Thanks again for the quick reply, Freon
In answer to your question there are 17 registers throughout the house. 7 on the topmost floor, 6 mid, and 4 lower (as it is a split-level home there is a lower basement as well where the furnace is located).
The make of the furnace is IDG. Input on the unit is listed as 90 MBTUH, output 68 MBTUH
I also says 115 V 60 cycles and 12 amps
Just as a quick test, I used aluminum duct tape to seal up that open heat duct, which comes out at the back of the unit pointing down between the unit and the unfinished framework of the wall behind. All it seems to do is make that area very hot. The furnace gives off enough heat in operation without needing it I think so I don't think the pipes are in danger of freezing. Even sealed up the basement feels like the warmest room in the house and it's definitely above freezing. With that duct sealed I think the registers all through the house are marginally hotter and stronger though still weakest on two registers on the upper floor furthest from the furnace. Those never worked well since we've been here.
Anyway, thanks again and let me know about the power output.