Useful Information - Safety Tips
How to Tell When Your Furnace Is Not Feeling Well
Scale: Flakes of rust, produced by the by-products of burning gas (carbon dioxide and water vapor). Scale may fall on the burners and impede gas flow. Over time, it can damage your furnace by harboring moisture, thereby fostering rust on a large scale. The solution: Your service technician can take out the burners and clean them. You can clean out excess rust flakes that fall to the bottom of the furnace housing.
Grinding, chattering sounds from relays (signifying electrical problems), a burner that huffs and puffs, banging (delayed ignition), or clunking and bumping (cracked belt passing over pulleys)? The solution: A good rule of thumb: if it's an unusual noise, it's a problem. Call your service technician. 
Carbon Monoxide: It's colorless, odourless and tasteless, and it can kill you if it's concentrated enough. It is caused by a lack of oxygen or a disruption of the fuel-burning process. The solutions: Your furnace breathes, just like you. Provide adequate ventilation to the unit and consider installing a fresh-air (combustion) intake. Use carbon monoxide detectors, combined with routine maintenance checks by qualified service technicians (mark them on your calendar).
Yellow Flame: That flame should be sharp and blue, clean and stable, burning as purely as possible. A yellow flame indicates dirt in the burner, which prevents it from mixing the gas and air properly. The solution: Call your technician to thoroughly test the system and clean it.
Dusty Smell: You turn up the thermostat and within minutes, your home is filled with a dry, dusty smell. The solutions:
- Don't worry; it's just burning the dust out of the combustion chamber. Change your filter
- If it's a constant odor, call your technician.
- If it smells like gas, call your utility company or the fire department and stay outside until no danger has been confirmed.
Backdrafting/Negative Pressure: Negative pressure results when you take air out of the house by using oxygen faster than air can enter the house. Backdrafting is a natural consequence of negative pressure; air rushes into the house through the chimney, effectively choking off the natural process of venting. The solution: Run a combustible air duct to the unit from the outside adapted from an article by David Carlson, Lennox Industries