April 2000 Edition

What Is A Heat Exchanger

We keep hearing about problems with furnace heat exchangers. Just what is a furnace heat exchanger?
Heat exchangers are normally found in gas furnaces around the Atlanta area. The heat exchanger is a compartment that encompasses the heat that is given off by the burners. The flames rise vertically into the heat exchanger from each burner opening. The heat exchanger is open at the bottom to allow fuel into the combustion area and it is open at the top to allow the exhaust gases from the combustion process to be routed to the flue and up through the roof to the outside of the house into open air.

Heat Exchanger

Heat exchangers operate at about 1600 degrees F. Comparatively speaking 1600 degrees is cool - hence the heat exchanger is thin enough to allow temperature near its exterior to rise to about 400 degrees F. There the room air is circulated by the blower across the bellows of the heat exchanger and then out the ductwork towards each room at a temperature of 100 to 120 degrees F. This is the critical area of a natural gas furnace. The heat exchanger must be thin enough to allow for the transfer of heat from the open flame, a thickness of about 3/32 of an inch. But also the heat exchanger must be durable enough to withstand heating and cooling multiple times each day for the life of the furnace.
A heat exchanger in the traditional furnace is made from rolled steel of two mirror image parts seamed together like a clam shell. Many furnaces fail by developing cracks in the sheet metal, cracks along the welded seams or holes due to rust or corrosion.
Occasionally holes formed by rust can be seen with the eye or with the aid of a mirror, but only 20% of the total surface of the heat exchanger is visible to view even with a mirror. Holes or cracks often are visible only when thermal expansion causes the cracks to open. Visual inspection is not normally possible when the furnace is in operation.
Many heat exchangers fail by becoming overheated. A heat exchanger is protected from overheating by a carefully adjusted upper limit device. The upper limit device causes the furnace to cycle to its off position when the temperature of the air in the plentum above the furnace exceeds the limit set by the mechanic.
Plugged air filters accelerate heat exchanger failures. A furnace filter neglected for several heating seasons will block the flow of air over the heat exchanger. The internal temperature of the furnace may exceed the continuous operating design temperature without reaching the high limit. Broken welds or cracks may result.
When a heat exchanger fails, the carbon monoxide given off by the burners leaks through the heat exchanger and is mixed with the air going over the heat exchanger. This air is then circulated to each room of the house through the ductwork. When this happens, carbon monoxide poisoning is possible. This is why it is important to have all furnaces, with heat exchangers over ten years old, inspected by a licensed heating and air conditioning contractor at least once a year.
 Carbon monoxide detectors will assist in protecting your family from faulty furnaces. We recommend adding at least two carbon monoxide detectors. One detector should be located in the furnace room around the flue pipe to alert you to backdrafting problems associated with a blocked or faulty flue pipes. This could also let you know if a clothes dryer, located in the same room, is drawing air down the flue pipe, into the room and out the dryer. This happens more often than you might think. The other detector should be located in or just outside the Master Bedroom sleeping area. The Consumer Product Safety Commission (CPSC) recommends purchasing a carbon monoxide detected that meet the requirements of Underwriters Laboratories (UL) standard 2034. Detectors that meet the UL 2034 standard currently cost between $35 and $80. Some models can be mounted on the wall or ceiling and some will plug directly into an electrical outlet with a battery backup.

Clothes Dryers Can Cause Fires

 The CPSC estimates that there is an estimated annual 15,500 fires, 10 deaths and 310 injuries associated with clothes dryers. Some of these fires may occur when lint builds up in the filter or in the exhaust duct. Under certain conditions, when lint blocks the flow of air, excessive heat build-up may cause a fire in some dryers. To prevent fires:
l Clean the lint filter regularly and make sure the dryer is operating properly. Clean the filter after each load of clothes. While the dryer is operating, check the outside exhaust to make sure exhaust air is escaping normally. If it is not, look inside both ends of the duct and remove any lint. If there are signs that the dryer is hotter than normal, this may be a sign that the dryer’s temperature control thermostat needs servicing.
l Check the exhaust duct more often if you have flexible duct. This type of duct is more apt to trap lint than ducting without ridges. If you have flexible plastic ducting, replace it with flexible metal ducts. Plastic ducts have been know to be a fire hazard.
l Closely follow manufacturers’ instructions for new installation. Most manufacturers that get their clothes dryers approved by Underwriters Laboratories specify the use of metal exhaust ducts only. If metal ducts are not available at the retailer where the dryer was purchased, check other locations, such as a hardware or building supply stores. If you’re having the dryer installed, insist upon metal duct unless the installer has verified the manufacturer permits the use of plastic duct.

Source: ASHI Reporter February 2000

If you have a question, change of address, comment, home tip or would like to send Home Tips to your clients, send your letter to Home Tips, Christian Building Inspectors, Inc., 1003 Star Court, Norcross, Georgia 30093. You can E-Mail your questions to us at rodharrison@christianbuildinginspectors.com. We reserve the right to edit questions for length.

Quote Of The Month

"A child on a farm sees a plane fly overhead and dreams of a faraway place. A traveler on the plane sees the farmhouse...and dreams of home"

- Carl Burns

A Tip Of The Hat To:

 Kathy Kim

Bestway Realty, Inc.

2505 Chamblee Tucker Road

Chamblee, Georgia 30341