High Efficiency Furnace Components

Many homeowners treat their furnaces with a sort-of "monster in the basement" attitude. This is certainly understandable considering the basic commodity a furnace supplies. Fire! This is not to be confused with the "fire" one enjoys in the backyard or in lets say a barbecue where the fire is usually contained within a ring or monitored while it burns, but this is fire inside our homes; burning most often while we go about our daily motions completely unaware of its many stages of operation.

Clearly this should bring a healthy respect to the topic of furnace safety but we should also be careful not to exaggerate our fears concerning this mysterious member of our household. Furnaces when properly serviced perform their many tasks with a remarkable safety record; however, according to the U.S. Fire Administration* approximately 55,000 residential home heating fires occur annually. The agency also stated that, "Many of these fires can be prevented through proper maintenance and proper use of heating equipment." So it seems the safest approach to this other member of the household may just be to get to know it better.

How does a high efficiency furnace work and what are its major components? We will start out by listing the basic components of a high efficiency (or condensing) furnace and describing their part in managing the "flame".
Heat Exchanger

In a high efficiency furnace there are at least two heat exchangers: A primary and a secondary heat exchanger. These components are similar to the backyard fire ring used to contain the wood, embers, and flames. The heat exchanger is like a metal pipe with both ends open. Flames enter the metal container (heat exchanger) on one side and exhaust through the other side. The exhaust on the outlet side of a high efficiency heat exchanger is expelled (vented) by a material called PVC piping. The main function of the heat exchanger is somewhat intuitive to its name. It exchanges heat from the combustion (burning gas) to the surrounding air that blows over it from inside your home. The important point to remember about a heat exchanger is that its primary function is to keep the exhaust fumes separate from the inside air of your home during the combustion process. When heat exchangers crack they no longer perform this function and need to be replaced.
Automatic Gas Valve

Automatic gas valves are devices within furnaces that automatically open or close and allow raw (unburned) natural or propane gas into the heat exchanger. When the thermostat sends a signal to the furnace for a heat cycle and all safety conditions within the furnace are met, the automatic gas valve opens and allows gas to flow. This gas flows into a device called a "burner" where it is ignited and sent to the heat exchanger.

Pressure Switches

These are safety devices that constantly monitor the internal pressure of the combustion area. If there were a problem with the vent pipe, vent fan, or heat exchanger the pressure switch is one component that eventually shuts off power to the automatic gas valve.

Vent Fan (induced fan motor)

This device draws the vented fumes out of the heat exchanger and into the PVC pipe that directs the fumes outside. If this device were to fail the automatic pressure control would sense the loss of pressure and shut down the combustion process.

Roll-out Switches

Roll-out switches are safety switches that automatically shut off the combustion process if they sense fire where it's not suppose to be. This condition could happen if the vent pipe became clogged or if the vent fan were to quit operating.

Indoor Blower Motor

The indoor blower motor moves air through the furnace. It is the noise you here as your furnace pushes air around the house.

Limit Control

Another safety control primarily concerned with shutting off the combustion process in case the indoor blower motor fails. This prevents the heat exchanger from overheating due to a lack of air over the heat exchanger because of a clogged filter or broken indoor blower motor.

IFC (Integrated Furnace Control) Board

Here is where the brain of the furnace resides. This electronic component monitors and controls all of the functions integral to the heat cycle of the furnace. If any of the safety control devices within the furnace report an unsafe condition it is the IFC board that interprets the signal and administers the appropriate response. The IFC board is also responsible for normal operation and self-diagnosis.
Should you have a high efficiency furnace in your home you should know that many years of design and testing have gone into the safety of this appliance. Having it serviced on a regular basis will help ensure its safe and efficient operation.