HVAC Unit 31 Gas Heat Review Questions

What are four factors that determine the efficiency of a furnace?

-Type of draft (natural atmospheric injection, induced, or forced) -Amount of excess air used in the combustion chamber -Delta T or temperature difference of the air or water entering versus leaving the heating medium side of the heat exchanger -Flue stack temperature pg 746

The typical manifold pressure for propane gas is _____ in WC.

11 in WC pg 715

What is the approximate oxygen content of air?

21% oxygen pg 716

Describe how a liquid-filled flame-proving system functions.

A liquid-filled remote bulb includes a diaphragm, a tube and a bulb, all filled with a liquid, usually mercury. The bulb is heated by the pilot flame and the liquid inside expands, causing the diaphragm to expand, which closes contacts wired to the gas safety valve. As long as the pilot flame is on, the liquid will remain heated and gas can flow. pg 736

What is a 100% shutoff system?

A system is 100% shutoff when there is a failure in the flame-proving device system and both the pilot valve and main gas valve are shut down. pg 748

What is a water and digital manometer?

A water manometer is used to measure gas pressure. It is a tube of glass or plastic formed into a U that is half full of water. Gas pressure is determined by the difference in levels in the two columns. A digital manometer also measures gas pressure. It does not have any fluid to spill. It can also measure both positive and negative pressures in inches of water column. pg 715

Describe the function of a draft safeguard switch.

Also known as a vent limit switch. They are NC switches usually mountd on the draft hood if the furnace has one. They monitor the draft hood temperature and open when there is any spillage of flue gas. pg 713

The purpose of the gas regulator at a gas burning appliance is to: ABCD

B) decrease the pressure to the burner. pg 718

Describe a flame rectification flame-proving system.

Because of the ionized combustion gases, a flame can conduct electricity. By placing the flame between two electrodes of different sizes, an AC current will flow better from one electrode than the other. This acts like a switch. As long as there is a flame, the switch is closed and will conduct electricity. If there is no flame, the switch is open and the electricity will not flow. pg 740

The specific gravity of natural gas is: ABCD

D) .60 pg 714

Name four types of gas burners.

Drilled-port burner Slotted-port burner Ribbon burner Single-port / inshot / upshot burner pg 726

T or F: All gas valves snap open and closed.

False - the valves are solenoid, diaphragm and heat motor. pg 719

Describe the difference between a forced-draft system and an induced-draft system.

Forced-draft have a combustion blower motor on the inlet of the heat exchanger. It pushes or blows combustion gases through the heat exchanger and causes a positive pressure in the heat exchanger. Induced-draft have a combustion blower motor located on the outlet of the heat exchanger. It pulls or sucks combustion gases through the heat exchanger and causes a slight negative pressure in the exchanger. pg 727

Describe why you should never use a flame to leak check a gas system.

Gas can be explosive. pg 759

What is a hot surface ignition system?

HSI uses silicon carbide which is highly resistant to electrical flow but will not burn up. It is similar to a glow coil. It is placed in the gas stream and allowed to get very hot. When gas hits the hot surface, immediate ignition occurs. The igniter operates off 120V line voltage. It is very brittle and breaks easily if touched. pg 738

Describe the function of a multipoise or multipositional furnace.

It can be installed in any position. It is very versatile. The configuration can be changed very easy. pg 712

Why is a metal vent often preferred rather than a masonry chimney?

Metal vents reach operating temperatures quicker than masonry vents do. Masonry chimneys tend to cool the flue gases. Masonry must be lined with a glazed-type tile to prevent corrosive materials from the flue gas from destroying the mortar joints. pg 756

Flame current is measure in _______.

Microamps pg 740

Name two types of fan switch.

Temperature controlled or time delay. pg 730

What is meant by the warm air blower's off-delay timing?

The amount of time that a fan will stay on after the burners turn off in order to cool down the heat exchanger. pg 731

Briefly describe a direct burner gas valve system.

The electronic module or IFC lights the main burner directly without a pilot flame. A spark, a hot surface igniter or a glow coil usually accomplishes ignition. pg 723

What is the first step a service technician should take when systematically troubleshooting a furnace that contains an IFC?

The first step in systematically troubleshooting a furnace that has an IFC is to obtain the electrical diagram of the furnace and the sequence of operation of the IFC. pg 761

Describe how a bimetallic flame-proving system functions.

