The most common reason why your heat furnace was blowing old air lies with the problem with your flame sensor.
1. Flame sensor looks like the picture below:
You should open up your heater furnace front metal and you should see something like this picture below:
See the white wire with the pink tip connector? You need to unscrew that flame sensor and using a semi rough Scotch Brite Scrubber to clean out the carbon dust on the long metal tip of flame sensor.
After cleaning the metal portion of the flame sensor, screw the parts back onto the metal plate and test to see if your heat furnace works or not. If it does not work, then this flame sensor may need to be replaced with a new parts. You can order the parts from Amazon.ca.
2. Your heat furnace air filter may be dirty and needs to be replaced with a new one. Usually heat filter can last for a month or two and needs to be replaced, if you don't want your heat furnace to work extra hard to get air circulation going through.
3. Check to see if all the rubber pipes inside the heat furnace is intact and that none of the rubber pipes had slipped or popped out on its own. In my case, i found a rubber pipes had slipped out on its own due to the regular vibration of the heat furnace. I reattached the rubber pipes and my heat furnace was working fine after that.
4. Sometimes you may need to change the position of the flame sensor to make it work.
Example: if you look at the picture above, the flame sensor is located at the left position. If it doesn't work, you can relocate your flame sensor to the right side of the metal frame if there is a hole for it to be screwed onto. For my heat furnace, i noticed that the flames on the leftmost position was starting at a very late stage and the flame sensor was sensing no flames and shutting the heat furnace right away. So i placed the flame sensor onto the right side position where the flames starts first. This way, the flame sensor will sense the flames first and will not shut down the heat furnace unit.
Flame sensor is a safety feature created by the manufacturer to prevent the unburned natural gas from entering the house vents throughout the house. Unburned natural gas can be potentially deadly if inhaled in large quantity. When the Flame sensor senses no flames present, then it shuts down the whole heat furnace unit.