Basic Gas-Fired, Forced-Air Heating Systems Flashcards Preview

Electrical Lab HVAC Period 2 SAIT-Alberta > Basic Gas-Fired, Forced-Air Heating Systems > Flashcards

Flashcards in Basic Gas-Fired, Forced-Air Heating Systems Deck (22)
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1
Q

Would you ever encounter a standard efficiency (65 - 75%) furnace in a new home?

A

No, in 1992 legislation was passed to ban furnaces lower than 78% efficiency. Old homes still have these.

2
Q

What is the purpose of a thermocouple?

A

It shuts down the gas if there is no pilot flame.

3
Q

Is a pilot safety switch 100% safe?

A

No, it will not shut down the pilot gas.

4
Q

Why does propane pool on the ground?

A

Because it is a heavier gas than air and therefore does not rise.

5
Q

What are three additional names for a safety gas valve (100% shutdown-[automatic pilot device])?

A

Thermomagnetic gas valve,
pilotstat,
safe-shut off valve.

6
Q

How does a diaphragm valve operate?

A

By the use of a bleed port to allow a pressure difference on either side of the diaphragm, based on a small electrical input.

7
Q

What is the most common Safety pilot device in use?

A

A combination gas valve.

8
Q

Do belt driven fans require regular maintenance?

A

Yes their bearings and belts must be checked and greased if needed.

9
Q

What are 4 thermocouple tests performed by an electrician?

A

Open circuit,
Closed circuit,
Response time,
Dropout point.

10
Q

In a small furnace (<400000Btu/h) what is the maximum response time of a thermocouple?

A

3 minutes.

11
Q

In a medium furnace (400000Btu/h to 1000000Btu/h) What is the maximum response time for a thermocouple?

A

20 seconds for propane and 90 seconds for natural gas.

12
Q

If a closed circuit test reads a very small mV value, what is the likely cause of the problem?

A

A weak thermocouple.

13
Q

Which thermocouple test is performed by a gasfitter?

A

Turn down test.

14
Q

Thermocouples have a generally slow response time, why are they still so widely used?

A

Because they are simple, durable and inexpensive.

15
Q

What is a typical thermostat setpoint for a Basic 24v FAH system?

A

20-25 degrees C.

16
Q

What is a typical High limit setpoint for a Basic 24v FAH system?

A

82-93 degrees C

17
Q

When does a high limit reset?

A

When the temperature drops 11 degrees below setpoint.

18
Q

When troubleshooting a 24v heating system, what is the first thing to check?

A

Voltage across the thermostat, normally and on a call for heat.

19
Q

Why do large unit heaters utilize step open gas valves?

A

To reduce the inrush of gas to the burners and reduce explosions.

20
Q

What is the major difference in a 120v thermostat?

A

It has no anticipator, which makes it suitable in all control circuits, except it will produce wider temperature swings.

21
Q

What makes troubleshooting unit heaters easier than a 24v system?

A

There is only one voltage to deal with in a unit heater.

22
Q

Who requires that the main exhaust fan be interlocked to start a furnace fan to make up the air in a house?

A

The Alberta building code.