The pilot heats a bimetal strip, which closes electrical contacts wired to the gas safety valve. As long as the pilot flame heats the bimetal, the gas safety valve remains energized and the gas will flow. pg 736

Briefly describe an intermittent pilot gas valve system.

The pilot is lit every time there is a call for heat by the t-stat. Once the pilot is lit and proved, it lights the main burner. When the call for heat ends, the pilot is extinguished and does not relight until the next call for heat. This creates an automatic and independent control of the gas flow for both the pilot and the main burner. pg 723

What is a flame rollout safety switch used for in heating applications?

The roll out switch is a safety device which senses heat and or flames. When a condition sets in where the flames want to roll backwards out of the firebox instead of into the firebox, the condition is sensed by the roll out switch. When a condition causes the flames to burner outwards towards you, they would be going backwards out of the firebox. A condition which is highly dangerous. The condition causes extremely high levels of unburned or partially burned fuel. Once this safety switch is tripped, it requires a service call by a technician to troubleshoot why the furnace has a flame rollout problem. pg 732

A _____ _____ sample is taken from the flue gas to check the efficiency of a gas furnace.

flue gas

When the limit switch circuit opens, it opens the circuit to the ______ ______.

gas valve. pg 732

The ______ ______ transfers heat from the hot gas to the airstream to heat the structure.

heat exchanger. pg 728

Where are inshot burners used in heating applications?

induced draft systems. pg 726

A _______ distributes gas to the various burners.

manifold. pg 726

Describe the difference between a single-rod (local sensing) and a dual-rod (remote sensing) system as they pertain to flame rectification.

- Single-rod - it consists of an igniter and a sensor all in one rod. This single rod accomplishes both ignition and sensing. There is only one wire running from the pilot assembly to the ignition module. The ignition pulse is accomplished on one half cycle and the sensing pulse current is accomplished on the other half cycle. - Dual-rod - it consists of a separate igniter and flame-sensing rod or electrode. The sparking is done with one rod and the sensing with the other. There are two wires running from the pilot assembly to the module pg 741

Describe the two types of pilot lights.

- The aerated pilot is mixed with gases before it enters the pilot burner. They are subject to periodic cleaning - The nonaerated pilot use only secondary air at the point where combustion occurs. Little maintenance is required and nonaerated pilots are found on most furnaces. pg 733

Describe how a modulating furnace operates.

It follows the heat loss of a structure and has longer run times and less cycling. This provides a more efficient furnace. They use a modulating gas valve instead of a staged gas valve. They use a variable-speed warm air blower to vary the speed and amount of warm air. They also have a variable operating solenoid or servo valve that varies the input rate of the furnace to allow a heating output range between 40 - 100%. pg 753

What is excess air?

It consists of combustion air and dilution air. Combustion air can be primary and/or secondary air. Primary air enters before combustion takes place. Secondary air happen after combustion and supports combustion. As excess air decreases, DPT increases. pg 745

What is an integrated furnace controller (IFC)?

It controls the ignition and sequence of operation of most modern gas furnaces. They have dip switches to vary control pg 747

When would a combustion blower ever interpurge a furnace's heat exchange?

An interpurge allows the combustion blower to operate for a certain time period between ignition tries. It is generally used when the furnace does not light or does not sense a flame. pg 749

What is the difference between a hard and a soft lockout?

A soft lockout is a time period build into a ignition module that allows for a certain time to light or relight the pilot or main burner. If this time period is exceeded, it will go into a semi-shutdown but it will keep trying to relight the system. It will go into hard lockout after several soft lockouts have been attempted. If it goes into hard lockout, the module will need to be reset by interrupting the power to the module. This can be done by a service call. pg 748-749

Describe how a thermocouple flame-proving system functions.

A thermocouple is connected to a shutoff valve and is heated by the pilot flame. As long as the electrical current in the thermocouple energizes the coil in the safety valve, the gas can flow. If the flame goes out, the thermocouple will cool off and stop the current flow. This will cause the safety valve to close and stop the gas flow. pg 735-736

The pressure at the manifold for natural gas is typically: ABCD

D) 3.5 in WC pg 715

Which of the following features does an automatic combination gas valve have? ABCD

D) all of the above. It has a pressure regulator, pilot safety shutoff, and redundant shutoff feature. pg 721

What is dilution air?

