Gas Furnaces Operation and Safety

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Questions and Answers

Thermostat calls for _____

heat

What happens when the gas valve opens?

Flame lights

Furnace gets ______

hot

_____ switch closes

Fan Signup and view all the answers

_____ runs

Fan Signup and view all the answers

Thermostat calls for _____ (newer inducement)

heat Signup and view all the answers

_________ fan starts

Inducer Signup and view all the answers

__________ switch makes

Pressure Signup and view all the answers

________ activates (spark, hot surface)

Igniter Signup and view all the answers

Gas valve (opens/closes) -- ______ lights

Gas valve opens; flame lights Signup and view all the answers

_____ has 3 seconds to verify

Sensor Signup and view all the answers

Fan starts due to _ time

PC Board Signup and view all the answers

The gas valve closes and flame goes off when the thermostat is satisfied.

True Signup and view all the answers

Limit switch powers (on/off) the system if the temperature around the heat ___________ gets too high.

exchanger Signup and view all the answers

Which can cause the limit switch to trip?

faulty fan blower Signup and view all the answers

When the furnace is off, is the pilot lit?

Not lit Signup and view all the answers

Define flame rectification.

The process of using a pilot flame or burner flame to change a small AC current to DC current. Signup and view all the answers

A thermocouple generates how much electricity?

25 millivolts Signup and view all the answers

HSI's are required to have a flame sensor.

True Signup and view all the answers

What are the two types of ignitions that use pilots to ignite the burners?

Standing pilot & intermittent-pilot Signup and view all the answers

Which type of ignition uses a high resistance heating element to ignite the burners?

Hot Surface Igniter (HSI) Signup and view all the answers

What typically happens when changing a furnace from natural gas to LP?

Change spud, pressure regulator spring, and the pressure switch. Signup and view all the answers

Study Notes

Gas Furnace Sequence of Operation

Older gas furnaces use a pilot light and follow a specific sequence for starting and heating.
Newer gas furnaces utilize an inducer fan and have a more complex starting procedure to enhance efficiency and safety.
Important components mentioned include the fan switch, gas valve, thermocouple, and limit switch.

Safety Mechanisms

Limit switches turn off the system when temperatures around the heat exchanger exceed safe limits to prevent gas flow.
Roll-out switches monitor flame roll-out situations, noting potential misalignment issues causing unsafe conditions.
Blocked-vent switches shut down operation in the event of backdraft conditions, crucial for safety.

Ignition Systems

Intermittent pilot ignition systems vary from traditional pilot systems, with no pilot flame present when the system is inactive.
Direct spark ignition does not use a pilot and ignites gas directly; offers lower maintenance needs due to simpler design.
Hot surface igniters (HSI) utilize a heating element to ignite gas, creating a glow that initiates combustion.

Efficiency Ratings

Annual Fuel Utilization Efficiency (AFUE) measures how effectively a furnace converts fuel into heat, rated in BTU.
Different efficiency levels (60%, 80%, 90%+) indicate variations in design components, heat exchangers, and combustion processes.

Troubleshooting & Maintenance

Common issues include a failed pilot light, blocked venting, or malfunctioning pressure switches which could necessitate investigation and repair.
Regular checks on wiring, gas pressure, and system components ensure safe operation and efficient performance.
When transitioning between fuel types, like natural gas to LP gas, specific changes to the gas manifold and pressure regulators are essential.

Combustion Principles

Combustion involves fuel chemicals and oxygen rapidly combining, often resulting in heat and byproducts like carbon dioxide.
Proper combustion requires precise gas pressure settings: 3.5" WC for natural gas, and 10-11" WC for LP gas.

Electrical Systems

Flame rectification is a critical safety feature that utilizes current flow to verify the presence of a flame.
Solenoid valves control gas flow based on signals from safety devices, and thermocouples sense flame presence to prevent gas leaks.

Component Functions

Spuds in gas manifolds regulate gas flow to burners and are essential for maintaining proper combustion levels.
The use of high-limit switches prevents overheating in the system, while roll-out switches protect against flame roll-out scenarios.
The gas manifold supplies fuel to multiple burners, ensuring an efficient heating process.

Pilot Types and Flame Detection

Standing pilot systems continuously burn gas to maintain ignition; intermittent pilots only ignite when needed.
Pilot systems (both standing and intermittent) rely on thermoelectric effects to maintain gas flow and ensure safety.

Important Terms and Measurements

Inches of Water Column (WC) is the standard measurement for gas pressure in HVAC systems.
Changes in altitude affect furnace performance, often requiring adjustments in orifice size and gas mixture for optimal combustion efficiency.

Electrical and Mechanical Interactions

High resistance heating elements in HSIs require significant voltage yet produce little electrical noise compared to DSI systems.
Understanding system wiring, including the role of sense terminals and electrical resistance in various ignition systems, enhances troubleshooting effectiveness.

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Description

Explore the operation and safety mechanisms involved in gas furnaces. This quiz covers pivotal components, ignition systems, and essential safety measures that ensure efficient functioning. Whether it's about older pilot light models or modern ignition systems, test your knowledge here.