Here are some non-professional opinions, so take them for what they're worth.

You can't add a stronger blower since that will upset the balance of your furnace. Not knowing the specifics of your furnace (model number), it produces x amount of heat. The blower (should be)is set to deliver a quantity of air,such that the difference between the return air temp and the air leaving the supply trunk is between specified limits.

By increasing the air flow (stronger blower), the temperature of the air leaving will drop, perhaps to an uncomfortable level. There is also the possibility of condensation forming which could cause corrosion of your heat exchanger.

I cannot comment on what size the new return should be, but more is better. In addition to better heat distribution, the additional return will allow more airflow, downstairs.

By adding the return, the static (return)pressure will drop, so if the system had not been putting out full CFMs, you might see an increase in airflow overall.

I would think the most direct (shortest) route would be better, but cannot comment on the legality of using the fireplace chase. Did you plan to use the chase itself as the 'duct'?

The whistle is caused by the high velocity of return air going through the grill. A second return will lower the velocity and might lessen/eliminate it.

I'm sure some PROs will chime in.

Good luck.

V

Also a non-professional, but I would think the airflow would be better if instead of tapping onto your existing return air run, you added a second run which tied in directly to the furnace return air plenum/box.

Also not a pro.

Ditto and ditto to the above. You can't have too much return. Any added ducting must be properly sealed with mastic and well insulated, especially if it runs through non-conditioned space.

Thanks everyone

to vesubotee - Yes the blower seems fine as is even with more return, but with the chimney option the structure of the chimney enclosure itself would not be changed, but instead get an additional chase added on to it, with the siding only being removed (leaving the sheathing barrier on) where the augmented width would be added, but I'm still concerned about max duct length.

to ryanhughes - In regards to attaching directly (or very near) to the furnace plenum, we were planning to do just that. The plenum appears to have an end sealed that could be removed or replaced with another duct outlet (but maybe not, due to the blower orientation). Currently the return duct enters the furnace right in the top end directly above the blower with a 16" diameter duct. I was looking at modifying this plenum to accommodate two returns, maybe adding plenum directly to the top with two duct outlets instead of the one it has now.

I'm just wondering what a typical long return branch length can be in the two story homes and what the trade-offs are in keeping the length short verses taking the longer route.

I would not enclose any sort of HVAC duct in the same chase as a flue. It is just too risky. If the flue developed any kind of leak and you did not have the return perfectly sealed--which is likely, even pros don't always get it right--you could draw flue gases into your furnace plenum and distribute them throughout your house.

Find out if what you are planning would meet code in new construction. If it won't, don't do it.

I'm not sure I got the 'message' across, but we were not thinking of entering the chimney chase - at all.

The chimney enclosure outside presents a location on which to merely add another corner on the outside of it (making sure the original chimney enclosure remains sealed and intact), and thus, having a new adjacent chase totally empty in which to possibly run an air return duct down if choosing that option. But, still I'm not sure code would even allow that.

While a friend of mine is pushing that option, I don't like this idea myself personally because of the length involved going to outside end of the house to go down, and then going back toward the center areas of the house in the crawlspace. That would add about 14 feet to the overall run. I'm just not sure what effect that would have on the benefits.

However, it has come to mind another idea I can try, if I'm really curious about the length or effects of it in a second return run. I can simulate the second return with minimal work and try it out. Simply connecting a second return duct to my modified furnace plenum (when I get that done), I will then run the full length of the second duct I plan to use, out the attic door and down the stairs and to a chosen location, simulating the turns and bends all inclusive of the enclosed run that I would wish to make. Then I would put a filter on the end of it (temporary mount), and run my furnace system in various tests. I have an anemometer coming in the mail, so once it gets here I can use it to test the flow pressure at each main branch, original upstairs, and then my test duct downstairs. I will also be able to feel the actual return pressures for myself and compare them. From this type of simulation I should be able to at least ascertain whether or not such a return system can be adjusted for balance, and what the net benefits might be after using such a length.

So that's what I plan on trying, without actually committing to any permanent options or modifying any structure yet. As always I still appreciate any thoughts and would like a pro to chime in anytime. Thanks again everyone.

Everything being equal, a long duct will return less air than a short duct. Since this will be an additional return duct, you should have excess total return capacity. The air velocity will be pretty low, and there should be a minimal difference between a long and short return.

I suggesting running the duct to the location you want to return air from and not worry about the length.

The new exterior chase could be a problem unless it was done just right. You would be running your duct in unconditioned space, which is not recommended. It would have to be very well sealed and insulated, especially since it would not be accessible for inspection or repair.