Older furnaces relied on physics to help eliminate harmful combustion products. These furnaces couldn't capture most of the heat from the combustion process, so their exhaust was often exceptionally hot. As a result, exhaust gases were hot and much less dense, allowing them to rise through the furnace flue and exit the home naturally.
Modern furnaces are far more efficient and can extract more heat than older models. These models rely on draft inducers to ensure that exhaust gases safely exit your home and clear the combustion chamber of old exhaust gases to improve efficiency. Without creating this negative pressure, your furnace won't be able to operate safely and will not turn on.
What Are the Symptoms of a Draft Pressure Issue?
Any furnace with a draft inducer will have at least one pressure switch. This pressure switch operates during your furnace's startup sequence, and its job is to prove the presence of negative pressure or "draft." Depending on the type of pressure switch your furnace uses, the switch will either open or close when it detects the pressure that it expects.
Since draft pressure is an essential safety element, your furnace won't turn on without the correct pressure. The draft inducer typically turns on after your furnace completes its initial startup safety check. The control board then waits for the pressure switch to indicate that everything is working as it should before moving on to ignition.
In practice, this startup sequence means that any problem with draft pressure will stop your furnace from running. You may hear the furnace turn on or the draft inducer start running, but the burners will not ignite. The furnace may shut off and try again. Depending on your particular model, there may also be warning lights or error codes indicating a pressure issue.
What's Causing Your Draft Pressure Problem?
Your furnace's draft exhaust system includes the draft inducer, pressure switch, and exhaust flue. An issue with any one of these parts can cause your furnace to shut down and refuse to work. The most obvious cause is a draft inducer that isn't working or that can't produce sufficient pressure, but the problem can sometimes be trickier to locate.
For example, a faulty pressure switch will also produce similar results. Pressure switches typically fail in a "safe" way, meaning that a bad switch will remain in its default open or closed state, indicating a draft pressure problem to the control board. A clogged exhaust flue is another problem. Debris, rodents, or other objects in the flue will prevent the inducer from producing adequate pressure.
Exhaust problems can potentially release harmful gases into your home, so you shouldn't try to override the safety switch or otherwise force your furnace to run. If your furnace won't turn on because of a draft pressure issue, it's best to call a technician as soon as possible to schedule furnace repairs.