Heating I Section#1 Furnace - Gas and Oil Quiz#3

This study session deals with the combustion process and equipment in a gas furnace.

The estimated completion time for this study session is ninety minutes.

Objectives
At the end of this study session, you should be able to:

• Write the theoretical combustion equation for natural gas.
• Determine the amount of combustion air required for a gas appliance, given its input capacity in BTU/hr.
• List and describe the two most common types of gas burners.
• Describe, in one sentence, the function of the gas valve, pilot light and thermocouple, on a residential furnace.
• List nine conditions that may be found with gas combustion equipment.

A 80,000 BTU/hr furnace and a 40,000 BTU/hr water heater are in an open unfinished basement. How much combustion air is required? How much dilution air is required?

Standard answers:

We need 15 cubic feet of air for every cubic foot of gas burned, for each of combustion and dilution air. One cubic foot of gas contains 1,000 BTUs. The total firing rate of the 2 appliances is (80,000 + 40,000)=120,000 BTU/hr. This means we burn gas at a rate of 120 cubic feet per hour, or (120/60 minutes) 2 cubic feet per minute. This means we need (15cubic feet of air/ cubic foot of gas x 2 cubic feet of gas/minute) 30 cubic feet of air per minute(CFM) for combustion air, and 30 CFM for dilution air.

My answer:

Typically, 10 cubic feet of are is required for a perfect combustion of 1 cubic feet of gas, which is equal to 1000BTUs. 5 cubic feet of access air is needed for perfect combustion. In conclusion, for 80, 000BTUs, 15 multiple 80 = 1200 cubic feet of air is needed for combustion, for 40, 000BTUs, 15 multiple 40 = 600 cubic feet of air is needed for combustion. In total, 1200 + 600 cubic feet air is needed for combustion. At least same amout of air is needed for dilution, so another 1800 cubic feet of air is needed for dilution

An 80,000 BTY/hr input natural draft furnace with a draft hood is enclosed in a closet. How big should the 

combustion air intake be? How big should the dilution air opening be?

Standard answers:

Each opening should be 80 square inches.

My answer:

80, 000 BTU/hr = 80 cubic feet of gas per hour which required 1200 cubic feet of air per hour = 20 cubic feet per min

The combustion air intake has to be able to at least take in 20 cubic feet per min. 

The same as dilution air, which has to at least take in 20 cubic feet per min. 

Describe briefly how the pilot light and thermocouple work to ensure safety.

Standard answers:

The pilot flame heats the thermocouple, which senses the flame, allowing the gas valve to open. If the flame goes out, the gas valve is automatically closed.

My answer:

The automatic safety control is the thermocouple. The thermocouple has a very special job. It sits in the pilot flame and senses whether the pilot is on or not. If the pilot is on, the thermocouple allows the electrical current to flow through to the gas valve so that when the thermostat calls for heat and closes its part of the circuit, the gas valve can open. If the thermocouple doesn't sense a pilot flame (because the pilot is out or the thermocouple is defective), the thermocouple will open the circuit and won't let electricity flow through to open the gas valve, even when the thermostat does call for heat. 

Explain the path the gas and primary air take through a ribbon burner. You should start at the gas valve.

Standard answers:

The gas travels through the manifold to the orifice. As it passes through the orifice it mixes with the primary air at the shutter, then the combination air/gas travels down the burner where it is burned.

My answer:

primary air goes through shutter and then mixed with gas flow

gas started at gas valve, then go through manifold, then pass gas orifice, then mixed with air and go into burner. 

List 3 reasons that a furnace may short cycle. After each one, note if the reason is a minor maintenance item or a major concern.

Standard answers:

Thermostat problem, defective high limit switch, cycling on high limit switch, (which is working correctly) due to poor air flow causing overheating or a failed heat exchanger.

My answers:

1. A thermostat is defective or has an inappropriate anticipator setting. Minor maintenance. 

2. A fan limit switch that is defective or not set properly. Minor maintenance.

3. Cycling on high temprature limit switch due to 

- A crack in the furnace heat exchanger allowing hot products of combustion to enter the house-air side of the furnace and trip the high limit switch every time the furnace come on

- most of the supply registers are closed. 

- the furnace is too large for the ductwork

- the ductwork is collapsed or obstructed. 

- the air condition coil is plugged, restricting airflow.

List 6 conditions you'll find on gas combustion equipment.

Standard answers:

rust, scorching, inoperative, delayed ignition, short cycling, poor flame color or pattern, flame wavers when house fan comes on, dirt or soot, gas odor or leak.

My answers:

1. Inoperative

2. Scorching

3. Poor flame color or pattern

4. Flame wavers when house fan comes on

5. Rust

6. Dirt or soot

7. Delayed ignition

8. Gas odor or leak

9. Short cycling

What are the three normal combustion products of natural gas?

Standard answers:

water vapour, carbon dioxide and heat

My answers:

Carbon Dioxide, water and heat.

What is meant by "spillage" on a gas furnace?

Standard answers:

Spillage (backdrafting) is when the combustion products flow out of the furnace into the room through the burner or draft hood, rather than up the vent. This is a life threatening situation.

My answers:

Spillage is also known as backdraft. This is also implied lack of dilution air. This means that combustion products can't go up the chimney or vents, but are dumped back into the room.

Incomplete combustion of natural gas produces:

nitrogen dioxide

carbon dioxide

ozone

carbon monoxide

sulphur dioxide

Standard answers:

carbon monoxide

Scorching is noted on the inside of the burner access door. This is generally a result of:

overfiring

underfiring

lack of combustion air

faulty high limit switch

chimney obstruction

Standard answers:

faulty high limit switch

The flame wavers over one burner in a gas furnace when the blower comes on. What is the likely cause of this?

the heat exchanger has failed

the gas valve is faulty

the blower motor is running too slow

airflow across the heat exchanger is insufficient

the vent is partially blocked

Standard answers:

the heat exchanger has failed