This Morning Nadia said that the first floor is so cold and we realized later that the Furnace in the basement is not working.
There's a great deal of Youtube video tutorials (This "Word of Advice TV" youtuber is very technical and detail) on Furnace. But since the red LED indicator on the furnace (after finding replacing the very old filter still didn't fix the problem, and opening the furnace cover), tells us (according to the error code chart in the back of the cover) that 3 flashes means Pressure Switch Stuck Open, I found a direct video on how to deal with the problem:
Basically just unscrew the Draft Inducer Motor, and clear the vent between the motor and the Pressure Switch that is connected with a rubber piping which can easily be removed. I must have jiggled the clog off as I took off the device because by the time I was ready to clean it, there didn't appear to be anything for me to clean, pipe was clear, the vent was clear as well, so I just briefly poked (paper clip) and gently blew those orifices. After that, hooked everything back and hold the safety switch where the cover would have held it when closed, to do a test run by seeing the ignitor lit up, followed by the burner firing. All good, after putting the cover back.