Upflow and Downflow furnace schematics (C) Carson Dunlop Associates Heating Furnace, Gas: Defects List & Home Inspection Education

  • DEFECTS LIST - HEAT FURNACE GAS - CONTENTS: Gas fired heating furnace inspection points, defects. Lists of important defects for residential buildings. What does a home inspector need to know? Home inspection training and education curriculum recommendations
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Gas furnace inspection curriculum& defects list.

This article lists significant gas fired heating furnace defects, definitions, and home inspection education topics.

The building defects and inspection points listed in these articles also guide homeowners and home buyers to building areas that merit careful attention and often point areas of safety concern or important maintenance and repair tasks. Page top illustration of a gas fired furnace courtesy of Carson Dunlop Associates.

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Home Inspection Education Curriculum - Heating, Gas Furnaces

Gas fired heating boiler burner soot means unsafe system (C) Daniel Friedman4.    HEATING, Gas Fired Furnaces Inspection Recommendations & Typical Defects Lists

4.1   Gas Fired Warm (or Forced air?  Here “warm air” is a gravity furnace) Air Furnaces

4.1.1 Knowledge Base for inspecting gas fired heating furnaces

1.  Describe the function of residential heating systems.

2.  Describe the common heating fuels (gas, oil, propane, electricity, wood).States GAS above ??

3.  Describe the combustion process with reference to the fire triangle, combustion air, ignition, and venting.

4.    Describe three methods of heat transfer(radiation, convection, conduction) and how each apply to residential heating systems.

5. Describe the common visually detectable defects in gas fired heating systems and identify safety hazards   

6. Describe the common types of heating equipment (furnaces, hot water boilers, steam boilers, space heating, radiant heating).

7. List the materials, function and location for the components of a gas fired  forced air furnace including

  • fuel delivery system,
  • burner,
  • gas valve,
  • pilot light,
  • thermocouple,
  • combustion air supply,
  • heat exchanger,
  • cabinet, fan (blower, air handler),
  • fan control,
  • high temperature limit switch,
  • thermostat,
  • draft hood,
  • vent connector (exhaust flue, C-vent, B-vent, vent pipe, stack pipe, flue pipe, chimney connector, breeching),
  • filter,
  • electronic air cleaner,
  • humidifier,
  • duct system,
  • registers,
  • grill,
  • induced draft fan,
  • electronic ignition system,
  • condensate handling system,
  • vent damper.

8. Differentiate between conventional, mid efficiency and high efficiency gas furnaces. Include a description of the normal operating cycle for each.

9. Describe the features of  adequate installation and repair technique for gas fired forced air furnaces.

10. Define the following terms with respect to gas fired heating systems: gas pressure regulator, gas meter, drip leg(dirt pocket), flex connectors, combustion air, draft air, dilution air, circulating air, spillage(back drafting, down drafting), natural gas, BTU content of natural gas, monoport burner, multi port burner, cross over igniter refractory (firepot), ,(Norrmally only in oil fired systems)  JDG gas valve, pilot light, thermocouple, fan/limit switch, heat shield(flame roll out shield), heat exchanger, indirect fired system, direct fired system, draft hood, blower(house fan, furnace fan, air handler, furnace blower), Fan door safety relay..JDG, up flow furnace, down flow furnace, horizontal furnace, summer switch, fan compartment interlock switch, thermostat, heat anticipator, vent connector, supply plenum, return plenum, filter, supply duct system, return duct system, supply registers, return grills, electronic air cleaner, humidifier, input capacity, output capacity, bonnet capacity, steady state efficiency, seasonal efficiency, steady state losses, off cycle losses, stack effect, AFUE, separation of circulating and combustion air.

11.Define the following terms with respect to mid and high efficiency gas furnaces: vent damper, intermittent pilot, direct spark ignition, hot surface ignition, flame rectification, natural draft burners, induced draft burners, forced draft burners, pre-purge, side wall venting, sealed combustion, direct vent, mid efficiency, high efficiency, condensing furnaces, latent heat of vaporization, neutralizer, differential pressure switch, heat switch(spillage switch), blower cover interlock, pulse combustion, combination furnaces(water heater and furnace), tempering valve (mixing valve).

12. Outline the life expectancies for gas fired forced air furnaces.

13. Identify the codes and standards which apply to gas fired appliances and warm air heating systems in your area.

4.1.2 Inspection Skills for Gas Fired Heating Furnaces - Warm Air Heat

1.    Describe the inspection procedure for conventional, mid efficiency and high efficiency, forced air gas furnaces.

2.    Identify the following common defects listed on the next page.

3.    Describe the implication of each defect.

4.    Identify safety issues for the inspector and occupant of the house (gas explosion, carbon monoxide poisoning, injury due to moving parts).

