Chapt. 41. Gas-fired heating systems

Gas pressure for liquid petroleum gas

10.5 inches of water column

Ultimate carbon dioxide content for natural gas

11.9%.

Ultimate carbon dioxide content for liquid petroleum gas

13.9%

Gas pressure for natural gas

3.5 inches of water column

National Standards Institute's (ANSI) maximum concentration of carbon monoxide permitted in fur- nace flue gas

400 parts per million (ppm)

thermal detection system

A system that uses a thermocouple, thermopile, or bimetallic element

What is used for natural gas applications

Black iron pipe & corrugated stainless steel (CSS) is also used

What needs to be measured to test a furnace's combustion efficiency

Carbon dioxide content. • Oxygen content. • Carbon monoxide content. • Gas pressure. • Stack temperature

flame rollout

Flame spills backward out of the burner.caused by a cracked heat exchanger, rust on burners, poor draft, or incorrect flue pipe height

Stack temperature formula

Flue Gas Temperature - Combustion Air Temperature= Stack Temperature

Gas furnaces ratings the devolop/ don't develop condsation

Gas furnaces rated over 83% AFUE develop condensation. below 83% AFUE do not develop condensation

Combustion air

Primary air and secondary air are commonly referred to together

High limit switch

Protects the heat exchanger from overheating

inshot burner

Single port directs the mixture of air and fuel gas through a large orifice to produce a large flame that is directed into the heat exchanger

bonnet

The sheet metal cham- ber where heat collects before being distributed. contains the heat exchanger

local sensing

When a system's electrode and flame rod are packaged together

integrated ignition control module

a control module that uses advanced electronics to pro- vide greater control and functionality than a nonintegrated control module

hot-surface igniter

a high-resistance heating element that produces a great deal of heat when current passes through

drip leg

a trap and collects possible moisture or sediment that may flow with the gas. installed at the furnace along with a manual shutoff valve. required at the outlet of the gas meter to prevent con- densate from running into the meter

Excess

any secondary air that exceeds the amount of air necessary for complete combustion

stack temperature in noncondensing furnaces

between 325°F and 500°F

soft lockout

briefly quits trying to ignite for a specified time before attempting ignition again. After a soft lockout's waiting period, the system automatically tries ignition again

intermittent-pilot ignition system

burns and monitors its pilot light only while the thermostat is calling for heat. control module does not attempt to sense the pilot

electromagnetic interference (EMI)

can disrupt electronics, such as the ignition control module on the furnace itself. keep electrical cables as short as possible

nonintegrated ignition control module

can open and close a gas valve, control a spark or hot-surface igniter, and monitor the interlocks

Flu pipe temp. Range

can reach a temperature between 500°F and 600°F

Height requirements of chimney or file pipe

chimney or flue pipe should extend at least 2′ (0.6 m) above the highest part of a roof. helps prevent flue gases from flowing back into the chimney or flue pipe under certain wind conditions

100% shutoff

closes the gas valve and the pilot valve when the flame rod does not detect a flame

non-100% shutoff

closes the gas valve but not the pilot valve when the flame rod does not detect a flame. shutoff times that are long enough for a dangerous amount of gas to dissipate before attempting to reignite the pilot

post-purge

combustion blower remains on for a given period of time after gas burner operation has ceased in order to vent any combustion gases in the heat exchanger

standing-pilot ignition system

continuously burning pilot that ignites the burners when there is a call for heat. Wired to the igni- tion control module, a thermocouple, thermopile, or bimetallic element

spark igniter

creates an electric spark across a gap between two electrodes when the ignition control module applies a high voltage to the electrodes

pre-purge

cycling on the combustion blower to vent any combustion gases remaining in the heat exchanger

Class B chimney

double-wall metal type

Slotted burners

feed a mixture of fuel gas and primary air through a series of narrow slots

Ribbon burners

feed the fuel gas and primary air mixture along the length of a burner producing a solid flame on top

gas-fired, forced-air furnace

fuel gas is piped to the furnace and kept under constant low pressure by a pressure regulator. low-pressure gas is controlled by a gas valve

elements necessary for combustion

fuel, heat, and oxygen

factors when installing venting systems

furnace capacity, heat load, type of flue pipe, length of flue pipe, rate of flue gas flow, and number of elbows

