HVAC For Beginners

Mobile Home Furnace

Understanding Your Heating System

The mobile home furnace provides heat through electric elements or the burning of a fuel such as natural gas or fuel oil. Non-electric units transfer this heat to the home's air via a heat exchanger. The air is circulated over and around the heat exchanger by a blower. The air is then sent into a duct system where it is delivered to the individual rooms.

The duct system is different from the system in a traditional residence.

Most mobile home duct systems do not include a return air duct system. The air is usually supplied to the blower through slots in the furnace access doors. The filters are usually located in those doors.

The mobile home furnace is typically installed directly above the supply duct.

A duct connector which is a special piece of metal duct connects the furnace outlet to the supply duct. The typical duct system is made from light gauge steel or aluminum. The duct typically runs the length of the trailer and is usually a single size. This type of system is commonly called a extended plenum system. Branch ducts are attached to this extended plenum and carry the conditioned air to the individual rooms. Registers in the rooms allow the air to exit the ducts and usually have a damper on them to control the amount of air flow.

A doublewide mobile home has two extended plenums. One runs down the length of each half of the home. They are usually connected by a flexible duct called a crossover duct.

A mobile home furnace uses a special venting system called a roof jack. This contains the entire intake and exhaust vent system in one package. Air for combustion is pulled in the intake vent which surrounds the exhaust vent pipe. The roof jack is made of metal and is used on all the units that have an efficiency (afue) rating less than 90%.

A high efficiency mobile home furnace (90% afue or above) has a second heat exchanger which cools the exhaust from the furnace. This allows the furnace to be vented with pvc pipes. Water is produced when the exhaust is cooled and is referred to as condensate. This water goes through a condensate drain system and is discharged.

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 set temperature.