5.    Communicate findings to client verbally and in writing, recommending corrective action where needed.

List of Typical Gas Fired Heating Furnace Defects

Combustion Air Hazards

  • Possible or obvious inadequate combuation air
  • Missing CO or Smoke Detectors

Gas Burners & Venting, Typical Defects at Gas Fired Heating Furnaces

Gas meters - general gas supply piping, regulators, controls

Gas regulators & control valves at the heating equipment

• Gas shut off and locked           

• Loose

• Ice

• Missing

• Leaks

• Rust

• Mechanical damag

• Scorched

• Poor access

• Poor location 

Heat shield defects at the gas burner - burns, rust, missing, damaged, leaks

Evidence of dangerous conditions

Soot - Carbon Monoxoide fatality risk warnings

flame leakage

rust or water or mechanical damage

Gas Furnace Venting Defects & Hazards

• Rust

• Undersized                        

• Connector extends too far in chimney                                  

• Draft hood spillage or backdrafting

 Gas Piping Defects

• Improper material

• Inadequate combustible clearance

• Copper tubing not properly labeled • Poor manifolding

• Improper connections

• Poor slope

• Inadequate support

• Poor support

• Inappropriate materials

• Poor connections

• Leak

• Rust, dirty, obstructed

• Missing shut off valves

• Vent connector too long

• No drip leg

• Vent connector too big or too small

• Piping in chimney or duct systems

• Plastic pipe exposed above grade 

Mid & High Efficiency Gas Furnace Defects

• Rust Corrosion.  JDG

• Airflow proving switch problems

Combustion Air Defects at Gas Fired Heaters                 

• Condensate problems

• Electronic ignition problems

• Inadequate combustion air

• Excess temperature rise

• Heat exchanger problems

Gas Burner Defects at Gas Furnaces

• Induced draft fan problems

• Poor vent location, arrangement

• Delayed ignition

• Poor combustion air intake location

• Dirt or soot

• Spillage switch problems

• Flame wavers when fan comes on

• Vent damper stuck

• Gas odor or leak

• Inoperative

• Poor flame color or pattern

• Rust Corrosion..JDG

• Scorching

• Short cycling

Warm Air (Forced Air) Heating Furnaces, Gas Fired: Defects

Heat Exchanger Defects       

• Cracks, holes or rust

• Excess temperature rise

• Soot or deposits

Mechanical Air Filter Defects

• Dirty

• Installed backwards

• Loose or collapsed

• Missing

• Wrong size

Electronic Air Cleaner / Filter Defects

• Damaged cells

• Dirty

• Improper orientation

• Inoperative

• Mis-wired

• Missing components

Air Handler Cabinet Defects                    

• Combustible clearances 

• Mechanical damage

• Missing components              

• Obstructed air intake            

• Rust                             

• Scorching                        

Thermostat Defects for Forced Air Gas Fired Heating Systems                 

• Anticipator problems

• Damaged

• Dirty                             

• Loose                            

• Not level                        

• Poor adjustment or calibration    

• Poor location                                                        

Humidifier Defects on Forced Air Gas Fired Heating Systems    

• Clogged pad, mesh or nozzle

• Dirty

• Inoperative motor or solenoid valve

• Leaks

• No duct damper

• Poor location

Blower Assembly Defects on Forced Air Gas Fired Heating Systems                                   

• Dirty

• Fan belt loose, worn, damaged    

• Inoperative                       

• Noisy                            

• Overheating                      

• Poorly secured                   

• Running continuously             

• Rust                              

• Too small                        

• Unbalanced or vibration                                           

Duct & Register Defects on Forced Air Heating Systems     

• Balancing damper adjustment

• Dirty, obstructed collapsed

• Disconnected or breached  JDG or damaged ducts

• Excessively long runs, excessive elbows

• Wire or pipes in ducts

• Registers or grilles (for return air) in garage

• Rusting or dirty

• In-slab ducts

• Mechanical damage

• Leaky joints

• Insulation missing, damaged

• Filter problems

• Undersized

• Weak airflow

Fan Limit Switch Defects on Forced Air Heating Systems

• Improperly wired                  

• Missing cover                    

• Scorching                        

• Set wrong or defective           

• Fan problems

Combination (water to air) Heating System Defects

• Cabinet problems

• Control problems

• Domestic water too hot

• Inadequate heat for house

• Inoperative, inefficient pumps

• Inoperative water heater

• Leaking, clogged coils

• Water control problems

Readers should see FURNACES, HEATING

and HEATING SYSTEMS for our complete list of articles on this topic.







Watch out: these inspection lists do not list all possible defects for the systems discussed, and not all home or building inspectors will examine all of the items listed here. CONTACT us to suggest corrections or additions to articles at this website.

These curriculae and building defect lists are based on smilar curriculum documents first prepared by Joe Scaduto, an ASHI member who prepared course material for Northeastern University's Building Inspection Certificate program in 1988, subsequently by DF, InspectApedia's editor, for New York University ca 1988 and later, with others, recommended to ASHI, the American Society of Home Inspectors. ASHI did not adopt this material though currently that association as well as others offer extensive HOME INSPECTOR EDUCATION material.

The curriculum and lists of defects are informed by additional analysis of the process of home inspection that was developed beginning Calgary, AB for Canadian and U.S. home inspector education and certification examinations in 1997. Other early contributors to home inspection education in the U.S. and Canada include Dr. Jess Aronstein, Alan Carson, Mike Casey, Mark Cramer, John Cox, Dwight Barnett, Douglas Hansen, Rick Heyl, Larry Hoytt, Bill Merrill, Kevin O'Malley, Dennis Robitalille, Keith Peddie, Pat Porzio, Roger Robinson.


Continue reading at BLUE vs YELLOW COMBUSTION FLAMES or select a topic from the More Reading links shown below.



Or see ANTIQUE HEATERS STILL in USE for an example of an antique but still-used gas fired floor/wall heater

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HEAT FURNACE GAS DEFECTS LIST at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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