With heating season upon us in Ottawa, Prescott-Russell & SD&G having a better understanding of how a furnace works will help prevent unexpected Furnace Repairs.
A lot of home owners in our service area like to be able to do a lot of repairs themselves and having an understanding of how a furnace works will help with that.
Here's the basic sequence of operation for a high-efficiency furnace:
*Before start-up*
1. Thermostat calls for heat: When the temperature in the room(ambient temperature) is lower than the number you set(setpoint), the thermostat sends an electrical signal to the furnace control board to begin start up operation.
*Furnace start-up*
2. Inducer motor start-up: The inducer motor (also known as the ventor motor) starts, creating a negative pressure in the furnace. This creates a positive pressure through the plastic venting pipes and blows to the outside. You've probably seen the white or black(older venting system) sticking outside your home.
*SAFETY CHECK*: The pressure switch will confirm there is properly flow in the exhaust system, and allow the next step to happen.
3. Gas valve opening: The gas valve has a big magnet in it, that will 'energize'; opening the gas valve and allowing gas to flow to the burners.
4. Igniter activation: The igniter gets powered (either a spark or hot surface ignitor) and ignites the gas, creating the flame.
*SAFETY CHECK*: The flame sensor ensures the flame is still burning, and wasn't blown out, which would pour gas into the house. Failure to 'sense' the flame, would cause the furnace to turn off. If all of the furnace's safety switches remain in the 'safe' position, it allows the next steps to happen and warm up your home.
*Main Operation*
1. Heat exchanger warms up: The heat exchanger(HX) absorbs heat from the flame, and once it reaches temperature, the board tells the blower fan to start. 2.
Blower motor activation: The blower motor begins to circulate warm air through the house via the ductwork.
*SAFETY CHECK*: The overlimit switch ensures the heat exchanger doesn't overheat. If the blower motor dies, or there's obstructions(like a plugged filter), it would trip this safety and turn off the furnace.
4. Exhaust venting: The inducer motor continues to push the exhaust gases outside through the venting system. The white pipes are called System 636. It's the only approved venting system for high efficient furnaces in Canada.
*Shutdown Sequence*
1. Thermostat satisfaction: once the temperature in the room reaches the setpoint, the thermostat removes the signal to the furnace control board, which shuts off the furnace.
2. Gas valve closure: The gas valve closes, shutting off gas supply.
3. Inducer motor shutdown: The inducer motor purges for an extra 30 seconds to a minute; ensuring all harmful gases are out of the home; and then turns off.
4. Blower motor shutdown: The blower motor runs for a bit longer(typically 1-2 minutes) to ensure all warm air is moved around the home.
That is the basics behind how a furnace works.
Join us next time when we dive into this deeper and start to discuss troubleshooting of these furnaces.