Dilution air is excess air after combustion and usually enters at the end of the heat exchanger. it is brought in by the draft hood of the furnace. pg 745

What is a direct-vented furnace?

It has a sealed combustion chamber where combustion air is brought in from the outside through an air pipe that is usually made of PVC. The combustion chamber never receives conditioned room air. This prevents the conditioned room from being depressurized or put into a vacuum which cuts down on the amount of infiltration air from the outside. pg 743

What is a redundant gas valve?

It has two or three valve operators physically in series but wired in parallel with one another. It is a safety feature that allows any operator (pilot or main) to block gas from getting to the main burner. pg 725

What is a smart valve system?

It is a gas valve and electronic control module in one package. It combines the features of an IP (intermittent pilot) system and an HSI (hot surface ignition) system in one. The pilot flame is considered one of the more reliable ignition sources in the industry. pg 751

What is meant by a furnace that has a positive pressure venting system?

It is a system whose flue pipe pressure is positive the entire distance to its terminal end. A positive pressure venting system means the pressure in the vent line is higher than the outside pressure, thus forcing your vent air to be pushed outside. pg 744

What is a variable output programmable thermostat?

It is a thermostat that can send a varying or proportional signal to the furnace controller of a modulating gas furnace. pg 753

What is meant by prepurging a heat exchanger?

It is to clear the heat exchanger of any unwanted flue gases, household fumes, or dust that may have accumulated. pg 749

What is the function of the servo pressure regulator in a modern combination gas valve?

It monitors the outlet pressure of the gas valve. Any gas outlet pressure change is instantly sensed and reflected back to the diaphragm. The regulator then repositions the diaphragm and disc to change the flow rate through the servo pressure regulator valve. It is a self-balancing system that works off gas valve outlet or working pressure and is independent of flow rate. pg 723

What is a non-direct-vented furnace?

It uses indoor conditioned air for combustion. This can depressurize a conditioned room. pg 743

Where are auxiliary limit switches used in heating applications?

They are used for counterflow and horizontal furnaces. They monitor room return air temperature and interrupt burner operation when the temperature gets too hot. pg 713

What are dual in-line pair (DIP) switches?

They are very small amperage single pole, double throw switch used in electronic circuits to set up the program in the circuit. An example is that they can tell a circuit board how much time the blower needs to stay on after the burners have shut off. pg 731

Explain why the preceding flame-proving systems are called safety devices.

They keep the gas from flowing through the main valve if the pilot flame goes out. pg 734

Describe how a two-stage furnace operates.

They use a two-stage gas valve and a two-speed combustion blower motor with two pressure switches (high and low pressure) to prove the draft. The first stage of heating puts out a manifold pressure of 1.75 " WC by energizing the first solenoid and operates about 50-70% of total output. The second stage puts out a manifold pressure of 3.5" WC by energizing the second solenoid gas valve. This provides 100% of the total furnace output. pg 752

Why is excess air supplied to all gas-burning appliance burners?

To make sure that all of the methane is burned. pg 717

T or F: A thermocouple develops direct current and voltage.

True - A thermocouple generates electrical current and voltage when heated by the pilot flame. 30 mv pg 735

T or F - An orifice measures the quantity of gas to the burner as well as directs the gas stream.

True. pg 726

The four types of gas furnace air flow patterns are _____, ______, _______ and _______.

Upflow, low-boy, downflow, and horizontal. pg 711-712

Define the dew point temperature as it applies to a condensing furnace?

When natural gas is burned, water vapor is produced. This vapor contains heat. If it can be condensed, latent heat will be given off. The dew point temperature (DPT) is the temp that the condensation process begins. In flue gas, the DPT varies depending on the composition of the flue gas and the amount of excess air. pg 744

Describe how a vent damper shutoff functions.

When the furnace is off, heated air can leave the structure through the draft hood. Automatic vent dampers that close when the furnace is off and open when the furnace is started can be placed in the vent to prevent air loss. pg 757

Describe the function of the draft diverter.

When wind conditions produce a downdraft, the draft diverter helps reduce the chance of the pilot flame being blown out and the main burner flame being altered. pg 755

An orifice is a drilled hole in a ______.

spud. pg 726