FC Furnace Installation Techniques 3 — Questions and Answers
Question 1: A furnace is installed in an attic with summer ambient temperatures reaching 130°F. What is the primary concern for the technician?
- Excessive heat may trip the high-limit switch during normal operation (Correct answer)
- The flue pipe may melt
- Combustion air oxygen content decreases at high temperatures
- The gas valve may open prematurely
Correct answer: Excessive heat may trip the high-limit switch during normal operation
Extreme ambient temperatures can cause the return air entering the furnace to be very warm, reducing the temperature differential and potentially tripping the high-limit safety switch.
Question 2: What does the term 'static pressure' refer to in a furnace installation context?
- The gas inlet pressure at the appliance
- The resistance to airflow in the duct system measured in inches of water column (Correct answer)
- The pressure in the combustion chamber during firing
- The draft pressure in the flue vent
Correct answer: The resistance to airflow in the duct system measured in inches of water column
Static pressure is the resistance the blower must overcome to move air through the duct system, and is measured in inches of water column (in. W.C.).
Question 3: When installing a Category IV (positive pressure, condensing) vent system, which vent pipe material is required?
- Type B double-wall vent
- Single-wall galvanized steel
- PVC or CPVC plastic pipe (Correct answer)
- Stainless steel Type 316L liner
Correct answer: PVC or CPVC plastic pipe
Category IV appliances exhaust at positive pressure with condensing flue gases, requiring corrosion-resistant plastic pipe such as PVC or CPVC as specified by the manufacturer.
Question 4: A technician is performing a combustion analysis and finds CO in the flue gas at 1,200 ppm air-free. What does this indicate?
- Normal operation for a natural gas furnace
- Incomplete combustion, likely from insufficient primary air or a cracked heat exchanger (Correct answer)
- Excess secondary air entering the flue
- High efficiency condensing operation
Correct answer: Incomplete combustion, likely from insufficient primary air or a cracked heat exchanger
CO above 400 ppm air-free in flue gas indicates incomplete combustion, which can stem from improper burner air adjustment, a cracked heat exchanger, or blocked flue.
Question 5: What is the correct sequence for lighting a pilot on a standing pilot gas furnace during a new installation startup?
- Turn gas on, light pilot, wait 30 seconds, turn to 'On' position
- Turn to 'Pilot', depress operator, light pilot, hold 30-60 seconds, release, turn to 'On' (Correct answer)
- Turn to 'On', light main burner directly, then adjust pilot
- Depress operator, turn to 'On', light pilot simultaneously
Correct answer: Turn to 'Pilot', depress operator, light pilot, hold 30-60 seconds, release, turn to 'On'
The correct procedure is to set the valve to Pilot, depress the safety operator to allow gas flow, ignite the pilot, hold the operator down for 30-60 seconds to heat the thermocouple, then release and move to On.
Question 6: Which installation practice is required when running CSST (corrugated stainless steel tubing) through a wall cavity?
- CSST must be sleeved in a metallic conduit through walls (Correct answer)
- CSST can pass through walls without protection if the wall is non-load-bearing
- CSST requires a plastic grommet at each wall penetration only
- CSST is prohibited from running inside wall cavities under any circumstance
Correct answer: CSST must be sleeved in a metallic conduit through walls
CSST must be protected by a metallic sleeve or conduit when passing through studs or walls to prevent damage from nail or screw penetration.
Question 7: What is the recommended maximum number of 90-degree elbows allowed in a standard PVC vent run for a condensing furnace before derating the allowable length?
- 2
- 3 (Correct answer)
- 4
- 5
Correct answer: 3
Most condensing furnace manufacturers allow up to 3 elbows before requiring the maximum vent length to be derated, as each elbow adds equivalent friction length.
FC Furnace Installation Techniques 3
A furnace is installed in an attic with summer ambient temperatures reaching 130°F.
What is the primary concern for the technician?