This section will help you understand the basics of how a modern furnace works. What needs to be emphasized here is we are a Bryant dealer, therefore we understand our products thoroughly, and all other products may differ.
The easiest way to help you understand furnace operation, is to go through the process of operation, step by step. We’ll describe each step as we go. These products are designed with many safety devices and other controls to aid in the development of very highly efficient heating units. These are very different from your grandparent’s furnaces 30 years ago.
Let’s assume you have:
A 20 year old Bryant furnace or newer (almost all other furnace manufacturers will be similar). This article will focus on forced air. Years ago some furnaces moved air with simple gravity. Hot air rises and cold air sinks.
Gas. Natural or L.P. We will not discuss oil, solar, or electric furnaces.
Electricity. Needs power to operate unit, 120 Volts
Working thermostat on the wall. It’s not uncommon for homeowners today to purchase and install their own thermostat. With the complexities of today’s furnaces just be sure you’re getting a compatible thermostat to match your furnace. Be aware of the terms; single stage, multi stage, variable stage or heat pumps.
Onto sequence of operation…
The first step needed to get the furnace running would be a call for heat. Make sure your thermostat has fresh batteries, (if recently changed and if your thermostat has one, make sure the “reset” button has been reset). Since there are many types of thermostats available, it’s impossible for us to get into specifics here. It bears mentioning that some thermostats have a reset button that needs to be found and pushed to reset. It may be easiest to look your model up online if you lost the operation manual.
Inducer motor. Usually the first element to show signs that something is happening. This device’s job is to get air flowing and to feed the furnace with oxygen for the fire that will raise the temperature to heat the house. Usually today’s furnaces have 2 plastic pipes routed to the outside.These are usually positioned through the sidewall, though going through the roof isn’t unheard of.
Once the fan speed reaches it designed speed (which will take approximately 90 seconds) the pressure switch should send a signal indicating that there is no obstruction. The pressure switch is designed to detect any obstructions in the vent pipes noted above. If there are any debris in the pipes (leaves, bee nests, marbles, toy cars, or snow), it will not allow the sequence to continue.
Once the pressure switch is proving to be functioning correctly, the pilot light is energized. Old style thermocouples that had an actual flame you could see are no longer used. Today’s pilot lights are commonly referred to as Hot Surface Igniters. Because that’s exactly what they are. A hot surface. These items are designed to get red hot, and to eventually ignite the gas. If your igniter is in working order it will get red hot for a specifically designated amount of time (30 seconds, for example) then the gas valve will open. Immediately following this segment, the burners should all show a blue colored flame at each burner. Very shortly after this, a flame sensor comes into play.
The flame sensor’s job is two-fold. Its first priority is to prove the furnace has flame. It’s 2nd job, after proving flame, is to send a signal to turn off the Hot Surface Igniter.
The next element in the process is the main blower within your furnace. This blower actually sends the heat throughout your house. The blowing process happens on a timed delay. Before the blower is activated, the furnace must get up to operating temperature. It usually takes about 90 seconds to reach that temperature. Once the furnace reaches its designated internal temperature or has met its time delay, the blower should start moving air throughout the space being conditioned.
Failure of any of these components listed above can shut down your furnace, or not allow it to start at all. Additionally the furnace may shut down during a heating cycle. For instance, you may watch your furnace go through the entire start-up routine, and all will seem to be going well…. just to have the furnace shut down before the house is completely heated up. This is usually caused by something as simple as a dirty air filter.
A convenient feature of the Bryant furnace is a fault code system. If your heater would experience an error, the unit will send out a code telling the homeowner or service technician where to start looking for problems. Here for instance the display could show a ‘pressure switch code’. A homeowner probably cannot diagnose a bad pressure switch, but as described above, it may not be a bad switch. The switch may be doing exactly what it was designed to do… such as finding little Johnny’s matchbox toy car he decided needed to be parked in the heating vent.