Troubleshooting Gas Furnace
Troubleshooting Gas Furnace
For the furnace to start blowing warm air into your house, the operation starts at the thermostat and goes all the way to the blower, turning on, forcing air into the rooms of your house.
First, we want to give a fair warning to anyone reading this. This article is for educational purposes. Red Deer Heating and AC do not recommend anyone other than a professional to start opening the furnace up and trying to diagnose the failure going on with the system.
There are high and low voltages which can shock a person. There are also lots of moving parts which can severely damage body parts, namely hands and fingers. The furnace also produces hot surfaces within the furnace compartments and around the housing which can cause severe burns.
There is an actual flame being produced by the ignition and startup of a gas furnace which can cause severe burns and damage to a person or property.
But before anyone can troubleshoot or actual check their furnace, they must first understand how the furnace works to better see where along the process the problem had occurred.
The Process
The Thermostat
When your house reaches a point where the heat needs to come on to keep you comfortable, a series of components of the furnace work in a certain order to produce the heat. The thermostat is the first part in the sequence which engages to make the furnace work.
There’s 24-volt power at the R terminal of the stat all ready. Within the workings of the thermostat, the 24 Volts closes a switch at the W terminal. The signal is sent to the control board back at the furnace.
The Control Board
The control board is a printed circuit board which has various switches, resistors and terminals acting as the quarterback of the heating system. It calls the plays as they need to happen. The control board has a terminal block, with screws on it, to hold down a set of thin low voltage wires coming from the thermostat.
Typically, the colours of these wires are red, yellow, white, green and blue.
- Red – Power
- Yellow – Cooling
- White – Heat
- Green – Blower Motor
- Blue – Common Terminal
Note the wire colours don’t matter here. They’re still copper on the inside of the Sheathing. So, if we use a brown wire for R at the control board, then brown needs to be hooked up to R at the thermostat.
Once the control board receives the signal from the thermostat to turn the heat on, it tells the inducer motor to come on. The inducer motor is a major component which removes the carbon monoxide from the flame of the gas furnace.
It draws the spent gasses into the metal or PVC flue pipe, which transfers those fumes from the furnace to the atmosphere through the roofline. You may have seen the metal pipe sticking out of the roof on top of your house in the winter and exhausting steam into the air.
The Pressure Switch
The pressure switch is a safety device which proves the inducer motor is on and doing its job properly. If it’s not, the sequence shuts down and retries again. This pressure switch is measuring the suction the inducer motor is producing and sends a signal back to the board, letting it know the start-up is working so far.
Meanwhile, other low voltages, safety switches are sending a signal of all-clear back to the board. There are a couple of roll-out switches and a high-temperature limit switch which have to prove to the control board about all is well there too.
The wires leading to the roll-out, high limit and pressure switches are usually all wired in the same series circuit with each other as a safety control. If any of these safety switches sense anything wrong with the start-up of the heating system, the sequence stops and retries again.
Starting the Ignition
There are three components engaged to light the flame and prove that it’s lit. When the pressure switches and other safeties tell the control board all is well, the board starts the ignition sequence.
First, the board sends a signal to the ignitor. This could be a hot surface ignitor, which glows orange or a spark ignitor which produces an arc between two metal forks and lasts for several seconds.
Whether the sparks or ignitor glows or not, 24 volts is sent to the gas valve which opens the diaphragm inside of it. It opens allowing natural or propane gas to flow through the metal burner assembly.
The gas now flowing through multiple orifices in the burner, assembly reaches the ignitor, which causes a flame to ignite and burn in a controlled fashion straight into the firebox or heat exchanger.
For this article, we’ll call it the heat exchanger. Crossover channels within the burner assembly allow the gas to flow from the first burner to the last one where the flame pours over a thin metal safety rod called the flame sensor.
The flame meeting the rod creates a millivolt DC signal to the control board which allows the gas valve to remain open. If no flame is being sensed, it means gas is flowing uncontrolled throughout the furnace cabinet, which is not good.
