Basic Gas-Fired, Forced-Air Heating Systems Flashcards
Would you ever encounter a standard efficiency (65 - 75%) furnace in a new home?
No, in 1992 legislation was passed to ban furnaces lower than 78% efficiency. Old homes still have these.
What is the purpose of a thermocouple?
It shuts down the gas if there is no pilot flame.
Is a pilot safety switch 100% safe?
No, it will not shut down the pilot gas.
Why does propane pool on the ground?
Because it is a heavier gas than air and therefore does not rise.
What are three additional names for a safety gas valve (100% shutdown-[automatic pilot device])?
Thermomagnetic gas valve,
pilotstat,
safe-shut off valve.
How does a diaphragm valve operate?
By the use of a bleed port to allow a pressure difference on either side of the diaphragm, based on a small electrical input.
What is the most common Safety pilot device in use?
A combination gas valve.
Do belt driven fans require regular maintenance?
Yes their bearings and belts must be checked and greased if needed.
What are 4 thermocouple tests performed by an electrician?
Open circuit,
Closed circuit,
Response time,
Dropout point.
In a small furnace (<400000Btu/h) what is the maximum response time of a thermocouple?
3 minutes.
In a medium furnace (400000Btu/h to 1000000Btu/h) What is the maximum response time for a thermocouple?
20 seconds for propane and 90 seconds for natural gas.
If a closed circuit test reads a very small mV value, what is the likely cause of the problem?
A weak thermocouple.
Which thermocouple test is performed by a gasfitter?
Turn down test.
Thermocouples have a generally slow response time, why are they still so widely used?
Because they are simple, durable and inexpensive.
What is a typical thermostat setpoint for a Basic 24v FAH system?
20-25 degrees C.
What is a typical High limit setpoint for a Basic 24v FAH system?
82-93 degrees C
When does a high limit reset?
When the temperature drops 11 degrees below setpoint.
When troubleshooting a 24v heating system, what is the first thing to check?
Voltage across the thermostat, normally and on a call for heat.
Why do large unit heaters utilize step open gas valves?
To reduce the inrush of gas to the burners and reduce explosions.
What is the major difference in a 120v thermostat?
It has no anticipator, which makes it suitable in all control circuits, except it will produce wider temperature swings.
What makes troubleshooting unit heaters easier than a 24v system?
There is only one voltage to deal with in a unit heater.
Who requires that the main exhaust fan be interlocked to start a furnace fan to make up the air in a house?
The Alberta building code.
