Mobile Home Furnace, How they work
We will begin by looking at the operation of an older style gas mobile home furnace. The heating cycle
begins when the thermostat senses that the room temperature is below the setting on the thermostat. On
most systems this thermostat is a simple mechanical snap action switch. When the switch closes, control
voltage (24 vac) is supplied to the furnace.
This control voltage passes through high temperature limit controls. These limit controls protect the
unit from overheating. There are two of them on the typical unit with one being located in the blower
compartment and one in the heating section.
If no over heating is present, the control voltage is applied to the gas valve. These older systems
used a small flame that was constantly lit to ignite the gas. This is called a standing pilot gas
furnace. A device called a thermocouple senses that the pilot is burning and opens a port in the gas
valve. When the control voltage is applied to the gas valve, the valve is opened and gas is allowed to
flow through the burner. It is lit by the pilot flame.
Newer furnaces that have an efficiency of 80% or higher do not have a pilot light. The control voltage
from the thermostat is delivered to a furnace control board.
The control board provides power to an inducer motor which establishes combustion air flow through the
heat exchanger. When the inducer gets up to speed, the pressure switch closes and allows the control
voltage to pass through it and the limit controls. The control board then either starts a spark or warms
a device called a hot surface ignitor. This ignitor heats up and glows orange. Then the control provides
power to the gas valve and gas flows through the burner. It is then lit by either the spark or the hot
surface ignitor. After the control board senses that the gas is burning, the spark generator or ignitor
is turned off.
After a time delay the blower is energized and air is circulated through the furnace. In older units,
a fan control senses the furnace temperature and energizes the blower at a set temperature.
When the thermostat senses that the air temperature is above the setpoint, the control voltage to the
furnace is turned off. This shuts off the gas. The blower continues to run for a set period of time to
cool off the heat exchanger. On older units, the blower runs until the furnace temperature is below a