter, the slab is does most of the basement heating (~50F), so we try not to go down there without a jacket.
A small amount of additional heat down there comes from the oil burner runs, which are controlled by Alexa to minimize oil use.

Since my three 200 watt PV panels weren't doing anything, I hooked up that 600 watts to a resistive load to make a little heat. I think it runs about 500w in good sun. 1700 BTUh, that has very little effect on the basement, but it might be keeping the floor near my chair in the dinning room about 1/1024th of a degree warmer.. Maybe placebo effect heating. :)

Anyways, my wife and I are getting really old. I've got RA in my hands and feet so bad that I should be living in SW Texas, instead of north of Boston.
So going down to mess around in my shop or ham station, isn't what I want to do in the winter time.
I looked over all those posts, and found the heating numbers so low, they were depressing.
The only really successful thing about this project is the basement fan is great for removing Radon..

Cheers,
Rich