FC Furnace Installation Techniques 2 — Questions and Answers
Question 1: What is the minimum clearance required between a furnace and combustible materials on the sides, unless the manufacturer specifies otherwise?
- 1 inch (Correct answer)
- 6 inches
- 12 inches
- 18 inches
Correct answer: 1 inch
Most furnace manufacturers require at least 1 inch of clearance from combustible materials on the sides, but always defer to the installation manual for model-specific requirements.
Question 2: When installing a furnace in a closet, which code requirement applies to the combustion air supply?
- No special requirement if the closet is large enough
- Two openings must be provided — one near the top and one near the bottom (Correct answer)
- A single large opening at the bottom is sufficient
- Combustion air must be ducted from outdoors only
Correct answer: Two openings must be provided — one near the top and one near the bottom
NFPA 54 requires two permanent openings in confined spaces — one within 12 inches of the top and one within 12 inches of the bottom — to supply adequate combustion air.
Question 3: During a gas furnace installation, the technician discovers the supply gas pressure is 0.25 in. W.C. at the manifold under full load. What action is required?
- No action; this is within normal range
- Increase the gas valve regulator setting
- Contact the gas utility to increase line pressure (Correct answer)
- Reduce the number of burners firing simultaneously
Correct answer: Contact the gas utility to increase line pressure
Natural gas manifold pressure should typically be 3.5 in. W.C.; 0.25 in. W.C. indicates severely insufficient supply pressure requiring the gas utility to investigate the supply line.
Question 4: What is the purpose of a furnace drip leg (dirt pocket) installed on the gas supply line?
- To regulate gas pressure before the valve
- To trap condensate and debris before they reach the gas valve (Correct answer)
- To prevent gas backflow during burner shutdown
- To measure gas consumption for billing purposes
Correct answer: To trap condensate and debris before they reach the gas valve
A drip leg is a short capped vertical drop installed upstream of the gas valve to catch pipe scale, moisture, and debris that could damage the valve.
Question 5: A high-efficiency condensing furnace is being installed. Where must the condensate drain line terminate?
- Into the flue vent pipe
- Into a sanitary drain or approved neutralizer (Correct answer)
- Into the return air plenum
- Outside through the exterior wall
Correct answer: Into a sanitary drain or approved neutralizer
Condensate from high-efficiency furnaces must drain into a sanitary drain; if the condensate is acidic (pH < 5), local codes may require a neutralizer before it enters the drain.
Question 6: Which type of pipe material is NOT acceptable for a natural gas supply line inside a residential structure?
- Black steel pipe
- CSST (corrugated stainless steel tubing)
- Copper tubing (Correct answer)
- Schedule 40 galvanized steel pipe
Correct answer: Copper tubing
Copper tubing is prohibited for natural gas in most US jurisdictions because natural gas can contain hydrogen sulfide that corrodes copper; copper is acceptable for LP gas only in some codes.
Question 7: When sizing a return air duct for a new furnace installation, which CFM rule-of-thumb is commonly used?
- 100 CFM per ton of cooling capacity
- 400 CFM per ton of cooling capacity (Correct answer)
- 1 CFM per square foot of conditioned space
- 2 CFM per BTU of furnace output
Correct answer: 400 CFM per ton of cooling capacity
The HVAC industry standard is approximately 400 CFM per ton of cooling capacity, which also ensures adequate airflow across the heat exchanger during heating.
FC Furnace Installation Techniques 2
What is the minimum clearance required between a furnace and combustible materials on the sides, unless the manufacturer specifies otherwise?