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NFI 200 Exam Review Questions
Fuel Delivery Design: Bottom Feed - Correct Answer โ โ pellets pushed to fire from beneath or behind burn pot
ashes pulled out into ash drawer by incoming pellets Fuel Delivery Design: Burnback - Correct Answer โ โ ASTM E1509 testing required
no smoke of charred fuel in hopper when tested under adverse conditions (combustion blower disabled, exhaust system blocked, electrical power lost) Bottom feed prevention methods include: - Correct Answer โ โ high temperature limit switch or thermistor on auger tube (isolation of auger tube)
Top feed prevention methods include: - Correct Answer โ โ pellets in auger tube separated from fire, high temperature limit switch on auger tube Fuel Delivery Design: Fuel Quality - Correct Answer โ โ Horizontal feed action pushes ash away (increases tolerance for ash content)
Top feed delivery does not affect ash removal (some units use improved grate design) Combustion Air Delivery (pellet) - Correct Answer โ โ mechanical draft (location of fan determines pressure, positive or negative, and volume of air linked to fuel feeds delivery)
natural draft (in some older systems, used mostly in Europe) Hopper (pellet) - Correct Answer โ โ holds fuel
Combustion Chamber (pellet) - Correct Answer โ โ combustion of pellets in fuel bed
secondary combustion of gases
baffles to slow exit of gases
gaskets to seal off-wanted dilution air
firebricks keep temperatures high Control board (pellet) - Correct Answer โ โ operation control (power switch, start button, fuel feed, convection blower, safety monitoring)
operation indicator lights
Technician adjustment screen Safety Systems (pellet) - Correct Answer โ โ mechanical switches that allow electricity flow under normal conditions (temperature, pressure, current, hopper lid position) Low Limit Switch (pellet) - Correct Answer โ โ monitors start up temperature
normally open (closes at minimum operating temperature)
bypassed for set period time during start up
exhaust temperature monitored
digital signal sent to control board
fuel feed shutdown in case of over fire or convection fan failure Thermocouples - Correct Answer โ โ monitors firebox temperature
two wires of different metals
produces 25-30 mV
- greater temperature difference between hot and cold junctions = more mV
- must be at least 300-400F differences Optical Sensor - Correct Answer โ โ optical sensor above burn pot
- relays sensor as long as light from fire sensed
- absence of flame or excessive smoke causes shutdown Ash recepticals - Correct Answer โ โ some sealed (limits air leakage)
some allow controlled amount of air
ash removal CRITICAL
- blocks air inlets
- accumulation on blower, heat exchanges, and venting systems Ash traps - Correct Answer โ โ spaces to catch fly ash
Combustion Requirements (3) - Correct Answer โ โ Air, Fuel, Heat Complete Combustion Produces (4) - Correct Answer โ โ Water vapor, carbon dioxide, heat, and non combustible ashes incomplete combustion produces (3) - Correct Answer โ โ carbon monoxide, hydrocarbons, and other gases rate at which fuel burns is controlled by - Correct Answer โ โ the amount of available air air to fuel ratio is a - Correct Answer โ โ critical factor in obtaining optimum fuel combustion BTU - Correct Answer โ โ British Thermal Unit, The amount of energy needed to heat one pound of water one degree Fahrenheit wood ignites at - Correct Answer โ โ 450-550 F natural gas ignites at - Correct Answer โ โ 1,150 F
propane ignites at - Correct Answer โ โ 920-1120 F
electric spark temp - Correct Answer โ โ 1,700 F hot surface igniter temp - Correct Answer โ โ 2,400 F
butane lighter temp - Correct Answer โ โ 3,200 F stick match temp - Correct Answer โ โ 3,000 F
cigarette temp - Correct Answer โ โ 500-800 F For complete combustion: air is ______ % oxygen + _____ % nitrogen - Correct Answer โ โ 20 : 80 for complete combustion, (QTY) molecules of (PART)_ need to be mixed with _(QTY) molecule of _____(PART)____ in the presence of (PART) - Correct Answer โ โ 2 oxygen : 1 fuel : heat
3 Ts of efficiency - Correct Answer โ โ Temperature, turbulence, time Temperature (efficiency) - Correct Answer โ โ minimum ignition temp Turbulence (efficiency) - Correct Answer โ โ fuel must collide with oxygen molecules (refers to the degree of fuel and air mixing that occurs in combustion zone) Time (efficiency) - Correct Answer โ โ amount of time fuel gases remain in the high temp zone. also known as "residence time" Heat Transfer Efficiency - Correct Answer โ โ refers to the percentage of heat generated by combustion actually transferred to room High heat transfer efficiency is normally associated with: (5 things) - Correct Answer โ โ Appliance having proportionally large exterior surfaces with good heat radiating properties
Long residence time of the hot gases in the Appliance
Adequate turbulence inside the appliance to property transfer heat from the fire chamber to appliance walls
Rapid heat transfer through the walls of the appliance
Adequate air movement (convection) outside 2 types of factory built fireplaces - Correct Answer โ โ radiating and circulating Radiant design - Correct Answer โ โ transmits heat energy from firebox opening by direct radiation to the space in front of it. (do not have grills or louvers on the face, heat moving blowers not avail.)
Most controlled combustion devices are made of (metals) - Correct Answer โ โ cast iron or steel
Cast Iron (fireplace) - Correct Answer โ โ made from molds, can be decorative Steel (fireplace) - Correct Answer โ โ welded joints that need no service, some use steel fireboxes inside decorative cast iron Soapstone (fireplace) - Correct Answer โ โ lower thermal conductivity, delivers heat slowly but retain and delivers heat longer Conduction - Correct Answer โ โ direct transmission of heat from one surface of solid material to another surface of solid material Convection - Correct Answer โ โ transfer of heat through a gas or liquid, including air, the transfer of heated air between areas of unequal density. Natural occurs between heavier, cooler air and lighter, warmer air
Forced convection occurs - Correct Answer โ โ By mechanical means, aka electric blower Radiation - Correct Answer โ โ emission of waves of infrared energy, converted to heat on contact with solid surfaces Radiant appliances - Correct Answer โ โ single wall, exterior wall is firebox wall. Generally required for large open spaces than small numerous rooms Convection appliances (3 things) - Correct Answer โ โ double wall construction, allows room air to circulate, convection currents, blower increases the amount and distance of heated air circulation
lower surface temperatures of outer wall exposed
less spacious area, may make immediate space too warm/stuffy
combustible floor during operation - indicates dimensions but no thermal requirements for floor protection Thermal Floor Protector - Type II (floor protection) - Correct Answer โ โ REQUIRES thermal protection, floor must have a minimum R value of 1.0, allowed ONLY if particular has an R value + pr greater than the minimum R value required NFPA - Correct Answer โ โ National Fire Protection Association ICC - Correct Answer โ โ International Code Conference
ASTM - Correct Answer โ โ American Society for Testing and Materials ANSI - Correct Answer โ โ American National Standards Institute Chimney not only removes exhaust but also responsible for - Correct Answer โ โ supplying combustion air that starts and sustains the fire
Natural draft (3 things) - Correct Answer โ โ gases flow from zones of high pressure to low pressure
describes and quantifies the force causing gases to flow
greater the temperature difference from heated gases in the flue and cooler air outside, more forces created Vent gas temperature (draft) - Correct Answer โ โ hotter = lighter, more forcefully they exit the venting system outdoor temperature (draft) - Correct Answer โ โ cold weather better for draft house stack effect - Correct Answer โ โ heated portion of the house taller than vent termination (heat rises)