Remote sensing

has an electrode for starting the spark and a separate flame rod for detecting the flame,

Conditions that an increase in gas pressure cause incomplete combustion

lack of air: high carbon dioxide content and low oxygen content in the flue gas

Stack temperature and condensing furnaces

less than 140°F

ignition system

light a burner safely and monitor for continued safe operation. controlled by a furnace's ignition control module

Class A chimney

masonry type. existing chimney must provide at least 1/2′′ (1 cm) space between the metal flex liner and the masonry inner wall

Combustion efficiency

measure of a furnace's combustion quality, which is essentially its ability to achieve complete combustion

annual fuel utilization efficiency (AFUE) rating

measurement of gas furnace efficiency

Category I furnace

negative-pressure venting, noncondensing furnace. flue gas temperature is at least 140°F (60°C) (higher than its dew point), so very little condensate forms. (Lower & mid efficient furnaces) (residential)

Category II furnace

negative-pressure, con- densing furnace. vented with stainless steel piping

Too lean

not enough fuel for combustion

Too rich

not enough oxygen available for all of the fuel present

combustion chamber

part of the heat exchanger containing the flames

measuring the carbon dioxide content in the flue gas

place the analyzer's sensor in the flue or vent pipe. Analyzer also measures oxygen content, carbon monoxide content, and stack temperature

Category III furnace

positive- pressure, noncondensing furnace. vented with stainless steel piping

Category IV furnace

positive-pressure venting, condensing furnace. flue gas temperature is less than 140°F (60°C) (lower than its dew point)high- efficiency condensing furnaces with secondary heat exchangers and PVC vent piping

Factors that affect pipe size

pressure drop of the piping, the specific gravity of the gas, and the amount of gas consumed by the furnace per hour

Flame rectification

process of using a pilot flame or gas burner flame to change a small electric current from alternating current to direct current

draft regulator!(vent dampers)

regulates the intake of indoor air into the flue to moderate or stabilize the flow of flue gas. controlled thermostatically, electrically, or barometrically

rollout switch.

safety device that monitors for flame rollout

Electric interlocks

safety devices that prevent operation of certain devices unless certain conditions are met

End switches

send a signal to the ignition control module to open the gas valve when the damper is turned open a certain amount or angle. connect to the end of a damper motor shaft

pilot light

small flame located near a furnace's gas burners that provides the initial heat to ignite the furnace

hard lockout

system shutdown for an unspecified amount of time after a furnace fails to light. Requires service call

What burner flame size gulated is based on

temperature difference between the thermostat set point and the room temperature

Stack temperature

temperature of the flue gas in the flue minus the temperature of the combustion air

What gas flow is affected by

the pressure difference between the combustion air entering the appliance and the flue gas leaving the flue

flammability limits

the range of fuel concentrations within which the fuel will burn when an ignition source is present

venturi effect

the reduction in pressure that occurs when a fluid flows through a constricted section of pipe

Ultimate carbon dioxide content

the specific amount of carbon dioxide by volume that is present in flue gas when the exact amount of air is supplied to achieve complete combustion

Conditions that a decrease in gas pressure can cause incomplete combustion

too much excess air: low carbon dioxide content and high oxygen content in the flue gas

sail switch

type of flow switch that uses a large paddle to catch a draft to open or close its contacts

power burner

type of gas burner that uses a blower to force both primary and secondary air into the burner tube. tube has angular deflector plates to spin or twirl the flame for more efficient burning

hot-surface ignition (HSI) system

uses a silicon carbide igniter to light the gas burners directly. No pilot

direct-spark ignition (DSI) system

uses an electric spark to ignite gas burners

atmospheric gas burner

uses the siphoning action of gas flow through the orifice to induce air- flow through the burner without the need for a blower

inter-purge

when a combustion blower cycles on during a soft lockout to vent any com- bustion gases remaining in the heat exchanger after a failed ignition attempt