This is a short video taken by one of our service technicians performing a routine annual furnace service for one of our customers. The furnace appeared to be operating normally, however, as we tested each control component and safety device individually, a very odd but important issue was discovered and it was determined that a furnace repair was required.
All mid and high efficiency furnaces have a safety device that tells the furnace circuit board that the induced draft motor is operating normally. This device is referred to by different names - proving switch, pressure switch, pressure differential switch, etc. When the induced draft motor is not operating within the designed parameters, this switch will not allow the burners to fire.
In this video, although its difficult to see everything clearly, you are able to see that the tubing to this switch has been disconnected by the service person, and therefore the burners should not be able to fire - yet they do. This is a concern that needs to be addressed immediately.
This point stresses the importance of having factory trained personnel inspect your heating equipment, as they know how your furnace should operate and can find the 'hidden' problems before they become a health or safety issue.