Order of Operations
Once the thermostat calls for heat what happens next
The inducer fan motor is energized
What causes rollout of burner flames?
Lack of secondary air or stopped up vent
Normal CO levels within a home should be below how many ppm?
Normal CO levels within a home are between 1 to 9 ppm.
What four different readings would an electronic combustion analysis test normally show?
Efficiency, oxygen level, carbon dioxide level, and carbon monoxide level.
What happens if the pilot fails to light during a trial ignition?
If the pilot does not light or if the pilot flame current is too low and not steady, the module will not energize the main valve and the burner will not light.
What happens after the inducer draft motor starts
The draft pressure switch closes
What can cause the blower to come back on for a short time after cycling off?
Blower not moving enough air or fan off setting too high
What does a high percentage of oxygen in the flue gas indicate?
This would indicate too much excess air for combustion
How does the presence of 7 to 9 percent CO2 in flue gas reflect heating unit performance?
It indicates that the unit is running efficiently.
An open door switch on a gas furnace will cause:
A complete system shutdown—no system operation at all.
When does the gas valve open on a HSI system?
right after the HSI is energized
What are some possible causes of delayed ignition?
Gas pressure too low, primary air too high, delayed valve opening
What is an indication of too much primary air?
Delayed ignition and noisy flames that lift off the burner.
The normal oxygen and carbon dioxide percentages in the flue gas of a properly operating gas furnace are approximately:
7.5 percent.
On a gas furnace service call a technician should never ________ before checking the microprocessor's LED indicator.
cycle power or remove the furnace door
What happens right after the gas valve is opened
Flame is rectified ( sensed)
What can cause a soot accumulation on the heat exchanger?
Too little air, too much gas, or flames touching heat exchanger
What is an indication of too little primary air?
The flames will be lazy and yellow, similar to having too much gas. If the primary air is just not open enough, the flames will be blue at the bottom and yellow at the tips.
What measurements are required to check combustion efficiency
Ambient temperature, stack temperature, flue gas O2%, and flue gas CO%
Which component failure could keep the fan from operating during the heating cycle on a traditional standing pilot gas furnace?
A defective fan switch
What happens when if the flame is not recitified (sensed) and how much time is given by the furnace to sense a flame?
Gas valve is deenergized. Less than 10 sec
On some furnaces little pops and pings can be heard before the blower comes on. What causes this noise?
Expansion and contraction of the heat exchanger as it heats up
What happens if there is improper venting of the furnace?
The combustion products will build up in the vent and the flames will start to come out of the combustion chamber.
What type of gas production is a definite indication of incomplete combustion?
Carbon Monoxide
When is the induced draft blower energized on an induced-draft furnace that uses hot surface ignition
On the initial call for heat