We have rollout switches and temperature sensors to prevent fires from spreading beyond the furnace. What about exhaust gasses? Almost every heating fuel we use produces deadly gasses. CO2, NO2, and more. If these gasses escape your furnace, you’re either going to leave the building or be killed by the gasses. It’s essential that we keep these gasses away from you. Your furnace has some very interesting ways of doing this and detecting a deadly hardware failure.
The Draft Inducer
Older furnaces rely on physics and chemistry to get the CO2 out. As your furnace runs, it creates hot gasses. The hotter a gas is, the higher it rises in the atmosphere. As long as the exhaust is hot, it should rise up into your chimney or exhaust flue and be vented safely outside away from people. This isn’t entirely ideal, the exhaust is limited by the temperature difference, slowing it down. It’s possible for wind and other atmospheric conditions to disrupt the exhaust flow. There’s no mechanism to detect and respond to a blocked exhaust path.
Modern furnaces don’t let the hot gasses just meander out the chimney. We use Draft Inducers, essentially blowers built just for furnace exhaust. These little blowers kick on a minute before ignition to vent the combustion chamber and usually remains on until after the furnace has shutdown. On the surface, this just makes the furnace burn cleaner. If there’s less carbon dioxide in the combustion chamber, there can be more oxygen. More oxygen means better burning with less fuel.
A nice bonus of the draft inducer is that it forces the exhaust gasses away. We don’t need to rely on temperature differences or much of anything except a clean flue pipe. This forcing out gives us something we can measure. It changes the pressure.
Of course, the draft inducer could fail, but the massive pressure differences it creates lets us detect and shutdown in the event of failure. When it’s working correctly, the Draft Inducer will create a lower pressure inside the furnace. As it runs, it sucks exhaust out, creating a lower pressure for more air to be pulled in. With a sensor, we can measure this pressure change.
In the event of a blocked exhaust way or failed inducer, this pressure sensor can signal for the whole system to shutdown. It might get a bit cool in the office, but you’re not going to get poisoned. Your HVAC Specialist can then take a look and fix the problem before it becomes a disaster.
This sort of approach doesn’t quite work on older furnaces without a draft inducer. On those furnaces, the pressure is essentially always going to increase in the combustion chamber. We can’t tell if things are working or not by their pressure, unless we’re altering the pressure in and can see a huge difference on system failure.