The Heat Exchanger
A delay occurs at this point to allow the heat exchanger to warm up so cold air isn’t sent through the ducts and into the air. The heat exchanger is a hollow metal box with individual chambers. The flame pours into each chamber, warming the metal to an extreme temperature.
Once hot enough, the air flowing over and around the metal box warms quickly from room temperature to about 100 to 140 degrees. The temperature is set by the manufacturer and must be closely adhered to, to keep the system operating safely and to proper specs.
Distributing the Warm Air
After this delay is finished, the blower motor starts up and sends forced room temperature air over the metal heat exchanger at the correct speed.
If the air is sent over too fast, the air entering the room won’t be warm enough. If it’s too slow of air or not enough air, the system will get too hot. And when the system gets too hot it means the high-temperature limit which is mentioned earlier, will open telling the control board something’s not right.
The blower motor has to be dialled in just right.
Troubleshooting Tips
Here are some things which can happen when the furnace isn’t starting up correctly. These next troubleshooting tips are not all-inclusive and are not to be taken as scripture which what is going with the furnace you’re working on is the problem. These are general problems only.
No power to the board.
If the unit is plugged in correctly and the breaker at the main panel is in the on position, there should be power to the furnace control board. There is also a transformer which can go bad between the outlet and the control board. These parts can and do fail regularly.
The board with proper power can send the high and low voltage signals it needs to to be the quarterback and run the system.
The Thermostat is working but not the inducer motor.
Low Voltage power is sent from the control board to the R terminal at the thermostat. Assuming you have 24 volts there, the thermostat closes the W switch, which between now has 24 volts applied to it.
If the 24-volt signal is getting back to the Control board’s W terminal, the control board is supposed to send the high voltage Signal to the inducer motor If voltage is getting to the inducer motor but it doesn’t run, you likely have a bad inducer motor or capacitor for the inducer motor if it has one.
If you’re not getting voltage to the inducer motor from the control board, you have a bad board or faulty wiring connection between the two.
Working control board, thermostat, inducer motor but no ignition.
If the inducer motor is running, the ignitor should start glowing or sparking. The gas valve should open allowing the gas to flow. The gas flame should crossover to the other burners in line and a signal should be received at the flame sensor, telling the board everything is good to go.
As with many components in troubleshooting, if the part is getting power but not operating, it’s likely failed. If it’s not getting power from the control board, it’s likely a bad board.
Working thermostat, inducer motor, ignition, flame Sensor, but no blower motor.
If everything works as it’s supposed to except the blower motor which has not turned on after the flame ignited and even after about a minute or so, something is going on in the blower motor.
If the motor is getting power but not working, the motor or its capacitor may have failed. If the motor is not receiving power from the board, the board is likely bad. Not all blower motors have capacitors either, especially on newer systems made in 2020. or later.
Working thermostat, inducer motor, ignition, flame Sensor, and blower motor but the system shuts down on high limit or roll-out.
Lastly, the blower motor comes on and the system starts heating but after a few minutes or even several minutes the system shuts down. This happens when the high-temperature limit switch opened, causing the system to retry again after the heat, exchanger cools off.
If the chamber housing the heat exchanger gets too hot, the high limit switch will shut the system down. This could be because of a few things:
- The blower speed setting is not correct.
- The air filter could be dirty.
- The ductwork could be too small or even collapsed.
- The evaporator could be clogged with dirt.
All of these have one thing in common: It is not enough air flowing over the heat exchanger which causes the inside temperature of the furnace to go over the recommended setting established by the manufacturer.
Although many things can go wrong with the gas furnace, sometimes in combination with each other, not much else can go wrong unless something in the sequence goes wrong.
Call Us!
If it is your first time troubleshooting a gas furnace, be careful and be safe out there. If you don’t know what you’re doing, just leave it to the experts – call Red Deer Heating and AC and we’ll fix it.